No glow plug light ONLY when cold (still starts)

[mikerancourt]

(I've been talking about this on the TDI Club forum, too, and I can't figure it out)

1998 MkIII TDI Jetta

Since it's started getting colder, my glow plug light doesn't come on when I turn the key (doesn't even flash the 1/2 second). However, the car starts up okay (the colder it is, the more cranking it takes, but never more than a second). After the engine gets warm, when shut it off then turn the key again (engine still warm), the glow plug light flashes as it normally should. Today, it's about 50 degrees, and when I went out, I turned the key on and off about ten times to see if the light would ever come on, but it didn't, so I cranked it, and the car started almost immediately. I let it run for only a few seconds, shut it off, turned the key, and the light flashed on.

Any idea what is going on here?

I have the following code:
- 00626 - Glow Plug Indicator Light

I've replaced the following:
- Four new Bosch Glow Plugs the other day
- New GP harness the other day
- Relay 109 in October (I don't think this is a problem since the car always starts)
- Relay 180 in June
- 50A fuse in June

I've tested the following:
- Resistance for all four GPs is the same, around 1.0
- Unplugging the coolant temp sensor causes no change
- The GP light bulb on the instrument cluster is good
- I can't test the voltage at the harness - the Glow Plug 101 tutorial on TDI Club doesn't account for the light not coming on, so testing this will only tell me the obvious: there's no voltage when the light doesn't come on.
- The grounds in the engine compartment seem to be good
- the harness at the ECU seems to be good and clean, but I don't really know how to test this and don't know where all of them are.

Please help if you can.
Thanks.
Mike

[chittychittybangbang]

50 degrees isn't cold enough for GP to have pre-glow. The GP have pre-glow after you turn the key and before engine start but it's dependent on cold.

Do you have VCDS? It can really help monitor these sensors and check things like glow plug adaptation. Maybe your instrument cluster is bad? I don't know what else could cause a faulty GP indicator light without digging through the service manual's wiring diagrams.

One other thing to check is chafed wiring. Under the airbox, near the starter, these are all places where the old wiring may be rubbing through. I don't think it should cause that error but maybe some weird stuff is happening which is causing some problem with the electrical system.

[mikerancourt]

I rigged up a test light to the GP harness (to each of the wires leading to it, first one, then the other), and it turns out I am getting power to the plugs after all. Thanks to the suggestion that I rig up thin wires to the blades on the relay then plug it in and start testing voltage, I was able to (if I understand the wiring diagram right – I’m looking at the one in my Haynes manual), determine that the relay is working. Well, I think so, anyway. For the first time I can recall, I was able to hear the relay click on when I jumped 30 to 87 (closing the circuit). Then, when I turned the key, I magically heard the click, as well. Maybe I just couldn’t notice it before, but I thought there used to be no click. Anyway, to be clear, that click is the sound of the relay doing its job and sending power to the glow plugs harness.
I am having trouble reading the wiring diagram, so I don’t know which wire is responsible for sending power to the GP indicator light in the instrument cluster (I know that it is 17C in the diagram, but I don’t know how to find 17C on the cluster harness, nor do I know if it’s something I can test while spliced into the relay.
I tested the bulb again, and it is good, so there is something else amiss. I don’t know why my indicator light isn’t going on, but at least I know the plugs are getting power.
Two things are of note here:
1. I clearly messed up by not testing the harness for power (I did do that on my old harness when I first started pursuing this problem, but not on the new one – perhaps I just failed to test the old one properly, or perhaps the new harness fixed one problem but not the one that’s specifically related to the light not coming on).
2. The actual glow time is about four seconds (it’s about 45 degrees here). I think my slightly hard starts were the result of not waiting long enough, not knowing that the glow time is more than a few seconds (though no start since adding the new harness has been in very cold weather, and no start was very rough, anyway).
3. I should mention that I have also found in recent weeks that both of my instrument cluster back-lights are blown, as well as my side marker lights. Could all of these bulb problems be related? I know the side marker lights are connected to the back lights, and of course the whole cluster uses the same ground. Could that be something I can or should check out? The wiring diagram says the ground to the cluster is E131, but I have no idea how to figure where that is in real life or where it actually connects to the frame. Any suggestions?

[Keithuk]

Welcome to the forum Mike.

Originally Posted by mikerancourt

I was able to hear the relay click on when I jumped 30 to 87 (closing the circuit).

You won't hear any click if you bridge 30 and 87 those are the main feeds in and out it will only click when you put a live feed on 86 and earth 85 when the coil is energised.

VAG Error Code: 00626

Fault Location:
Glow plug warning lamp (G29)

Possible Cause:
Wiring.
Glow plug warning lamp.

