Glow Plug Light On? Troubleshooting Starting Problems

Hello,

I have the following problem with my Passat Variant TDI, manufactured in 1994, with a 90 HP engine, 1Z engine code, and 334,000 km on the odometer:

'Sometimes, the preheating indicator light doesn't turn off, and the car won't start. I have to preheat it for a while, sometimes just 5 minutes, but sometimes even two hours, then the indicator light goes off, and it starts. This happens for a day or so, and then I don't have any problems for a week or two. The other day, while I was driving, the preheating indicator light suddenly came on, and the engine immediately shut off. I had to preheat it for half an hour on the side of the road, and then it started again. After about 20 kilometers, the same thing happened, but it hasn't happened since. The error memory only shows something about the accelerator pedal position... I've already replaced relays 109 and 103, but it didn't help. Of course, when I take it to the workshop, it never happens; they think I'm too stupid to start my own car.'
Does anyone have any ideas?

Thank you.

Jens.

Hello,

If the vehicle itself is telling you what's wrong, why is it being ignored?
Just take a closer look at the throttle position sensor.

Hi,

Quote:

Sometimes I have to preheat the oven for just 5 minutes, but sometimes it takes as long as two hours.

You definitely aren't glowing for two hours... after that, even a large battery would be almost completely drained. The light from this LED indicates, as stated in the manual, not only that it is glowing, but also that there is an error.

Otherwise, I completely agree with Marco.

Best regards,

Jan.

Hello,

The workshop supposedly tested the sensor, and according to their statement, it is working correctly.
Furthermore, I can't understand the connection between this sensor and the starting behavior.
'I understand that I don't need to warm it up for two hours; the indicator light stays on for that long, that's what I meant.'
I am also aware that this warning light is an indicator for the error memory, which is why I had it checked. However, I don't understand the connection between the accelerator pedal position and the starting behavior – and neither does the workshop, apparently.
Since my Passat also has the typical jerking issue, which has already been extensively discussed here, I related this error message to the jerking.

Thank you and best regards,
Jens.

smils wrote:

Sometimes, the preheating indicator doesn't turn off, and as a result, the car won't start.

Recently, while driving, the preheating indicator suddenly came on, and the engine immediately shut off. }The error memory only shows something about the accelerator pedal position... }I've already replaced relays 109 and 103, but it hasn't improved anything. }When I take it to the workshop, it never happens; they think I'm too stupid to start my own car.}
I suspect there's a copper wire somewhere in the wiring harness, possibly even in the central electrical system.

That's going to be a real pain to find. You could start by carefully checking for any cracks or areas of wear, as those are usually the easiest to spot.

Alternative: DWA may be spinning (if applicable).

Hi Jens,

Perhaps you could share the exact error message with us? "Something about the accelerator pedal position" isn't particularly helpful when troubleshooting. The exact wording, along with your preheating indicator, might reveal a pattern.


Regards,
Sure, here is the translation of the text from German to English:

"Micha"

Hello,

Unfortunately, I don't remember the exact wording of the error message. Since it hasn't started this morning, and definitely not within an hour, I need to figure out how to get it to the workshop. They will read the error codes, and I will post the exact wording here.
First of all, thank you very much for your efforts!

Greetings,
Jens.

Hello,
Okay, I now have the exact error message:

00777 027
Throttle position sensor - G79.
implausible signal
sporadically

'I had this error message read out at the VW dealership. Immediately after that, my speedometer went all the way to the maximum, and the lighting in the instrument cluster on the temperature gauge went out. Of course, VW denies any connection. I was there with a colleague who saw that they exchanged something in my fuse box after the diagnostic check. Naturally, VW denies this! It's frustrating when you take your car to a brand-specific workshop and get it back with even more defects than before.'

Maybe you have some ideas on how to solve this problem.
Based on VW's recommendation, I had already replaced relays 109 and 103, but unfortunately, without success. I can't even understand why I replaced relay 103, because the voltage supply to the glow plugs has nothing to do with the error code.

Best regards,
Jens.

Apparently, some old, vulnerable 109s have been spotted in the parts warehouse.
Perhaps they've now discreetly replaced it with a working 109...

Is it running now?

