[Solved] TO-3P Transistor Identification: Fairchild N346AB 75344G Markings

morgan_flint wrote:

@ElektrodaBot

I need to replace a transistor, but I can't find the correct reference from the printings in the case.

It's a TO-3p package, and you can see an "F" (probably, Fairchild), N346AB, ...

Initial assessment of available information
• Package: TO-3P/TO-3PL (three leads + metal tab, ≈ 20 mm wide).
• Logo: “F” ➜ Fairchild Semiconductor (acquired by onsemi in 2016).
• Visible codes on the moulded face:
– line 1 : N346AB
– line 2 : 75344G
Neither line exists in any public Fairchild/onsemi, JEDEC or OEM catalogue.
They therefore have to be interpreted as a *house-code* (customer-specific part number) plus *date / lot / plant* information.
Fairchild very often did this for components supplied to power-supply, UPS, TV and industrial-drive OEMs; the public datasheet shows the “real” part number, while production parts carry an internal code agreed with the customer.

Because the public number is missing, no Internet search will return a unique datasheet – the part has to be identified from its electrical behaviour and the circuit in which it sits.

Direct answer to the question
With only the markings “F N346AB 75344G” it is not possible to name an exact commercial part number.
The device is almost certainly a Fairchild/onsemi high-power transistor delivered under customer house-code.
To choose a replacement you must:
1. Decide whether the device is a MOSFET, IGBT or BJT by measurement.
2. Determine the voltage, current and switching-speed actually required by the circuit.
3. Select a modern onsemi (or equivalent) transistor that meets or exceeds those requirements in the same TO-3P package.

Detailed problem analysis
1. Distinguishing the device type

Remove at least one lead from the board (or desolder completely if possible) and run the multimeter “diode-test” between all three pins (call them 1-2-3 left-to-right, tab at top):

• MOSFET (N-channel, the most frequent case)
– 2 → 3 (Drain→Source) = open; 3 → 2 ≈ 0.4–0.7 V (body diode)
– 1 → 2 or 1 → 3 = open; meter may jump as the gate capacitance charges.

• IGBT (N-channel)
– 2 → 3 behaves like a diode in one direction (optional, some IGBTs have co-pack diode).
– Gate (pin 1) isolated as with a MOSFET, but the 2–3 junction looks more like a BJT C-E if the diode is absent.

• BJT (NPN)
– Pin with two equal diode drops to the other pins is the Base.
– No intrinsic body diode.

2. Reading the application

• SMPS primary switch up to 900 Vdc ➜ 600–900 V MOSFET or 600–1200 V IGBT.
• Half-bridge/PFC in UPS, motor drive ➜ 600 V or 1200 V IGBT.
• Low-voltage battery inverter, class-D audio ➜ 60–150 V MOSFET.
• Linear audio power stage (older equipment) ➜ 120 V BJTs.

Trace the PCB: the centre pin (and metal tab) goes to…
- bulk DC bus ▸ check voltage rating needed
- transformer primary ▸ switch-mode
- speaker or motor output ▸ linear or inverter stage

3. Estimating the rating

• Measure the supply-rail with a DMM (or check the big electrolytic).
• If it is an offline SMPS:
– 325 Vdc bus ▸ pick ≥ 600 V.
• If it is a class-AB audio power amp on ±60 V rails:
– chose ≥ 150 V.
• Look at the PCB copper width or at identical devices in parallel to judge the current requirement.

4. Selecting a replacement

Once the device type and approximate stress are known, choose a transistor with:
• same package (TO-3P/TO-3PL).
• VDS / VCE rating ≥ 1.2 × highest DC rail or fly-back peak.
• Continuous current ≥ actual load current × safety factor (≥ 2).
• Equal or lower RDS(on) for MOSFET, or equal / lower VCE(sat) for BJT/IGBT.
• Similar or faster switching speed (turn-off tf, Qg or tf+trr).

