Golf III TD: Strange Electrical Problem (Wagon)

Hello forum!

I have a Golf III TD (estate/wagon) with 75 horsepower and the following problem:

Despite having sufficient oil, the oil warning light is flashing. The charging indicator light is dimly lit at approximately half intensity, as are the airbag warning light and the rear window defroster light (when the defroster is turned off). Additionally, the tachometer is not working.

My suspicion is that a 'parasitic' electrical path to the vehicle's ground has formed somewhere in the wiring harness, causing current to flow 'backwards' through, for example, a light bulb or relay coil, and then through the aforementioned indicator lights.

Does anyone have a wiring diagram for the electrical system, or has anyone else observed this phenomenon? A wiring diagram would be necessary to locate the source of the defect.

Best regards and thank you in advance - Dudelman.

Hello,

Please check the grounding cables of your engine, starter, and alternator.

Best regards,
Christian.

Hello,

Thank you for the tip, but unfortunately, I couldn't find anything obviously wrong.

During my troubleshooting, I also noticed that when the rear window defroster is turned on, the entire instrument panel illumination comes on (even with the parking lights off). This indicates a short circuit somewhere, and resolving it will likely require a detailed wiring diagram.

Thank you for the tip, Dudelman.

During my search, I also noticed that when the rear window defroster is turned on, the entire instrument panel illumination turns on.


It's probably not related to the wiring harness for the tailgate...

dudelman wrote:

During my search, I also noticed that when the rear window defroster is turned on, the entire instrument panel lighting comes on (even with the parking lights off). Therefore, there must be a short circuit to ground in an unintended location

.
Hi,

If so many consumers, which shouldn't have anything to do with each other, are malfunctioning, the cause is often a missing common ground (or positive) connection.

I might be able to email you the SLP.
What year is your car?

... when the rear window defroster is turned on, the entire instrument panel lighting comes on ...

You've essentially already given yourself the answer. The main ground connection, which includes the rear window, has failed. You can now take the complicated route and try to pinpoint the exact location of the fault. Or, you can take the workaround approach and run a new ground wire to the rear window defroster and/or the instruments. It doesn't matter to you where the break in the circuit is.

You can, of course, connect the mass anywhere on the chassis, which should make the workaround significantly easier.

Regards,
-Private

It doesn't matter to you where the point is that is now defined.

Great tip! That's definitely shoddy workmanship!

I'm quite certain now that the problem lies in the cable routing to the tailgate: The switch for the trunk light is also located in the tailgate, which explains the connection with the interior/instrument lighting. Sometimes the wires are broken beneath the insulation. Often, multiple wires are exposed, which can lead to unwanted connections.

Bye.

Jörg.

That's shoddy workmanship, isn't it!
Hello Jörg,

I agree with you, as long as we're talking about a different type of wire. However, a power wire that is frayed, broken, or torn and is making contact with the chassis poses less of a risk compared to signal or 12V wires.

I agree with you that Dudelman should check the area around the tailgate opening. However, if the problem isn't there, I would suggest (at least for testing purposes) trying an additional ground cable.

Regards,
-Private

Unfortunately, I haven't made much progress yet. I'll probably have to approach the problem more systematically. The result would certainly be better than a quick-fix workaround.

I will report on the results here!

Best regards and thank you for the tips - Dudelman.

Okay, the first major issue has been fixed: In the left cable harness to the tailgate, all the insulation was broken in several places (the plastic is extremely hard and therefore brittle). Two of the cables (sw [rear window] and gr/gn [license plate light]) were melted together, which explained why the instrument lighting turned on when the rear window heater was activated. This problem no longer occurs.

In the circuit diagram, some cables are terminated with a number inside a rectangular box. Does anyone know where these cables lead, or does anyone have a list of the terminations?

The other symptom is still present: The charging indicator light is only dimly lit (constantly), the oil pressure light is flashing brightly, and the tachometer is not working. I suspect that this is a fault in the instrument panel or its power supply, as there is no other explanation for this.

1.) It is very likely that the tachometer will shut off in the event of undervoltage.
2.) The low voltage is plausible due to the dimly lit charging indicator light.
3.) A flashing oil pressure light can also be caused by undervoltage, especially if the flashing electronics have their own dedicated power supply. In this case, the voltage supplied to the oil level sensor would be too low, and the control light would falsely indicate a low oil level, causing the oil warning light to flash.

After I charged the battery, the diesel engine started again. Before starting, the battery had approximately 12.3V, and after starting, it was even a little lower (11.7V). Perhaps the voltage regulator is also defective?

The question is: What causes undervoltage? Could the voltage regulator be defective? How is that normally tested?

Best regards and thank you in advance - Dudelman.

In the circuit diagram, some cables are terminated with a number inside a rectangular box. Does anyone know where each of them leads to, or does anyone have a list of the terminations?

The number inside each box indicates the power line number that it connects to. You will find a box with the number of the destination power line. So: a box with the number X in power line Y, and a box with the number Y in power line X. The power line numbers are numbered from left to right at the bottom.

It is possible that, even with intact insulation, the conductor wire within the cables passing through the rear door frame may be broken.

Before starting, the battery had approximately 12.3V, and after starting, it was even a little lower (11.7V).

'11.7 V with the engine running is definitely too low, as the alternator is not charging. It must first be 'pre-excited' (essentially started) via the charging indicator (or a parallel resistor). If something is broken or damaged in or around the combination instrument, this will no longer work.'

Jörg.

OK, I finally got it!

The charging indicator light only glows dimly because one terminal is directly connected to the (+) of the battery, while the other is connected to ground through the alternator (LiMa). If the alternator has a fault (e.g., a broken serpentine belt), the rectifier diodes cause a voltage drop, resulting in a ground connection with a voltage drop of a few volts, which causes the indicator light to glow at approximately half brightness.

'In the Golf III (I'm not sure if it's the same in earlier models), the oil pressure control electronics are powered by the alternator (presumably to ensure the oil pressure light illuminates even when the engine is not running). Once the engine starts and the alternator provides power, the light goes out.'

3.) For the airbag warning light.

4.) The tachometer receives its speed information from the unbalanced three-phase signal of the alternator. Therefore, my tachometer was not defective, but rather displayed the speed it measured (which was zero).

Conclusion: The ignition coil or its regulator is defective.

After installing a working starter motor, everything is working perfectly again!

Best regards and many thanks to everyone involved for all the helpful tips - Dudelman.