Preheating System Failure: Complete Breakdown

Hello everyone,

Yesterday, I had the following problem with my car (A4, AFN, model year 1996):
During the day, with a temperature of 20°C, I tried to start the engine, but the glow plug indicator didn't light up at all. The engine just cranks and cranks.
Following the wiring diagram, I observed the following:
The preheating relay has ignition voltage (+) on one side of its control coil. On the other side, it receives either ground (GND) from the control unit (ECU) if preheating is required (which activates the relay), or it receives positive voltage (+) from the ECU (which prevents the relay from engaging).
So far so good: So, I disconnected the coolant temperature sensor from the radiator hose. However, it still didn't pre-glow (not even for 1 second).
After that, I disconnected the battery, unscrewed the control unit, unplugged the motor controller, and checked the connectors (to the pre-glow relay and the water temperature sensor). Everything seemed to be okay.
Then I reconnected the control unit, connected the battery, and lo and behold:
The preheating system runs for approximately 20 seconds and then switches off again via a different relay. Then, I reconnected the coolant temperature sensor. Ignition on, the preheating coil illuminates for 1 second, and the engine starts normally.

Now, the question for me is: Did I have a bad connection somewhere (e.g., at the connector of the control unit), or does the control unit occasionally malfunction? Do you have any other ideas about this? Does anyone know how the data from the coolant temperature, etc., is processed in the control unit?

I will check in the near future to see if the error occurs again, or if it was just a loose connection, and I will post an update.

Best regards,
Ernst.

Hello Ernst,

It seems to me more likely that your engine control unit or parts of the electronics didn't receive any power. The glow plug system isn't used at 20 degrees Celsius (unless the sensor is disconnected, which would mean extremely low temperatures, resulting in the maximum glow plug time). The brief illumination at the beginning is more likely a functional check than an actual glow plug cycle, and that didn't happen.

In my opinion, this whole issue has nothing to do with the preheating process; it simply lacked power. This could be due to a contact problem (which you probably fixed by disassembling it), or the infamous relay 109, which is specifically responsible for supplying power to the EDC (Electronic Diesel Control).

Best regards,

Jan.

Jan6K wrote:

... or the famous Relais 109, which is currently intended for powering the EDC.

Hi everyone,

If the preheating indicator remains completely dark when you turn the ignition on, it's almost a sure sign that the EDC (Electronic Diesel Control) is not receiving power, which is usually due to relay 109.

While faulty glow plugs or LEDs (or other interruptions in this circuit) cannot be completely ruled out, they have, to the best of my knowledge, never been reported in this context.

So, first, follow Jan's tip .

Hello fellow enthusiasts,

Thank you for the quick response.

The issue with the power supply for the EDC sounds plausible. I suspect that the EDC "hung" during a brief interruption of the power supply. However, it was still able to provide a 12V signal to the glow plug relay, even with the coolant temperature sensor disconnected. That's how I noticed that something wasn't right, and I apparently fiddled with the right parts in the electrical box... Well, sometimes luck plays a part too.

I'll take a closer look at relay 109 at some point; I don't want to be stranded somewhere.

Best regards and have a pleasant evening.
Ernst.

It was, however, capable of providing a 12V signal to the glow plug relay, even with the coolant temperature sensor disconnected.

I see it differently: The control unit likely has an NPN open collector output. When it doesn't output GND, its level is 'floating,' meaning it takes on an arbitrary level. In your case, the output is connected to the glow plug relay coil, which is at +12V.

Therefore, the control unit did not output +12V, but rather received +12V.

I also believe that your control unit wasn't receiving any power at all. So, it could be a loose connection or the infamous relay 109.

Regards,
-Private