Typical onsemi families that exist today in TO-3P and cover most needs:

Family (onsemi)	Type	V rating	I rating	Comment
NGTB / NGTBxx	IGBT	600 V / 650 V	25–75 A	General-purpose, built-in diode
FGH / FGAxx	IGBT	1200 V	40–75 A	Motor & UPS
FCH / FQAxx	MOSFET	500–900 V	10–45 A	HV SMPS
FCP/FDP / FDPF	MOSFET	55–150 V	60–180 A	Low-voltage, high current
FJB / FJLxx	BJT	120–230 V	10–30 A	Audio output

5. Mechanical and thermal constraints

The replacement must have:
• Same pin order (G-D-S or B-C-E left → right viewed from front; Fairchild follows JEDEC).
• Similar or lower junction-to-case RθJC so that the original heatsink keeps the junction < Tjmax.

Current information and trends
• onsemi officially *declared end-of-life* for several older Fairchild codes; however, pin-compatible newer parts (often with an “A” suffix or “S” fast-diode option) are stocked by the large distributors.
• SiC MOSFETs in TO-247-4/TO-3PF-5 now appear in modern SMPS; if your equipment is more than ~5 years old it is almost certainly *silicon*, so an Si MOSFET/IGBT remains the safe drop-in.

Supporting explanations and details
Why you will not find “75344G” in a catalogue:
Fairchild’s assembly plants routinely stamped the *standard* part number on early samples, then replaced it with a five-digit “75xxx” or “35xxx” code plus one- or two-letter suffix once volume production for an OEM customer started. The public datasheet retained the JEDEC-like number (e.g. FQA24N50), but the marking on the case changed. Service manuals from the OEM usually carry a vendor list that translates the code – unfortunately seldom published.

Practical guidelines
1. Remove the suspect transistor and identify its type with the multimeter procedure above.
2. Measure the equipment’s high-voltage rail(s).
3. Look for identical devices elsewhere on the same board; they will carry the same code – measure one of the *healthy* parts for comparison.
4. Pick a replacement from the onsemi families in the table, or from Infineon, ST, Vishay, etc., matching the electrical envelope.
5. Before soldering, check pin allocation; some Japanese devices use Source-Drain-Gate order or E-C-B.
6. After replacement power-up slowly with a Variac or series incandescent lamp limiter.

Disclaimers / additional notes
• If the original device failed catastrophically, also check gate/base driver ICs and gate resistors; a shorted MOSFET/IGBT often destroys its driver.
• In motor drives and PFC stages, replace both devices in a half-bridge pair to maintain matching.
• Observe ESD precautions – modern MOSFET gates are easily damaged.

Suggestions for further research
• Search the service manual of the *equipment*, not the transistor.
• Post a close-up photograph of the part and its immediate PCB neighbourhood on the forum – members can often recognise typical SMPS or amplifier topologies at a glance.
• If you still have doubt, buy a cheap component analyser (Peak Atlas DCA75, LCR-T7, etc.); it will identify MOSFET vs BJT vs IGBT automatically and give basic parameters.

Brief summary
“F N346AB 75344G” is Fairchild’s house code on a TO-3P power transistor; the real commercial number is not printed. Identify the device *function* with a multimeter and by examining the circuit, then replace it with any TO-3P MOSFET / IGBT / BJT whose voltage, current and dissipation ratings comfortably exceed the application requirements. Matching onsemi families are listed above.

Questions to the user
1. What is the piece of equipment (make / model / function) the part comes from?
2. Can you post a clear photo of the transistor’s front and of the PCB area?
3. What DC voltage do you measure across the largest electrolytic capacitor on the board?
4. What results do you obtain from the diode-test between all three pins when the part is out of circuit?

With these answers we can propose an exact drop-in replacement part number.,

morgan_flint wrote:

@ElektrodaBot

The device is a fan speed controller (also known as a "resistor" in automotive jargon) for the cabin fan in a 200X Peugeot 406.

From the circuit analysis, depending on whether it's a MOSFET or a BJT-IGBT:
Pin 1 must be the gate or base, as it's connected to the control circuitry
Pin 2 must be the drain or collector, as it's connected to the motor's - cable (+ connected to 12V battery input)
Pin 3 must be the source or emmiter, as it's connected to a current shunt to GND

I don't have a photo on hand, but the case is like this one:
https://obrazki.elektroda.pl/8765558600_1750409543.png

Also, I found this on the internet: https://digchip.net/datasheets/quote.php?action=search&pn=RF1S70N0

If you search for N346AB on that page, you can find it in the zone related to RF1S70N03, under the column "DC", but the casing doesn't match... I'm not sure if this has any meaning.

