Testing coolant glow plugs.

[chonchibelly]

Hello everyone.

I have searched what I can find on testing the coolant glow plugs and system on an '98 A3 Jetta. From previous threads all I have been able to come up with is that it is similar to testing the diesel glow plugs. I think that I may be having issues with mine as the vehicle seems difficult to start below 20 deg. or so. I have tested the Ohms on the plugs and am getting a reading of around .3. The control unit/fuse is somewhat melted as seems typical, but am getting 12V on the input posts. How do I then check the output? What is the proper way of testing voltage to the plugs and what else should I be looking for? What Ohm reading should I be getting from the plugs?

Thanks for the help.

 

[AndyBees]

Coolant GPs have not affect on starting nor does the ECU monitor them. Their purpose is to aid with a fast warm-up of the engine as well as the cabin.

I wouldn't worry about the ohm reading.

I'd remove them one at a time to see if they are working. You'll need to hot-wire them. I wouldn't apply voltage no more than two or three seconds without them being immersed in coolant or water.

Keep in mind, vehicles of your vintaged that were automatics did not have the Coolant GPs.

 

[chonchibelly]

Thanks Andy. Is this the only way to check voltage? Seems like a chore considering I would need to partially drain the coolant correct?

From what I have read, the plugs also pre-heat the coolant, no? This is important to me as I have had moments where the car would not start no matter what. I have a frost plug or headbolt heater installed which takes care of the problem as long as its had a couple hours plugged in. The winters are cold here in interior AK and I need everything I can get as you can literally watch the temp gauge drop on downhills.

 

[Vince Waldon]

They don't pre-heat the coolant to aid in starting, if that's what you mean, but they do improve the warmup time slightly and help defrost the front windows a bit sooner. The additional load on the alternator also improves warmup time. If you want to dig in a bit there's a couple good threads here, including one with some before-and-after graphs.

In any event, as Andy pointed out they are not monitored by the ECU so testing manually is the way to go:

- they should all read about the same resistance... somewhere less than an ohm. To me yours seem a bit low at 0.3, but that could just be the sensitivity of your ohmmeter... what reading does your meter read when you short the two leads together?

- if you have a higher capacity ammeter you can measure the running current. Connect the ammeter between a plug and the positive terminal of the battery. You should see an initial surge of just under 10 amps that then drops to about half that as the plug warms up.

- they should get a solid 12V but only when they are triggered...the conditions of which are a bit of a mystery...it's not covered in the service manual. I have a monitoring LED on mine and they only come on when the outside temperature is below a certain point *and* the coolant temperature is also very cold... they then stay on till the coolant temp gets to 50C. They are also run in configurations of 1, 2, or 3 at a time depending on if the engine glow plugs are also running (which they do for the first 2-3 minutes after the engine starts, summer or winter)..so you won't see any voltage at a couple of the coolant plugs for the first couple minutes of engine running, even in the dead of winter.

Can't remember the trigger point exactly, but it was well below freezing...so right now they may not be coming on at all... even in Alaska.

They tend to get corroded terminals and then melt the harness due to the increased resistance at the corrosion, so a good cleaning of all terminals is a wise idea.

And once again, just to reiterate: they have *nothing* to do with starting the engine. If you have problems starting a cold TDI in the winter unfortunately your problems lie elsewhere: retarded timing, weak starter, weak battery, low compression, defective glow plugs (although the ECU usually traps those).