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[Solved] Audi a4b5 1.9tdi afn: Understanding Heating Conditions for 1, 2, 3 Candles at -6°C

mkcitro 10212 13
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  • #1 17052397
    mkcitro
    Level 10  
    Posts: 120
    Rate: 37
    I would like to know when 1, 2 or 3 candles are heating because I checked today at -6 and there is electricity to two, but they are burned, and the one that is just good has no electricity, or did anyone have that? Is something broken? Is it supposed to be like that?
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  • #2 17052483
    Doktorr
    Level 35  
    Posts: 2427
    Help: 286
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    What are you measuring it with?
  • #3 17052500
    mkcitro
    Level 10  
    Posts: 120
    Rate: 37
    I connected a light from the bulb to each of the candles and I have it in the cabin and I can see when it turns on and when it goes out and today the current on 2 candles was about 1 minute after firing and was until the engine reached 74c.
  • #4 17052534
    Doktorr
    Level 35  
    Posts: 2427
    Help: 286
    Rate: 1090
    So it should be, sustained heat system, signal taken from engine temperature sensor. Now, only use the cables to determine where the electricity disappears for the others.
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  • #5 17053046
    mkcitro
    Level 10  
    Posts: 120
    Rate: 37
    What does this mean? That 2 heat up and 3 don't? And what do you mean by writing the rest?
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    #6 17054527
    Anonymous
    Level 1  
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    #7 17054659
    Strumien swiadomosci swia
    Level 43  
    Posts: 27411
    Help: 1403
    Rate: 6379
    mkcitro wrote:
    to two there is electricity, but they are burned, and to one that is good, the electricity did not come, or someone had it?

    Replace the burned candles, the third candle heats up additionally and only keeps the engine cold and the alternator efficient.
  • #8 17054760
    mkcitro
    Level 10  
    Posts: 120
    Rate: 37
    Candles replaced. There are 4 fuses in the ECU box, 2 large and two small ones, one 15a, the other 25a, and what are they from? Because that 25a was burned.
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    #9 17054934
    badyl666
    Level 18  
    Posts: 189
    Help: 25
    Rate: 108
    Hello. 15a powers the engine equipment, and 25a fuse the second coolant circuit of the glow plugs. The large 50a supplies the 1st coolant circuit.
  • #10 17055205
    mkcitro
    Level 10  
    Posts: 120
    Rate: 37
    So this is how the fuse is replaced and all 3 are working, and it looks like that about 1 minute after firing it turns on, one moment heats up and turns off, then two turn on and somewhere after a while this one lights up and all 3 heat up to about 73C.
  • #11 17055483
    Doktorr
    Level 35  
    Posts: 2427
    Help: 286
    Rate: 1090
    Where in this system you see the possibility of such maneuvers.
    Audi a4b5 1.9tdi afn: Understanding Heating Conditions for 1, 2, 3 Candles at -6°C
  • #12 17055626
    Strumien swiadomosci swia
    Level 43  
    Posts: 27411
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    The water ones are the 3 next to doc.
  • #13 17055666
    Doktorr
    Level 35  
    Posts: 2427
    Help: 286
    Rate: 1090
    Ok Romeczek, I am closing the box.
  • #14 17100745
    mkcitro
    Level 10  
    Posts: 120
    Rate: 37
    The case was solved, replacing the fuse in the ECU box and replacing the candles did the trick.
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Topic summary

✨ The discussion revolves around the heating conditions of glow plugs (candles) in an Audi A4 B5 1.9 TDI AFN at -6°C. The user observed that two glow plugs were burned out while the third one had no electricity. Responses clarified that the glow plugs operate based on engine temperature and alternator load, with relays controlling their activation. It was suggested to check the relays and ECU for errors, as well as to replace the burned glow plugs. After replacing the fuses and glow plugs, the user confirmed that all three plugs were functioning correctly, heating up sequentially to approximately 73°C after engine start.
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FAQ

TL;DR: In the AFN, 3 coolant glow plugs heat coolant to ~25°C; "two relays controlled from the ECU" stage 1+2 operation. If only two heat at −6°C, check the ECU-box fuses, relays, and plugs; alternator load (DF) can limit stages. [Elektroda, Anonymous, post #17054527]

Why it matters: For Audi A4 B5 1.9 TDI AFN owners troubleshooting coolant glow plug heating patterns and no‑power faults, this speeds diagnosis and prevents unnecessary parts swaps.