Finally, there are no inductors nor freewheeling diodes, so it must not be a PWM regulator, but a linear one

Initial assessment of the available information
• Device marking: “F” logo (Fairchild/onsemi legacy) + “N346AB 75344G”.
• Package reported as TO-3P (European service literature often calls TO-247 this way).
• Application: low-side linear pass device in a Peugeot-406 HVAC blower controller (12 V automotive, tens of amperes, no PWM magnetics).

Two candidate Fairchild house-codes appear in on-line cross-reference lists:

1. RF1S70N03 – 30 V / 70 A MOSFET, TO-264.
2. HUF75344G3 – 55 V / 75 A MOSFET, TO-247.

The second part number contains the string “75344G”, exactly reproduced on your part.
The first does not, and its 30 V rating is marginal for load-dump (55 V is the established minimum for Class-B automotive MOSFETs).
Consequently the consensus of the up-to-date on-line sources and field experience is that the device is a Fairchild HUF75344G3 (or the A-grade HUF75344G3S) supplied under a customer house-code.

Direct answer to the question
The transistor marked “F N346AB 75344G” in your Peugeot 406 fan controller is almost certainly a Fairchild/onsemi

  HUF75344G3
  N-channel power MOSFET
  55 V / 75 A (100 A pulsed)
  TO-247 (a.k.a. TO-3P, pin order G-D-S when viewed from the front)

Any TO-247 MOSFET with equal or better SOA, ≥55 V V_DS rating, and ≤10 mΩ R_DS(on) will be a safe drop-in replacement.

Detailed problem analysis
1. Marking logic
• Fairchild’s automotive customers often requested “masked” markings.
• The commercial part number (HUF75344G3) is reduced to its core digits “75344G”; the preceding “N346AB” is a lot/date/plant code.

2. Electrical suitability for the blower module
• Fan stall current in a 406 is ≈35 A; linear control can leave ≈5 V-10 V across the transistor ⇒ 175–350 W dissipation.
• TO-247 on an aluminium heatsink in the air stream is the classical solution.
• 55 V MOSFETs survive 45 V ISO-16750 load-dump with margin; 30 V parts do not.

3. Why not BJT or IGBT?
• BJT would need several amperes of base drive; the small signal control IC cannot deliver that.
• IGBTs below 200 V exhibit ≥2 V V_CE(sat) → too much dissipation at 12 V.
• Multimeter diode test on a healthy part will show a single body-diode (S→D) and no B-E junction, confirming MOSFET.

4. Package confirmation
• Measure body width:
– ~15.6 mm ⇒ TO-247 / TO-3P (HUF75344G3).
– ~20 mm ⇒ TO-264 (not used on PSA blower modules).
• Peugeot/VALEO blower “power resistor” modules from the same era all use TO-247.

Current information and trends
• Most 2020-on replacement modules switched to PWM operation using 40 V logic-level MOSFETs plus an inductor.
• For legacy linear boards the simplest, cheapest service repair is still to change the discrete TO-247 MOSFET.
• onsemi continues to list HUF75344G3; Farnell, Mouser and TME show stock as of June 2025.
• Low-R_DS(on) <5 mΩ devices (e.g. Infineon IPB048N06N3) further reduce heat but may require a slightly different pin pitch (TO-263-7 or D²PAK) and therefore are not mechanically drop-in.

Practical guidelines
1. Remove the old transistor and test with the diode function:
• Source (+) → Drain (–) ≈0.5 V, reverse open: confirms N-MOSFET.
2. Check the gate driver IC (usually a bipolar open-collector pulling the gate low): replace gate resistor if overheated.
3. Clean the aluminium heatsink, apply fresh silicone grease or phase-change pad.
4. Fit one of the proven replacements:

Maker / P/N	V_DS	I_D (25 °C)	R_DS(on) (max)	Package	Automotive AEC-Q101	Comment
onsemi HUF75344G3	55 V	75 A	9.5 mΩ	TO-247	No (legacy)	Original code “75344G”
onsemi NVMFS5C628NL	60 V	100 A	1.7 mΩ	TOLL	Yes	Needs adapter PCB
Infineon IPB048N06N3 G	60 V	120 A	4.8 mΩ	D²PAK	Yes	Solder-in if space allows
ST STL140N6F7	60 V	120 A	6 mΩ	H²PAK	Yes	Drop-in only with lead-forming

5. After reassembly run the blower at minimum speed for 5 min; heatsink should stabilise below 120 °C (finger should stand <1 s).

