Troubleshooting the glow plug relay

[MrDave]

I have a recurring dtc:
17068 Unknown Error Code
35-00

which is actually:
Glow Plug Control Module to PCM Circuit (J271) Range/Performance

which I understand to be:
"glow plug relay to ECU"

So I was troubleshooting my relay last night, and no wonder my startup is a bit rough, my glow plugs aren't working!

I made an extension harness so I could probe the relay's connections, and I still haven't found out much.

From the Bentley:
Relay Pin#1 to pin T121/42 ECU
#2 +12, main power in
#3 br ground
#4 to glow plugs 1/2
#6 to glow plugs 3/4
#7 to pin T121/33 ECU
#9 to ECU controlled power ('blue circuit', relay 109)


I have confirmed all my connections, and even with my water temp sensor unplugged, I never get +12 on the GP outputs. In fact, I never get anything on the GP outputs.

I have ground on ground,
+12.5V on the main input,
+12.5V on the 109 relay circuit (with the key on),
and resistances of ~.8ohm on each of the GP circuits.

The ECU connections give me 11V and 1/2V when the key is on
(can't remember which was which).

I haven't started the engine yet to check to see what changes when the engine gets warmed up and the glow plugs shut off completely.

I assume the relay is toast, but it's also quite new, and rather expensive.

Does anyone have insight or ideas?

What are the ECU outputs supposed to be?

-Dave

 

[Jimmy]

If you have gone this far, it should be possible for you to activate the relay "manually" the n do a continuity test of the contacts that deluver current to the GP's. All you should need are some alligator type jumper wires and a 12V supply. This way you isolate the problem from the car or ECU. If you are not aware, there is a 50A fuse on top of the battery to protect the GP circuit. Check also the heavy terminals on the bottom of the relay rack, and somwhere under there is also a main ground for the dash.

 

[MrDave]

Swapped in a second relay, didn't make a difference.
I'm led to believe that the Bentley wiring diagram may be wrong.

Looking at the wiring diagram, I'm wondering if the ECU wires are mixed up, or if some of the other pins are.

From yesterday's post:
Relay Pin#1 to pin T121/42 ECU
#2 +12, main power in
#3 br ground
#4 to glow plugs 1/2
#6 to glow plugs 3/4
#7 to pin T121/33 ECU
#9 to ECU controlled power ('blue circuit', relay 109)

On the relay itself,
pin #1 is labeled "ST"
pin #2 is the real large pin, guaranteed the +12V input
pin #3 is labeled "31", so it's ground
pin #4 and #6 are labeled "GP", and they are regualar sized pins, so guaranteed for the glow plugs
pin #7 is labeled "D1"
pin *9 is labeled "86".

Standard wiring convention is that pin 86 is one side of the relay coil, and the wiring diagram seems to support this.

Of "ST" and "D1", which one would allow me to manually activate the relay?

For those that don't know, I'm installing an ALH TDI from a 2002 Jetta into a 1982 Rabbit.

-Dave