@slmils

Hi Jens, my father-in-law also experienced the problems described in your first message! The vehicle's year of manufacture and mileage were almost identical.
It was also diagnosed, but a search or replacement of components yielded no results. He then tried to pinpoint the problem using the wiring diagram. In the end, he discovered it was the immobilizer. It's a black box located under the instrument panel. This "box" has the function of preventing the engine from starting in case of theft. He removed it and checked for cold solder joints. He found one and resoldered it, and since then there have been no more problems.
You might want to have that checked and repaired by a professional, or consider buying a new immobilizer.

Best regards, Maik.

Hi,
'No, unfortunately, it's not working. The error persists and is getting worse, and we're also seeing additional issues (the speedometer is maxed out and the instrument cluster lighting is defective) that appeared after VW read the error codes.'
Ich gehe bestimmt nicht wieder zum VW Zentrum Chemnitz, dort bekommt man sein Auto nur noch kaputter zurück als es so schon ist! War dreimal dort und dreimal haben die entweder den Fehler nicht gefunden oder Mist gebaut. Ein Servicemeister meinte, dass ich den Minuspol der Batterie mal längere Zeit abklemmen sollte, dadurch würde zumindest der Tacho wieder funktionieren. Häää, habe ich den Tacho vielleicht kaputt gemacht??? Als ich mein Auto dort abgegeben habe funktionierte der Tacho noch! Fahre ich in eine Fachwerkstatt um mein Auto kaputt machen zu lassen und mir dann dümmliche Vorschläge anhören zu dürfen? Sorry für meine Ausdrucksweise, aber ich habe die Nase voll von VW-Werkstätten! Bisher habe ich damit NUR negative Erfahrungen sammeln dürfen. Ich bleibe in Zukunft bei meiner freien Werkstatt, oder mache es selber. In der 'Freien' sind die Leute nicht überbezahlt und haben noch Spaß an ihrer Arbeit! Die beschäftigen sich wenigstens noch mit einem Problem und tauschen nicht einfach nur ganze Komponenten der Reihe nach aus - was nur viel Geld kostet und doch nichts hilft, außer VW den Umsatz zu erhöhen. Nur ist eben meine 'Freie' mit dem aktuellen Problem überfordert. BiNo one has been able to help me so far, and VW certainly hasn't – quite the opposite.


In my opinion, the problem cannot be with the immobilizer, because if the car simply dies while driving, it has nothing to do with that. It also shouldn't be the relay 103, because the glow plugs don't need power while driving. I've already replaced relay 109 for the third time, so they can't all be defective. The displayed error message (pedal position, see above) is also not related. For me, the only possible explanations are a grounding problem or a short circuit. But how do I find the fault? I can't possibly trace and test every single cable.
Do you have any other ideas or suggestions, perhaps even regarding the speedometer?

Thank you and goodbye.

Jens.

Has the correct seating of the connector on the throttle potentiometer been tested and ensured? If an infinite resistance is reported from there to the control unit due to a connector that is not properly seated (or a cable break, etc.), the entire system would behave very similarly. I am 100% certain that the error message from the throttle sensor is closely related to your problem...

If I am wrong, please feel free to correct me.

The error is guaranteed to be found in the accelerator pedal potentiometer – everything else could just be secondary issues, and it's best to check the accelerator pedal potentiometer even in those cases.

Hmmm, actually, the self-study program 153 says something about the donor G79.

'... In case of a defect, the engine runs with an increased idle speed, allowing the customer to reach the next workshop....'

That would, however, contradict my aforementioned assumption . Therefore, I suspect a broken cable or a short circuit in the area of the wiring leading to the throttle potentiometer. However, it might just be another issue that is unrelated to the engine stalling or the 'not wanting to start' problem. In that case, my suspicion would be more towards the immobilizer.

If there is a fault with the accelerator pedal potentiometer, the engine control unit (ECU) will go into a limp mode, which means it will operate at approximately 2000 RPM continuously. Additionally, this type of error is almost always stored in this vehicle model.
(Temporarily abandoning tolerance).
Here you can see your mistakes: https://community.dieselschrauber.org/en/viewtopic.php?t=6575&highlight=
I suspect your vehicle doesn't have an immobilizer, so the problem might be related to the DWA (Doppelverriegelung mit Wegfahrsperre - double locking with immobilizer).
Best regards,
Josi.

Hello,

It has an immobilizer. However, according to Volkswagen, the problem cannot originate from the immobilizer because the car also simply turns off while driving.
I'm going to try that with terminal 15.

Bye.
Jens.