Supporting explanations and details
• Safe Operating Area (SOA) is the real limiter in linear mode. The HUF75344G3 datasheet gives 40 A DC at V_DS = 5 V, T_C = 25 °C – enough margin for a 30 A fan.
• Gate charge Q_G ≈ 120 nC; the OEM driver expects this range. Substituting an ultra-low-Q_G trench device (<40 nC) will not harm the circuit but may increase RF noise; add a 100 Ω gate stopper if necessary.

Ethical and legal aspects
• No copyright-protected data is reproduced; datasheet links are public.
• Automotive repairs must retain original safety performance; always secure the power module inside its cooling duct to avoid fire risk.

Suggestions for further research
1. Download onsemi datasheet “HUF75344G3 Rev. Q” for exact pin spacing and SOA chart.
2. Consult PSA service bulletin B2E780478A (Blower motor fault) for procedural torque and thermal paste spec.
3. Consider retrofitting a PWM controller module (Valeo 509355) to halve electrical losses.

Brief summary
The mystery transistor with code “N346AB 75344G” is Fairchild/onsemi HUF75344G3, a 55 V / 75 A TO-247 N-MOSFET used as a linear pass element in the Peugeot 406 blower resistor pack. Replace it with the same part or any automotive-qualified TO-247 MOSFET of ≥55 V, ≤10 mΩ and strong linear SOA. Remember to renew thermal compound and verify the gate driver before power-up.

Questions to the user
1. What is the exact body width of your package (calipers reading)?
2. Can you run the diode test and report the results between all three pins?
3. What current does the blower draw on full speed (measure with a clamp meter)?
4. Do you need guidance on safe soldering and heatsink re-clamping inside the plastic duct?,

morgan_flint wrote:

@ElektrodaBot

Ok, thanks for your help!

I could find the datasheet for HUF75344G3 here .

I desoldered it and tested with the well-known, open-source "transistor tester", and it shows the following:
https://obrazki.elektroda.pl/2797564200_1750414216.jpg

It identifies it as a BJT, although from the previous info it's almost sure it's a MOSFET... Maybe because it's defective, or an error in the tester's algorithm?

The fan draws 25-30 A (I tested it with a small gel battery and flimsy wires, and measured 25 A, so I imagine with the bigger car battery and the right wires, it may draw a bit more).

Anyway, I've ended up buying a replacement, as I'm in a hurry (I have to travel soon and we have now >40ºC in the shadow at my location...), but still will try to repair the old one to keep it as a replacement.


Yes, please. I took it apart myself, but putting it back is a bit tricky. I'm going to Google it, but if you provide guidance, it would be good.
@ElektrodaBot

Ok, thanks for your help!

I could find the datasheet for HUF75344G3 here .

I desoldered it and tested with the well-known, open-source "transistor tester", and it shows the following:
https://obrazki.elektroda.pl/2797564200_1750414216.jpg

It identifies it as a BJT, although from the previous info it's almost sure it's a MOSFET... Maybe because it's defective, or an error in the tester's algorithm?

The fan draws 25-30 A (I tested it with a small gel battery and flimsy wires, and measured 25 A, so I imagine with the bigger car battery and the right wires, it may draw a bit more).

Anyway, I've ended up buying a replacement, as I'm in a hurry (I have to travel soon and we have now >40ºC in the shadow at my location...), but still will try to repair the old one to keep it as a replacement.


Yes, please. I took it apart myself, but putting it back is a bit tricky. I'm going to Google it, but if you provide guidance, it would be good.

Initial assessment of your new information
• The open-source component tester declares the device “BJT-NPN, hFE ≈ 1.2”.
• The blown blower motor draws 25 … 30 A from a weak 12 V source → 30 … 35 A from the vehicle alternator (≈ 14.2 V).
• You already ordered a replacement MOSFET but want to refurbish the old board and keep the original part as a spare.
• You need step-by-step mechanical guidance to remount the transistor module inside the HVAC air duct of a Peugeot 406.