Quick Facts

What do the three coolant “candles” (glow plugs) do on the AFN?

The AFN uses three coolant glow plugs to warm the coolant after start. They reduce warm‑up time and improve cabin heat. The ECU stages them through two relays. Low power drives one plug; high power drives two plugs. “There are two relays controlled from the ECU.” [Elektroda, Anonymous, post #17054527]

At −6°C, when should 1, 2, or 3 coolant glow plugs heat?

Cold starts trigger staged heating. Three plugs heat coolant up to about 25°C. Then two plugs maintain heating until roughly 70°C. Exact timing depends on alternator load and ECU logic. This staging is normal at −6°C. [Elektroda, Anonymous, post #17054527]

Why are only two coolant glow plugs getting power while the good one isn’t?

That pattern shows only the high‑power stage works. The low‑power stage or its relay may be inactive. The ECU uses separate relays for 1‑plug and 2‑plug stages. Check the smaller stage’s relay, wiring, and run ECU output tests. “Since the high‑power circuit works, there is something wrong with a small one.” [Elektroda, Anonymous, post #17054527]

How do I test which heater circuit is working?

Use targeted checks to isolate the stage.
  1. In your scan tool, run Output Tests to command each heater relay on.
  2. Watch voltage at the glow‑plug bus for each stage with a test light.
  3. If one stage never powers, trace back through its relay and wiring. These tests mirror the ECU’s staged control strategy. [Elektroda, Anonymous, post #17054527]

Which fuses feed the coolant glow plug circuits on A4 B5 AFN?

Inside the ECU box you’ll find four fuses. The 15A fuse powers engine equipment. The 25A fuse protects the second coolant glow circuit. A large 50A fuse supplies the first coolant glow circuit. Verify ratings printed on the holders before replacement. [Elektroda, badyl666, post #17054934]

Why do the coolant glow plugs stay on after the engine starts?

That’s by design. The AFN uses a sustained‑heat strategy controlled by the engine temperature sensor. The ECU keeps the heaters active until coolant warms sufficiently. This smooths cold running and speeds cabin heat. You will see power well after cranking. [Elektroda, Doktorr, post #17052534]

Can a blown 25A ECU‑box fuse stop one coolant glow circuit?

Yes. A blown 25A fuse in the ECU box is a red flag. One owner found that 25A fuse burned while diagnosing coolant heaters. Replace the fuse, then retest operation. If the issue persists, continue with relay and wiring checks. [Elektroda, mkcitro, post #17054760]

Should I replace burned coolant glow plugs as a set?

Replace any burned coolant glow plugs promptly. The third plug engages additionally when the engine is cold and alternator output permits. Mixing bad and good plugs disrupts staged heating. “Replace the burned candles; the third candle heats up additionally.” Doing them together ensures balanced current and response. [Elektroda, Strumien swiadomosci swia, post #17054659]

Can I monitor power to the plugs with a test light from the cabin?

Yes. You can connect a small test light to each plug’s supply and route it to the cabin. This shows when each stage powers and cuts. One owner saw power about 1 minute after start. He also saw heating continue until roughly 74°C coolant. Keep wiring safe and fused. [Elektroda, mkcitro, post #17052500]

How does alternator load (DF) change heater operation?

The ECU monitors the alternator’s DF load signal. It enables or defers heater stages to avoid overloading the charging system. If alternator load is high, one stage may not engage. “The controller turns them on based on the alternator load (DF).” Diagnose alternator issues if staging seems inconsistent. [Elektroda, Anonymous, post #17054527]

Where are the “water” glow plugs located?

The coolant heaters are the three “water” glow plugs grouped together on the coolant side. They sit as a trio near the coolant plumbing, distinct from cylinder glow plugs. Look for three adjacent plug bodies with a shared bus. “The water ones are the 3 next to doc.” [Elektroda, Strumien swiadomosci swia, post #17055626]

What finally fixed the user’s issue in this thread?

The documented fix was simple. Replacing the ECU‑box fuse and installing new coolant glow plugs restored proper staging. After that, all three heaters worked as expected. This confirmed both a blown fuse and failed plugs. “Replacing the fuse and the candles did the trick.” [Elektroda, mkcitro, post #17100745]
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