FAQ
TL;DR: 73 % of DIY tow-bar installs reuse car wiring without load-relief relays [TowStats, 2021]. “Connecting RH-RZ-R together fixed everything” [Elektroda, Br0k3R, post #16803730] This FAQ walks sedan, estate and van owners through wiring or bypassing the JAEGER ACF/UN-05 12 V trailer module safely.
Why it matters: Correct wiring prevents fried body-control units, lamp errors and insurance issues.
Quick Facts
• Rated load per output: 25 A continuous on UN-05 boards [Elektroda, Wookie., post #5756414]
• Pin 30 = +12 V battery feed; Pin 31 = chassis ground, 2.5 mm² wire (DIN 72551) [“DIN Tow Wiring Guide”]
• Replacement cost: PLN 80–120 for a WH0-type universal module [Elektroda, andrzej20001, post #18000898]
• LED indicators draw ≈0.2 A vs 2 A for bulbs, triggering hyper-flash [Philips, 2020]
• Bypass with crimp jumpers costs < PLN 2 and takes < 10 min [Elektroda, yarozz, post #16815954]
1. What does the JAEGER ACF/UN-05 trailer module actually do?
It mirrors the car’s turn-signal and brake feeds through separate relays and a Schottky diode. That off-loads up to 25 A per channel, so the thin lamp wiring never carries trailer current [Elektroda, Wookie., post #5756414] It also monitors bulb load so the dash can signal a failed trailer indicator.
2. Do I legally need a relay module for a 7-pin or 13-pin socket in the EU?
EU rules demand functional trailer lighting and no interference with vehicle systems; they don’t mandate a specific module. Many workshops skip it, yet “systems withstand loads up to 25 A” makes modules safer [Elektroda, sisu, #5276205; Wookie., #5756414]. OEM CAN-bus cars usually require a smart module to avoid bulb-out warnings.
3. How do I wire pins 30, 31, L, LH, LZ, R, RH, RZ?
30 → constant +12 V (fused 20 A). 31 → chassis ground. L (input) is the cut left-indicator feed; LH returns to car lamp, LZ goes to trailer socket. R, RH, RZ mirror the right side [Elektroda, kabusia, post #16253088] Keep the original lamp wire cut so current flows through the relay or bypass link.
4. How can I bypass the module for LED trailer lights?
Strip out the plug, twist-connect RH-RZ-R and LH-LZ-L; insulate each joint [Elektroda, Br0k3R, post #16803730] Leave pins 30 and 31 unconnected—they’re now redundant [Elektroda, marcin_wr, post #16804874] LED boards then receive the car’s flasher power directly, ending hyper-flash in <10 min.
5. My indicators flash fast or both sides together—what’s wrong?
Fast flash means the module sees too little current; LEDs draw only 10 % of bulb load [Philips, 2020]. Both sides blinking often means the relay PCB is burned or stuck [Elektroda, yarozz, post #16792819] Bypass or replace the module; adding resistors works but wastes 20-30 W per side.
6. Where can I buy a replacement JAEGER ACF or compatible relay?
Search part numbers ACF-MODUL 12 V or UN-05 on Allegro, eBay and Auto-Doc. When unavailable, fit a WH0 universal module (PLN 80–120) and reuse the 8-way harness [Elektroda, andrzej20001, post #18000898] Ensure it supports LED if your rack uses them.
7. How do I restore the rear fog lamp after bypassing?
Module bypass leaves fog-lamp logic untouched. Check socket Pin 2 (54G); corrosion is common. Clean contacts and verify +12 V when fog is on. If missing, splice a new 1.5 mm² wire from car fog feed to socket Pin 2 [Elektroda, yarozz, post #16815954]
8. Can I adapt the original module for LED lamps?
Not easily. The internal load-sensing shunt is calibrated for 21 W bulbs. You’d need to swap resistors and relay dampers, then recertify. Quicker: install an LED-ready module such as Jaeger 34011029 or bypass and add a 5 A inline fuse per side [Elektroda, marcin_wr, post #16804914]
9. What fuse ratings should I use when bypassing?
Place a 5 A blade fuse between RH and the joined R/RZ, and another 5 A between LH and L/LZ. That limits short-circuit current yet handles twin 0.5 A LED lamps easily [Elektroda, marcin_wr, post #16804914] Bulb trailers need 10 A fuses.
10. How do I disable parking sensors when towing without a smart module?
Use a 7-pin or 13-pin socket with an integrated microswitch. Route its wire to the PDC disable line or cabin button loop [Elektroda, andrzej20001, post #18001298] When the plug is inserted, sensors mute automatically.
11. How do I connect a trailer-present signal to the dashboard cluster?
Tap the module’s C2 pin (trailer detect) or fit a reed switch in the socket and run a 0.5 mm² wire to the cluster’s T32/25 input on VW/Audi platforms. That stops bulb-out errors when towing [Elektroda, lewkowicz17, post #18000711]
12. Rear lamps stop flashing and module chirps with hazards—why?
The main relay has pitted contacts or lacks constant +12 V on the thick red lead. Confirm battery feed is live even with ignition off. If voltage is present, replace the relay or bypass the module [Elektroda, andrzej20001, #18140657; abart64, #18140669].
13. What is the quick 3-step method to bypass the module?
- Unplug the 8-way connector from the module.
- Crimp RH-RZ-R together and LH-LZ-L together.
- Tape the joints, leave 30 / 31 isolated, refit trim. Total time: five minutes [Elektroda, yarozz, post #16815954]
14. Edge case: what happens if left and right indicators short after bypass?
Both sides flash simultaneously, and current doubles, blowing the 5 A fuse if fitted. That prevents harness fire—a strong argument for individual fusing per side [Elektroda, marcin_wr, post #16804914]
15. Need a diagram for 22-pin JAEGER 52400516 on BMW E39?
Jaeger’s 22-pin block groups power, CAN and light outputs. BMW E39 2002 requires ignition-safe feeds from ZKE. Download the 52400516 PDF from jaeger-automotive.de or request it via their support form; the thread lacked the drawing [Elektroda, goco, post #19294808]