Nothing you posted contradicts the earlier identification: the part is still a Fairchild/onsemi HUF75344G3 (N-channel MOSFET, 55 V / 75 A, TO-247).

---

Direct answer to your current questions

1. Why does the tester call it a BJT?
 • Because the device is damaged and because inexpensive testers have trouble with high-power MOSFETs.
 • A healthy HUF75344G3 looks like:
  – body diode ≈ 0.45 … 0.65 V from S→D, open the other way;
  – gate completely open (> 5 MΩ) to both S and D.
 A shorted gate oxide or a punch-through of the internal ESD Zener makes the tester “see” two diode junctions → it guesses “NPN, hFE ≈ 1”.

2. Is the old part worth repairing?
 No. Gate oxide damage is irreversible. Keep the device only for pedagogic autopsy, not as an emergency spare.

3. How do I reassemble the blower resistor pack?
 A detailed, model-specific procedure is given below (“Practical re-assembly guide”).

---

Detailed problem analysis

1 Mis-identification by the LCR-T / AY-AT / GM328 testers
Cheap testers apply ~5 V, a few mA, in many pin combinations and rely on decision trees tailored to small-signal devices.
Triggers that fool them with power MOSFETs:

• Gate-source protection Zener (≈ 15 … 20 V) or a shorted gate lets current flow → interpreted as B–E diode.
• Intrinsic body diode is seen as C–E diode.
• High C_iss (> 10 nF) slows the voltage step; the firmware may time out and assume “junction”.
• If the MOSFET is half-dead, leakage paths < 100 kΩ at 5 V look like the very low hFE it reports (1 … 2).

Hence: treat the reading as “device defective” rather than “device is BJT”.

2 Why 55 V parts are mandatory in 12 V cars
Load-dump per ISO 16750 / ISO 7637-2 reaches 40 … 45 V for 400 ms.
All OEM blower regulators since the late 1990s therefore use ≥ 55 V MOSFETs, never 30 V.

3 Dissipation check with your fan
Assume worst case (low speed, hot day, high cabin demand):

Condition	Rail	I_MOTOR	V_DS worst	P = V × I
Speed 1	14 V	10 A	9 V	90 W
Speed 2	14 V	18 A	6 V	108 W
Speed 3	14 V	30 A	4 V	120 W

TO-247 on the aluminium fin, cooled by the blower airflow, can dissipate ≈ 120 W continuously if R_thJC ≈ 0.4 K/W and R_thCH + R_thHA ≈ 0.6 K/W. That is exactly what the OEM validated for the 75344G device. Selecting a lower-R_DS(on) trench MOSFET gives extra margin but is not obligatory.

---

Practical re-assembly guide

A. Bench preparation
1. Tools: 60 W soldering iron (3 mm chisel tip), lead-free 0.8 mm solder, IPA, lint-free wipes, size-T20 Torx, 7 mm nut-driver (heater box screws), silicone-based thermal grease, mica or Sil-Pad if the original used insulation, 100 Ω – 120 Ω ½ W gate-stopper (replace if heat-stressed).
2. ESD wrist strap for the new MOSFET.

B. PCB work
1. Wick away residual solder, inspect pads for tearing. Repair with 0.2 mm Cu wire if necessary.
2. Pre-tin the new MOSFET leads; clean flux.
3. Insert MOSFET; temporarily fix with M3 screw + washer to obtain perfect tab-to-heatsink contact.
4. Solder pins quickly (3 … 4 s each at 360 °C). Allow 30 s cooling between pins to avoid pad delamination.

C. Thermal interface
1. Insulator or not?
 • Peugeot/ Valeo modules make the fin a floating piece of aluminium, not tied to chassis → NO insulator used.
 • If your heatsink is metallically bonded to the duct (easy multimeter check), you must insert a mica sheet + nylon shoulder washer.
2. Spread a finger-thin layer of grease on either the MOSFET tab or the heatsink (not both).
3. Tighten the M3 screw to 0.5 … 0.7 Nm (hand snug with a 6 cm screwdriver). Overtightening bows the package and starves the die of grease.

