2000 TDI Random mystery no-start

Hi all,

I've been reading for a bit trying to solve a problem with my 2000 Jetta TDI and could really use a few suggestions. For seemingly no reason, once in awhile the coil light does not light when the key is turned so of course the engine cranks but does not fire. If I leave it and do nothing for a bit (hour, 2 hours, overnight, no telling really) then often it WILL start and will run and start perfectly for months. It never dies when it's actually running.

The difficult thing here of course is that a.) it's tough to diagnose and solve when it's working perfectly and b.) when it fails I don't have my tools or am not in a position to work on it, then when I get it home and I'm ready to go it works fine.

After reading here I first thought it was Relay 109, changed that with no difference made (I actually thought that was it, as it started after I changed it but then it did it again a week later). Checked all the fuses, nothing seems to be amiss. I bought a Ross Tech to help try and figure it out and the only code that appears is:

Address 17: Instruments Labels: 1J0-920-xx0-17.lbl
Part No: 1J0 920 900 L
Component: A4-KOMBIINSTR. VDO V56
Coding: 07242
Shop #: WSC 00066
VCID: 2B55FB8CD9DD61461E5-5140

Subsystem 1 - Part No: IMMO
Component: IDENTNR: VWZ7Z0X1517788
IMMO-IDENTNR: VWZ7Z0X1517788

1 Fault Found:
01314 - Engine Control Module
49-10 - No Communications - Intermittent

-------------------------------------------------------------------------------
Address 19: CAN Gateway Labels: 6N0-909-901-19.lbl
Part No: 6N0 909 901
Component: Gateway K<->CAN 0001
Coding: 00006
Shop #: WSC 00066
VCID: F0EFACE0B07BC29EEB7-515E

1 Fault Found:
01314 - Engine Control Module
49-10 - No Communications - Intermittent

Is the first thing to try basically replacing the ECM with a new one? I'm thinking that it couldn't be the harness, because why would it not work and then 20 minutes later work fine for months without the car moving or any changes made? It's also a bit hard to test the harness, because like I said it won't STAY broken long enough to get things apart and get a meter in there to check voltages.

Any help or suggestions would be great, it's driving me crazy not knowing at any given time if a key turn is going to be successful.

Change your 109 relay......if it's black it's the old style.....get the new one which is grey.

The 109 is the main power wire to the ECU. In a pinch you can take the old one apart and look at the base of the pins and you will see circular cracks around the solder joints. Reflow the solder and it will work......then order a new one?

csstevej said:

Change your 109 relay......if it's black it's the old style.....get the new one which is grey.

Click to expand...

Yep, I changed the Relay 109 one with one from the dealer (40 bucks, but I wanted it to WORK).

Put the new relay in and I thought i'd solved it as the car started. But a week later it mysteriously would not light the coil light again for about 4-5 key turns. Then it started working again. Since then it's happened one more time in the morning, got a ride to work and when I got home it started again.

Wire connections at the relay base. Fuse connections on fuse ?34?. Or is I it 32?

You could put a jumper wire from the battery positive to the fuel shut-off solenoid on the injection pump and see if it starts.

whitedog said:

Wire connections at the relay base. Fuse connections on fuse ?34?. Or is I it 32?

You could put a jumper wire from the battery positive to the fuel shut-off solenoid on the injection pump and see if it starts.

Click to expand...

I'll try running the wire next time it fails on me. If it starts when I run the wire to the solenoid, what does that indicate?

It indicates that you are losing power or ground signal in the relay 109 circuit. when it doesn't start, you could check to see if you have power on fuse 32 and 43. If you don't, you aren't getting power out of the relay 109. If not, then you need to check for power at pin 30 for R109 and check for ground at pin 85. If you have both of those, I'm not sure...

Possible bad ignition switch?. Would have swore it was a 109 issue ....but you said you changed it....unlikely a new faulty relay.......but possible.

maddmatt21 said:

Yep, I changed the Relay 109 one with one from the dealer (40 bucks, but I wanted it to WORK).

Put the new relay in and I thought i'd solved it as the car started. But a week later it mysteriously would not light the coil light again for about 4-5 key turns. Then it started working again. Since then it's happened one more time in the morning, got a ride to work and when I got home it started again.

Click to expand...

$40? Ouch... $9.95 on idparts. I just bought one the other day to keep in my glove box. I hope you fix your issue, I have no experience or knowledge of this.

whitedog said:

It indicates that you are losing power or ground signal in the relay 109 circuit. when it doesn't start, you could check to see if you have power on fuse 32 and 43. If you don't, you aren't getting power out of the relay 109. If not, then you need to check for power at pin 30 for R109 and check for ground at pin 85. If you have both of those, I'm not sure...

Click to expand...

I understand checking for power at fuse 32 and 43. Where are pin 30 and 85? Are these pins in the ECU plug?

I ask just because I want to make sure I find these locations in advance so I can keep things disassembled (or loosely assembled) to the point where I can find em, then carry my test meter with me so I can test everything no matter where I am the next time it happens.

30 and 85 are in Relay 109 (R109) connection.

I bought a brand new 109 relay for my 2000, it turned out to be bad from the moment I plugged it in. At $10 from idparts it might not be a bad idea to change it out again.

Nice to know you found it.....weird that a new part failed but it does happen.