Audi A3: No Start, Starter Works, Glow Plug Off?

Hello,
I'm having problems with my Audi A3 Caddy 1.9 TDI; it won't start.
I've already browsed your forum and found some posts, but I haven't found a solution yet.
"So, the preheating light isn't working, not even briefly. Sometimes it does work, and then the car starts without any problems."
The other lights turn off when I try to start it.
Starter and battery are new.
"The VAG-Com diagnostic tool indicated an error with the plus contact of the needle lift sensor, but it shouldn't be the cause of the starting problem. Disconnecting it didn't help, and the resistance value is present."
The starter motor engages even when the clutch pedal is not depressed, but the clutch switch itself seems to be working correctly.
Diesel is also available.
The fuses are all okay, but relay 109 clicks when the ignition is turned on.
Thank you, Georg.

burschi wrote:

Hello, I'm having problems with my AHU CADDY 1.9 TDI; it won't start. Bitte gib mir den deutschen Text, den du übersetzt haben möchtest. "So, the preheating light isn't working, not even briefly. Sometimes it does work, and then the car starts without any problems." Bitte gib mir den deutschen Text, den du übersetzt haben möchtest. The fuses are all fine, but relay 109 clicks when the ignition is turned on.

Hi,

It's likely that the main switching gear (MSG) isn't receiving power despite the relay clicking. This can sometimes happen if the contact points are dirty or if the relay itself has a faulty solder joint in the load circuit.
A cable break in the current-carrying path is also a possibility.

Just measure if there's voltage at the disengaging point when it doesn't start; that will give you more information.

First of all, thank you, Ulf. I will try my best.
My father tinkered with it again, and suddenly the car started working again.
Then I unplugged the valve lifter sensor, and it still worked, so we can rule that out.
At some point, it stopped working again. I tried a new ignition switch, but that didn't help. Even the spare key didn't work.
I then cleaned some connectors and contacts in the engine compartment.
After that, I bypassed relay 109 and the car started again.
While disassembling, I found a cold/broken solder joint on the relay 109.
I just got myself a brand new 109, and it's the newer version.
Then I read and cleared the error codes.
I thought the problem was fixed.
Just now, as I was driving home from work, the same thing happened again: the starter motor turns, but the engine doesn't start.
I bypassed the number 109 fuse again, but it didn't help. I put it back in and tried starting the car several times, and suddenly the glow plug light came on again, and it started running.
I immediately drove home, and now it's working again. Let's see what happens tomorrow morning.
I just remembered what Ulf said: when you turn the ignition on, you can hear a clicking sound from the fuel injection pump, but only if the glow plug light is illuminated. If the glow plug light isn't on, there's no clicking and it won't start.
"However, I have already cleaned the plug and the grounding contact."
I'll try it again tomorrow, though.
Thank you, Georg.

Hello,
I tried a few more things today.
It still wouldn't start. After several attempts, I removed and reinstalled relay 109, and the preheating light came on again, and it started up.
I then tried a few things, systematically disconnecting the valve lift sensor, fuel injection pump, glow plug system, and several sensors, turning on the ignition each time, and in each case, the glow plug light came on briefly, which is normal.
Based on this, it's unlikely that any of these components are faulty, because if they were, the glow plug lamp wouldn't have illuminated.
I then tried disconnecting the immobilizer coil and starting the engine. The engine started briefly but then shut off after about 5 seconds, so I don't think it's the immobilizer either.
What I noticed today while I was out and about is that after being stopped for about 8 hours, it wouldn't start again. I turned the ignition on and off a few times and eventually gave up. Then, I turned the ignition on again, and after about 2 minutes, the preheating light came on briefly, and the car started running again.
Now it's not working again, and I really don't know what else to do. I don't think it's a broken wire either, because even when I disconnect individual components, the preheating light still comes on, which is essentially the same thing as a broken wire.
I am grateful for any help, Georg.

burschi wrote:

Now it's not working again, I really don't know what to do anymore. I think it's also not a broken cable because when disconnecting individual components, the preheating lamp still turns on, which is essentially the same as a broken cable.

"I think it's quite possible that a broken cable is causing the problem, leading to the MSG (presumably a piece of equipment) temporarily not receiving power: specifically, whenever the preheating lamp doesn't turn on."

Or with which observation do you want to rule it out?

@Ulf,
You're right, a broken cable in the control unit cannot be ruled out. I was referring to the other sensors and the fuel injection pump, where it is unlikely because even when those components were disconnected yesterday, the glow plug light came on. I didn't have the engine control module (MSG) in mind when I made my statement.
I just measured the current draw at the shut-off point, and it's only present when the car is actually running (it's working now).
And just to be sure, I unplugged the glow plug module (MSG) and lo and behold, it didn't work – the preheating light didn't come on, but the starter motor still turns over.
I think Ulf's diagnosis was correct, meaning that the MSG (Main Switching Gear) is intermittently losing power.
That also explains why it doesn't record any errors.
Okay, so how do I find the spot where there's no electricity flowing through it?
Are there any specific areas that should be checked first, or can the entire wiring harness be disassembled?
Thank you, Georg.

burschi wrote:

Now, how do I find the spot where no electricity is flowing through? Are there any specific areas that should be checked first, or can the entire wiring harness be disassembled?