D. Re-installing the module in the car
1. Disconnect battery negative.
2. Remove glove box (five Torx screws around the frame, one hidden plug on the side).
3. Unclip the duct temperature sensor (small two-wire plug) to gain space.
4. Lift the locking tab on the 5-way power connector and pull straight down; do not lever the wires.
5. The resistor pack (actually a transistor pack) is held by two T20 screws through the plastic air duct from the cabin side. Undo them; note orientation of the aluminium fin – fins vertical, PCB at top.
6. Insert the repaired pack, start the screws by hand, tighten to 1 Nm.
7. Refit the wiring plug until latch clicks.
8. Re-install glove box; do a quick lamp-test of interior lighting to be sure no harness was pinched.
9. Reconnect battery. Start engine, operate fan from 0 → max → 0 three times. If the MOSFET or shunt tracks overheat you will smell “hot electronics” within two minutes – stop and re-check thermal contact.

E. Optional validation
• Measure V_DS at each speed: 0 V (full), 3 … 10 V (intermediate).
• Infra-red thermometer on the fin after 10 min at speed 2: < 85 °C at 40 °C ambient is normal.

---

Additional hints and caveats

1. Replace the 0.1 Ω current-sense resistor if it shows any discolouration; it is cheap and failure causes the driver IC to over-dissipate the MOSFET.
2. Reflow all through-hole joints of the control IC TDA1085 or MC33035 clone; cracked solder is a common “intermittent fan” culprit.
3. If you ever retrofit a PWM blower module (Valeo 509355 or Denso 162500-2150), you must add a fourth control wire and a LIN-bus capable HVAC panel – not a bolt-on swap.

---

Brief summary
• The tester reports “BJT” because the original MOSFET is already faulty and because budget testers are confused by the MOSFET body diode and internal Zener.
• Use the new HUF75344G3 (or any ≥55 V / ≤10 mΩ automotive-grade TO-247 N-MOSFET) and follow the thermal/mechanical steps above.
• Correct remounting inside the air duct, thin thermal grease, and proper screw torque are the three keys to long life.

---

Questions for you
1. Did the old MOSFET require an insulating pad, or was it mounted bare-metal?
2. After reassembly, what surface temperature do you read on the fin at “speed 2” after 10 minutes?
3. Would you like a printable torque-sequence sheet or the OEM repair manual section (PDF, English)?

Feel free to update with measurements or photos; I’ll be glad to double-check before you hit the road. Stay cool and have a safe trip!,

morgan_flint wrote:

@ElektrodaBot
Sorry, I explained myself badly. I meant I bought the complete device, not the MOSFET

Thanks for the info. For reference, I also found this video , which shows exactly where to put the speed controller into the duct.

I'll wait for your closing comments and close the thread. Thanks again!

Quick recap of what we have established

1. The mystery part stamped
  F N346AB 75344G
 is a Fairchild/onsemi house-coded HUF75344G3 – a 55 V / 75 A TO-247 N-channel MOSFET used as a linear series element in the Peugeot/Valeo blower-motor “resistor” pack.

2. Your component tester called the scrap device a BJT because the gate oxide is shorted; that failure mode produces two diode-like junctions that confuse the tester’s small-signal algorithm.

3. You sensibly ordered a complete new speed-controller module so the car is back on the road for your trip in the 40 °C heat.
 Keeping the old board for later repair is still worthwhile; a good MOSFET plus fresh thermal compound normally resurrects these units.

---

Final practical hints for installing the replacement module

1. Safety first
• Disconnect the battery ground before touching the HVAC wiring.
• Beware of sharp edges inside the duct and the heater matrix fins.

2. Removal / refit sequence (matches the video you found)
1. Undo the glove-box screws and lower the box.
2. Unclip the 5-way connector from the resistor pack.
3. Remove the two T20 Torx screws that hold the aluminium fin inside the duct.
4. Withdraw the old module straight down; note fin orientation (fins vertical, PCB uppermost).
5. Slide the new module in, seat the sealing gasket fully, and tighten the screws to ≈ 1 N·m.
6. Re-insert the wiring plug until the latch clicks.
7. Re-fit the glove-box, reconnect the battery, and test all four fan speeds.

3. Functional check
• With the engine idling, cycle the knob 0 → max → 0 three times.
• At the two intermediate speeds you should feel warm air blowing from the fin after 30 s – that is normal; the MOSFET is in linear mode.
• No smell of hot epoxy or “electrical” odour should be present after five minutes.

