Engine Not Starting? Troubleshooting Guide.

Volkswagen Golf 3 TDI.
Year of manufacture: 1997.
Engine: AHU
154,000 km

Okay, here's the problem:


The preheating function is not working properly. It sometimes works, and sometimes it doesn't.
If it doesn't work, it looks like this:
- Turn the key to pre-glow.
- There are no preheating indicator lights illuminated on the dashboard, even with a cold engine (outside temperature 2.2°C).
- The engine is running, but it won't start.

If possible, please use the following method:
- Engage reverse (or reverse gear).
- Turn the key to start (No preheating light appears on the dashboard).
- Car sputters and stalls.
- The light bulb in the fixture turns on.
- Engine starts.
It's clear that this method of starting the car isn't the best.

Incidentally, this problem occurs regardless of the engine temperature.
The problem appeared suddenly. Once the engine starts, everything seems normal so far.


- The battery seems to be in good condition.
- It probably can't be the glow plugs, right?
- Normal driving behavior.


What do you think about it? Which areas should I take a closer look at?

Thank you for any tip or suggestion.

What do you think about that? Which areas should I take a closer look at?

Relay 109 and the general power supply of the control unit. Fuses, cables, connectors. Don't forget the ignition switch...

CU Gremlin.

Okay, I'll check that out tomorrow.

I was also thinking about the relay. I'll measure it to see if it's working properly.

I will report on any new developments in this case tomorrow.

Do the checks for oil pressure and alternator turn off when you start the engine and it doesn't start?

Yes, the other indicator lights are working fine.

I drove the vehicle twice today.
Every time, the preheating light would come on, and then it would start up as usual.

There has to be some kind of intermittent issue somewhere if it's only happening occasionally, right?

By the way, yesterday I stood in heavy snowfall for over an hour on the Feldberg mountain in the Black Forest, waiting for the ADAC (German Automobile Club).

He then checked the battery, and it was okay.
And then he used what's known as the 'bump start' method to get the engine running again. After a few attempts, the engine started right up.

Okay, so...

We were now able to localize the problem. (thanks to Don & Gremlin ).

Specifically, there's a problem with the central electrical system. There's a loose connection on the socket for relay 103 (the preheating relay).

If the car doesn't preheat when starting, I briefly jiggle the relay. After that, everything works fine and it runs perfectly. Even the vibration from closing the door can trigger the loose connection.

At the next opportunity, the central electrical system will be disassembled and all connections will be checked.

By the way, the relay has been tested and is working perfectly.