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
- Architecture: Two relays from the ECU stage the heaters (1 plug = low power, 2 plugs = high power) and react to alternator DF load. [Elektroda, Anonymous, post #17054527]
- Temperature thresholds: 3 plugs heat to approx. 25°C coolant, then 2 plugs continue to about 70°C. [Elektroda, Anonymous, post #17054527]
- Sustained heating: Post‑start heating is normal; the engine temperature sensor governs on/off behavior. [Elektroda, Doktorr, post #17052534]
- Fuse map (ECU box): 15A = engine equipment; 25A = second coolant circuit; 50A = first coolant circuit. [Elektroda, badyl666, post #17054934]
- Proven fix in thread: Replacing the ECU‑box fuse plus glow plugs restored full operation. [Elektroda, mkcitro, post #17100745]
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.
- In your scan tool, run Output Tests to command each heater relay on.
- Watch voltage at the glow‑plug bus for each stage with a test light.
- 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, #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, #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]