As taken from my Free EOBD II Error Codes software

[mikerancourt]

Thanks for the welcome.

Originally Posted by Keithuk

You won't hear any click if you bridge 30 and 87 those are the main feeds in and out it will only click when you put a live feed on 86 and earth 85 when the coil is energised.

My relay has four blades, three of which are numbered: 30, 85, and 87. The un-numbered one seemed to be ground. 85 and 87 showed 12.8 volts or something like that at all times. When I connected 30 and 87, the relay clicked. When I connected 30 and 85, I heard a rapid clicking coming from somewhere in the clucter area, perhaps the ignition. I'm guessing that one is what appears to be labeled 60 on the wiring diagram, going to the ignition. I can't explain why that seemed to have 12+ volts even with the ignition off.
The wiring diagram in the Haynes manual shows 30 and 87 on opposite sides of a switch. It also shows "31" going to ground. I see nothing labeled 85 in there.

Oh, and electrical systems are confusing to me. Wiring diagrams help only to an extent.
Thanks.

[mikerancourt]

I did a lot of fun work this morning:
1. I took out the cluster and took it apart to visually inspect and mechanically test all the connections working backward from the bulb to the cluster wiring harness. I had no problem with the ground when I applied 12v to various places along the circuit on the board leading up to the GP light, and when I applied 12v to pin 20 on the harness connector, I got a light. This means the cluster is in working order. I get no voltage at pin 20 when I turn the key, so it looks to me like the problem is in the wiring from the cluster to the fuse panel unless I'm failing to understand the whole circuit somewhere. Unfortunately, there are about 15 white wires bundled in that harness, and they disappear behind the fuse panel. I don't see how I could possibly get in there to try to figure out which wire is #20 (GP light) and trace it all the way to its source.
2. I did test the voltage at each of the four sockets on the GP harness, and I got 12v at each. I also made sure they were all snugly connected. They were.

To be clear, I can live without a GP indicator light. I am satisfied that the plugs are glowing and that the car will start even if I find myself in very cold weather. I've thought of a way to rig up a indicator with a wire spliced into one of the wires leading to the GP harness, then run through the firewall and spliced into wire #20. This will not, however, solve my code error (00626) and my CEL. I don't think I would be able to pass inspection next time around (I don't know when that would be since I've never owned a new enough car to need one) if I have a CEL (though maybe I can do it in the summer, and maybe the light will work normally in warm weather). So, how should I go about figuring out this wiring situation? Is there actually something I don't know about this circuit that might make the solution easier than taking apart every damn piece of the mess under the dash around the fuse panel?

Thanks for the great help, folks.

[mikerancourt]

I just realized that everyone may have thought I was testing my GP relay (doesn't actually say "180" on it, but rather Bosch 0281003013). Actually, it was my ECU relay (109). Today, I tried to hook up thin wires to my 180, but there are 7 blades, and I found it very difficult to keep them from falling off or touching each other. Consequently, I can't say I got accurate results, but I think I found the following:

30 - constant 12v (key on or off)
86 - when I turn the key, it flashes on, then stays off for a few seconds, then has 12v
ST - When I first connected a test light to this, lit it up for a second, then went off, and then there was a whine/buzz coming from inside the dash somewhere. However, there is no whining/buzzing when I don't put a test light on it. At some point, when I checked the voltage on this, it was about 10.8 and at some point, it failed to light the test light, though I think the ground was still good.
D1 - With the key on, this shows .5 volts.
31 - At first, this turned on 12v with the key, and that disappeared when the relay clicked off.
G1/2 - Ground
G3/4 - Ground

I'm giving up before I break something. Maybe tomorrow I will feel differently.

To review:
I have no GP light when it's cold (no half second flash, no sustained light when the plugs are on). I have a CEL (00626, GP Indicator light, wire). New 109, relatively new 180 (June), new GP harness, four new GP's, relatively new and working 50A fuse. I do hear the relay click on and off. I have power to the harness, resistance is good on each GP, and the GPs seem to be working because when I wait for the relay to click off before cranking, it starts right up. There is no voltage going to the wire in the instrument cluster harness. When I do apply 12v to the pin (#20) at the harness connector, I do get a GP light, so the cluster circuit is working. The problems seems to be just the wiring from the fuse panel to the cluster, but I don't see how a person would be able to trace that wire, which is supposed to be white/green but is actually just one of 15 white wires going between the fuse panel and the cluster. I'd be content to string a new wire in its place if that would make the CEL go away, but I don't know how to get to it.

A few people have sent me different kinds of wiring diagrams, but I don't know how to read them or how they would help. They don't seem to match my car, or perhaps I just don't know how to translate the code of wiring diagrams.

Thanks.