Car Won't Start? Troubleshooting Guide

...Hello.
I installed an aftermarket heated seat system, including the entire wiring harness, and now the engine won't start... I have a 1.9 diesel engine... I'm desperate... what could be the problem?

...there is no error message...

...I just noticed that the glow plug unit isn't working before I try to start the engine... it can't be due to the recent modifications, right???

Okay, have you checked the basics? Is the battery dead? Does the interior lighting turn off when you try to start the car?

Quote:

...I just noticed that the glow plug unit isn't turning on before you start the engine

.
More likely, the battery is drained if the starter motor isn't turning. I think your seat heater might also be heating up while the car is stationary.

No, none of that is the case... as I said, the preheating unit isn't activating. Could it be related to terminal 30, from which I took the positive connection for the seat heater?

Then you should probably provide some more information about the vehicle, the modifications, and things that have already been tested.

...is a 4-door Golf TDI...

Yesterday, I installed a wiring harness for the seat heaters, and since then, the car won't start because the glow plug unit isn't activating when I turn the ignition key.

I assume this is only indirectly related to the wiring harness installation. You must have connected the cables to the potential distribution block in the fuse box, right? If so, I would say that relay 109 is faulty and is no longer switching due to your tinkering. Relay 109 is known for having cold solder joints, and is often a source of errors. Just try bridging the load circuits of this relay; if it then works, you'll know what the problem is. If it doesn't work, you can rule it out.

...this relay isn't located in the fuse box, but rather in a socket underneath the steering wheel, where I screwed the positive wire to terminal 30, right?

hoepfelchecker wrote:

...this relay isn't located in the fuse box, but rather in a socket underneath the steering wheel, where I screwed the positive wire to terminal 30, right?

Yes, that's what I mean. On the relay holder, there are also fuses. I expressed myself a little clumsily above.

...and what should I do next?
So, is it not at terminal 30 where I'm connecting the positive wire, or what?

hoepfelchecker wrote:

...and how am I supposed to proceed now?

Read what I've written and do it.

...hmm, yes, and how do I handle the bridging?

Take a piece of cable, strip the insulation from both ends, and insert it into the contacts of the load circuit. But please don't ask me now how to strip a cable.

"Remove the relay and first check if there is voltage at terminal 30. If there is, then bridge terminal 30 to terminal 86."

schrauber2904 wrote:

if so, then you bridge terminal 30 to 86.

Terminal 86, according to DIN standards, is the control terminal on a relay. Terminal 87 is the load terminal. However, this is also usually indicated on the relay itself.

Okay, I've bypassed it now, but unfortunately, that's not leading to any results either; the engine still won't start.

...Thank you everyone, the problem is now solved... Fuse 29 was not quite properly inserted now it's working again.