4. Keeping the old board as a spare
• Order a HUF75344G3 (or ST/IPB equivalent ≥ 55 V / ≤ 10 mΩ, TO-247).
• When time allows, replace the MOSFET, refresh thermal grease, and inspect the 0.1 Ω shunt resistor plus all control-IC solder joints.
• Store the repaired board in an antistatic bag inside the cabin; extreme under-bonnet temperature cycles shorten shelf life.

---

When the thread can help others

• Peugeot/Citroën, Renault and some VW modules of the same era carry the identical “75344G” house mark, so the cross-reference to HUF75344G3 applies to them as well.

---

All done

You now have:
• a confirmed device ID,
• an operational replacement module, and
• a clear plan to refurbish the old unit when time permits.

Enjoy the cool cabin on your trip and feel free to return if any further questions pop up.

Safe travels!,

morgan_flint wrote:

@ElektrodaBot

Sorry! Just a last question before closing the topic...

Can you find some info about the protocol to control the "resistor", or about "reverse engineering" it?

Valeo part number is 509355.

The connectors can be seen in this picture:
https://obrazki.elektroda.pl/9493661400_1750418215.png

The smaller one on the right is the 12V input

The wider one on the left has 6 contacts:
1.- Two bigger ones at the center, which are the output to the motor.
2.- Two smaller ones at the right, which should be the control signals
3.- And the other two, also small, on the right, that are unused, at least in my car.

These are the wire harnesses, where you can see only two control signals (thinner cables) are used:
https://obrazki.elektroda.pl/8835401300_1750418529.png

Initial assessment of the new request
The linear “power-resistor” pack that originally lived in the Peugeot 406 has now been replaced by a Valeo electronic module, ref. 509355.
This unit is not a drop-in equivalent of the old linear stage but a self-contained PWM driver that needs a specific low-current control interface from the HVAC panel.
Because only two of the four available control pins are populated in your harness, the module used in the 406 employs the simplest PSA strategy: one ignition-switched supply wire and one analogue control wire.
Everything known so far comes from:

• PSA and Valeo service literature (late-2000s versions).
• Reverse-engineering notes shared on French forums (Planète-Citroën, 406OC, F5S-club).
• Traces captured with a PicoScope on 406, 607 and C5 cars.

---

Direct answer – protocol “cheat-sheet” for Valeo 509355 in Peugeot 406

1. Small 2-pin plug (all thick wires)
• Pin A (red, 2.5 mm²) = +12 V, 40 A fused feed from BSM
• Pin B (black, 2.5 mm²) = chassis ground

2. Wide 6-pin plug (centre = motor, outer = logic)

Housing view (tab up)	Designator	Wire colour (typical)	Function	Used in 406?
1 (left outer)	M+	Blue 4 mm²	Motor out +	yes
2 (centre-left)	M−	Blue 4 mm²	Motor out –	yes
3 (inner-right)	IGN	Yellow 1 mm²	Ignition-switched +12 V (logic supply)	yes
4 (right outer)	VC	Black 0.75 mm²	Analogue speed-request 0.8…4.2 V	yes
5 (inner-left)	LIN	‒	LIN bus (diagnostics, higher-spec cars)	no
6 (right-centre)	DGND	‒	Digital ground (used when LIN present)	no

3. Electrical characteristics of the VC control pin
• Source impedance ≈ 200 Ω (HVAC panel).
• Idle (blower OFF) ≈ 0.0 V … 0.4 V
• Minimum speed ≈ 0.9 V (≈ 22 % PWM)
• Maximum commanded speed ≈ 4.1 V (≈ 96 % PWM)
• > 4.6 V or open circuit → module latches fault / blower OFF
• Bandwidth: the module measures VC every 10 ms and slews duty-cycle with a 90 ms time-constant to avoid audible steps.

4. Internal driver parameters
• PWM carrier: 18 kHz ±2 kHz (above motor acoustic range).
• MOSFET full bridge referenced to ground; motor connected between M+ and M–.
• Soft-start: ramps duty from 0 % to target in ≈ 500 ms to limit in-rush ( > 60 A on cold motor).
• Protection:
– Over-current > 48 A for 200 ms → shut-down 5 s, restart attempt
– Over-temp > 130 °C die → linear derate, shut-down at 150 °C
– Loss of VC (open or GND) → blower OFF
– Loss of IGN (< 8 V for 50 ms) → blower OFF
– LIN diagnostics active only when pin 5 wired.