I'm not familiar with the Caddy-SLP.

Instead of spending a long time searching for the problem, I would set up some LED traps for the "wobbler," meaning I would tap into the power supply line at all the plugs and run temporary wires from there to control LEDs inside the device.

If the error occurs again, you will immediately see where the voltage is coming from.
You can try connecting the wires to the last lit LED and the first dead one as a temporary fix, which should restore power to the MSG (permanently) if the wire gauge is sufficient; I would recommend using at least 1.5 mm².

You can then do the final repair at your leisure, when you have the opportunity.

Hello.
I've now started looking for the source of the power issue. When the car was working again, I pulled out all the relays and fuses one by one to see what components are responsible for supplying power to the MSG (Main System Module).
Only the 109 department was responsible for that, but it's been reorganized now, and nothing helped to fix the car except getting a new one.
I then drove a continuous distance of 200 km, and had done so on several previous occasions as well, even when the problems were already present, and there were absolutely no issues.
After the car had been sitting for a while, it still wouldn't start.
I once unplugged the MSG connector and plugged it back in, but it didn't work.
After repeatedly turning the device on and off, it suddenly started working again.
Before the problem first appeared, I had been vacuuming the inside of the car and had left the ignition on for about half an hour while listening to the radio. I accidentally turned the key one position too far. Before that, everything was working normally.
It first appeared when I tried to start it.
Could it be that the MSG has been damaged in the process?
Does anyone know the pin configuration of the MSG (Main System Gateway)? There are two slightly larger pins on the right and left sides. When the car was still working, I checked both of these pins for voltage, but neither had any power.
I need to know which pin(s) are responsible for power supply so I can determine whether there is no power reaching the device or if there is power but the control unit is not functioning.
In the event of a broken cable, any shaking or vibration while driving should theoretically cause the power to cut out, which would shut off the car. Has this happened yet?
Currently, after multiple attempts to start and stop the engine (approximately 30-50 times), it starts without problems and runs frequently. It can be started and stopped repeatedly, sometimes even after hours, although not always – sometimes within 5 minutes. I haven't been able to find any correlation between whether the engine is warm or cold; sometimes it starts when warm, other times not, and the same applies when it's cold, which again suggests a potential wiring issue.
Thank you, Georg.

Hello Ulf,
I wrote my last post while you were replying to me, and I posted it before I saw your response.
That's a good idea. I took a look at the cable routing in the car, and it seems like there's no more connector between the control unit and the fuse box, except for the area near the splash panel. I didn't get a close look at that specific area.
I already asked about the pin configuration earlier so I can check if there's power or not. However, as Ulf suggested, I'll thoroughly inspect the entire wiring harness again to see if there's a connector blocking something. I'll also ask around in my circle of friends to see if anyone has an AHU engine so I can try using a different control unit.
Thank you, Georg.

Last week, a friend of mine fiddled with the fuse box under the steering wheel, and the car worked perfectly for a week again. He thought it might have been a loose connection.
On Monday, the problem reappeared. I disassembled and cleaned the fuse box, checked all the connectors and reinstalled them. Additionally, I inspected the wiring harness leading to the control unit for any further connectors or damage, but found nothing.
Okay, I've now disconnected the connector from the MSG and checked each individual pin for voltage.
If the car isn't working, I've only got a single pin providing 12 volts of power.
If it works, I have 12 volts on 4 pins and approximately 0.15 volts on some other pins, which I believe are signals from the individual sensors.
I thought it might be a grounding issue, so I removed the ground cable, cleaned it, and reinstalled it with battery terminal grease -> no improvement.
Based on the above measurement, not only the control unit but the entire engine electronics should be without power, which suggests a problem with relay 109. However, this relay is new, and bypassing it doesn't help either.

Is there another relay or component in between that is either conducting electricity or not?
"Right now, if the car won't start, turning on the ignition and waiting for one to three minutes sometimes works. Eventually, the preheating glow plug light will briefly illuminate, you'll hear a click from the engine compartment (which indicates the fuel injection pump disengages and a vacuum valve opens), and then I can usually start it."
Thank you very much, Georg.

Hi,

We've been having the same problem with our Touran for a week now.

Has anyone found a solution to this problem yet?
Thank you.

burschi wrote:

If the car isn't working, I only have one pin with 12 volts plus. If it works, I have 12 volts on 4 pins and approximately 0.15 volts on some other pins plus

.

Is the voltage present at the connected engine control unit, or at the pins when the engine control unit is disconnected?

Do you have a wiring diagram? There should be a common power supply for the three pins that are not receiving power somewhere in the diagram. I would rule out three simultaneous failures. If the pins are completely independent, then, depending on the answer to my first question, I would suspect the motor control unit itself.