No start. No power to pump fuel solenoid

This diagnosis of no start took quite a bit of time to find. I thought
I had bad fuel as engine ran rough at first. Eventually stalled and died.
I changed fuel filter and purged. Still no start. Eventually vag comm'd it
ad note :
17946/P1538 - Fuel Shutoff Solenoid (N109): Open or Short to Ground.
I can jumper directly to battery and she fires up.
( Noted that Solenoid draws well over an amp. Is that typical?)



Fuse is OK
Glow plug light comes on (I think this absolves the 109 relay but I
have seen some very rare posts indicating that this is not 100.00% always true.


As far as tracing this back to the ECU, can someone confirm the pin # on the ECU that I should look at? I have seen pin 53 and pin 120
Is solenoid really directly connected to the ECU? ...'cause I would
think that would be a bad design asking an ECU to directly drive an inductive load, plus as previously noted that solenoid draws ~1.5A
which may not be much for a solenoid, but a hefty load for an ECU.
Wiring is OK to first plug, then it goes under battery and not
sure what other connectors that wire goes to before finding its way
to ECU.


Any online wiring diagrams that I can zoom in on and trace this?


Thanks for your attention

Now remove the wire going to the solenoid and again jump power to just the solenoid. Does the car start and run?

Yes, the problem is just the wire to the solenoid.

No, it's deeper than that and by jumping power to the solenoid, you are powering up the other things that are powered at the same time.

Which car is this?

Checking my Bentley, the solenoid is powered directly from the ECU. It would have circuitry in it that can handle the load. The pin could be T80/79 or T121/81 or something else if it's the Passat.

Sorry I should have mentioned it, but car is a 2003 Mk4.

So back feeding that solenoid line goes back to the ECU and back feeds other components as well---interesting.

whitedog said:

Checking my Bentley, the solenoid is powered directly from the ECU. It would have circuitry in it that can handle the load. The pin could be T80/79 or T121/81 or something else if it's the Passat.

Click to expand...

When you quote the pair T80/79 is 80 the signal and 79 the return?

almus said:

When you quote the pair T80/79 is 80 the signal and 79 the return?

Click to expand...

Since yours is the 2003, T121 is the connector and the other number is the cavity.

Does pin 121 of the ecu service that solenoid exclusively? Or does it go elsewhere? I pulled the plug off the ecu and it shows a finite
resistance to ground with the solenoid disconnected.

Any source of online wiring diagram for that ?

I think you can buy Mitchell on demand for just the one car you are working on. There might be a time limit on it too. I like Mitchell's wiring diagrams.

almus said:

Does pin 121 of the ecu service that solenoid exclusively? Or does it go elsewhere? I pulled the plug off the ecu and it shows a finite
resistance to ground with the solenoid disconnected.

Any source of online wiring diagram for that ?

Click to expand...

I have a Bentley and I found it in there. I was looking at wrong pin.
It looks like it is T121/120 if I read it correctly. I see no evidence that the line goes anywhere except to the N109 solenoid It looks like it is an "open"--- according to diagram there is only one connector it goes through.

I had seen some reports that the ECU could be faulty; hopefully
it is just an open wire.

Open green corroded wire in middle of harness. Looked like the
stuff I am used to seeing in a marine environment.

She started right up

Thanks for help

Yeah, that wire was compromised some time ago. As these cars continue to age more and more of these things will start popping up: even the wire looms themselves start eating/chafing the wires/insulation.

burn_your_money said:

I think you can buy Mitchell on demand for just the one car you are working on. There might be a time limit on it too. I like Mitchell's wiring diagrams.

Click to expand...

Agreed, it's real wiring diagrams like I used back in the 70's.
If you do rent it, it is time limited.
The annual cost is to high for home use.
The diagrams are in Adobe format so you should be able to save them.