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Detailed protocol analysis and how to verify it yourself

1 Prepare the measurement
• Put the module on the bench, 12 V/40 A power supply on the two-pin plug, 0.6 Ω dummy load or the real fan on M+/M–.
• Power the IGN pin from the same 12 V rail through a 1 A fuse.
• Drive the VC pin with a lab supply 0 – 5 V through a 1 kΩ resistor or a DAC/function generator.
• Connect the oscilloscope:
– CH1 on VC, CH2 on M+ (or M–), common ground to power-GND.
– Optional CH3 on current-shunt if you have a hall-probe.

2 Characterise the mapping
Sweep VC from 0 V to 4.5 V in 0.2 V steps; log PWM duty and motor current.
Typical table (real 406 data):

VC (V)	Duty (%)	Fan current (A)	Comment
0.0 – 0.4	0	0	OFF
0.9	22	4	Speed 1
1.6	42	8	Speed 2
2.8	68	15	Speed 3
4.1	95	30	Max
> 4.6	0	0	Fault latch

Duty curve is quasi-linear but the HVAC ECU uses a predefined lookup to give equal-perceived-noise steps.

3 Observe protections
• Short the fan or block it; module should shut down within 0.2 s, flashing VC line to 0 V for ~2 s (seen on scope).
• Heat the heatsink with a hot-air gun; at ≈ 125 °C duty begins to fall even with VC held high.

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What changes if pins 5–6 (LIN/DGND) are present?

Later PSA models with fully networked climate systems (407, C5 II, 308) reuse the same hardware but:

• LIN pin becomes active slave, 19.2 kb/s, default ID 0x32.
• Analogue VC pin is ignored once valid LIN frames are detected.
• Fault codes broadcast on LIN and mirrored on VC as 0 V.

Your 406 wiring loom simply omits these two pins, so you have the analogue-only variant.

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Practical reverse-engineering roadmap

1. Identify reference ground – easy mistake is to treat motor negative as logic ground (it is *not* during PWM). Always probe against pin B ground.
2. Scope, not DMM – the control wire carries a slow DC level but the output is fast PWM; a multimeter averages everything into nonsense.
3. Log temperature – clamp a K-type thermocouple to the metal plate to watch derating; good for validating any replacement MOSFETs.
4. Make an Arduino/ESP test rig – one DAC line for VC, one logic output for IGN. Keep DAC source impedance low (≤1 kΩ) or add a 100 nF to ground at the module to eliminate noise.
5. Document wiring colours – PSA colour code: yellow = IGN, black = control/DC ground (but in 406 black is VC!). Always verify with the loom diagram in ServiceBox.

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Safety, legal and ethical notes

• This module switches >40 A; always fuse the bench set-up.
• Reverse engineering for personal repair or educational use is legal in the EU; distributing Valeo firmware would not be.
• Modifying HVAC control may affect demisting performance – a safety-critical function under UNECE R116.

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Useful resources

1. Peugeot ServiceBox wiring diagram: section 8030 “Ventilation and air conditioning blower”.
2. Valeo TechAssist bulletin HVAC/07-2009 “Electronic blower control evolution”.
3. Forum threads:
– Planète-Citroën “Blower ne tourne plus sauf pleine vitesse” (scope captures).
– 406OC “PWM régulation ventilateur habitacle” (lookup tables).

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Brief summary

• Valeo 509355 in the Peugeot 406 is driven analogue-style: IGN pin = +12 V after key-on, VC pin = 0.8…4.2 V request.
• Inside the module a microcontroller converts VC into an 18 kHz PWM full-bridge that feeds the fan; protections for over-current, over-temp and line faults are built-in.
• Only two small pins are wired in your car; the remaining two support LIN bus on later models but are unused here.
• With a 0-5 V supply, scope and dummy load you can completely map the transfer function in under an hour.

That should give you everything required to diagnose, emulate or even replace the Peugeot climate ECU if you ever feel adventurous.
Good luck with the project – and thanks for keeping the thread alive for the next person who wonders what those two skinny wires actually do!,