Originally Posted by 777funk

To do this it's got a 5 wire harness with somewhat large wire (maybe 12AWG) going to the GPs. You can either pop the top off of this harness to expose the pins (Look at the wire harness and you'll see it has a snap lid on the back side of it) and measure on one of the 5 pins while it's plugged in to the GP relay or measure at the GP terminal itself.

What you're looking for is 12V at the GP cycle (first minute I believe after turning the key). Measure with a voltmeter (multimeter set to measure at least 12V). Put the black on the battery ground and the red on the battery Red just to check and make sure you're measuring 12V. Then have someone turn the key to Glow and measure at the GP relay's 5 pins or the Glow plugs themselves. If you have loop (series) style glow plugs your circuit will be a little different. If so you should measure voltage across the two wires on the GP but it won't be 12V. Rather 12V divided by the number of GPs in series.

I do not get voltage. The diesel is a 190D Year 1987.

Now if you don't measure voltage at the GPs, don't blame the relay yet--- here's why:

1. The relay is designed to NOT provide the GPs with power if more than 2 are burned out. (measure ohms across the GP -with it out of circuit- and you should see .7ohms approx on the new style (not old series type-if it is series type you'll see still close to this and Open Circuit still equals dead GP). If it's an Open Circuit or very high resistance, the GP is dead... you can also just use jumper cables on the GP and see if you get a red glow if it's not the series style GPs)

All my GP's at the wire harness measure about .6 ohms after correcting for the wire leads resistance.

2. The connections on the relay may not be tight. Check them.

The somewhat larger wire that is attached (via a brass bolt) to the preglow relay is showing voltage to the relay and is a good clean conection.

There is no voltage from the 5 pins on the relay after turning the key to the point where the glow plug light should light. The four pin connector does not appear to be real tight. I took a pair of needle nose pliers and made them a bit oblong. This did appear to help.

Only one of the four pins receives voltage while the key is engaged. Does this sound correct. What do the other three do?

3. The connections at the GPs may be shorted or not in contact. Check them.

This is not indicated by the ohm meter.

4. The FUSE on the GP relay may be shot. It's a big 80A (I think) silver colored strip fuse (about 1 1/4" by 1/4") that has a screw on either side. Measure across this to make sure it's good. You should see 12V on either side of the fuse or you can measure OHMS across it and see close to '0' ohms.

Checks out fine.

5. The wires could be shot. But this I'd doubt. Again though you can measure across them with an ohm meter and should get close to ZERO ohms.

But that should help you.

Here's more of a description of how your GP relay works if you're board:

It has some simple logic inside that does the following:

-Is the key turned to Glow?

Yes
-Do I measure a resistance that says I've got all but at least 1 or 2 glow plugs still working? (my guess as to how it knows the GPs are still good is by resistance)

All check out fine.

-Has the starter engaged (the purple wire on the 4 wire harness senses the starter's voltage signal when it's energized)

This must be the one I am receiving voltage from.

-If YES to the first TWO and NO to the THIRD question, send 12V to the glow plugs for 1 minute.

What do you mean by this? Unplug the harness and send 12 volts to the corresponding pin.

I've fixed GP relays with corroded Relay copper terminals. All you need is a screwdriver to take the relay apart and sandpaper to scuff the terminals clean.

I did this a couple months ago (after having hard start issues) and cleaned all the terminal points and the everything worked including the glow plug light. Last week the car would not start (B100 does not help). The glow plug light did not light up, not even for a second. I took the relay apart and the one of the 4 contact pins was completely burnt off.

I bought a new preglow relay and installed it thinking it would be an easy fix.

I even think I can feel and hear the relay make contact. Could I have a defective preglow relay even though it is brad new? MB has glued or done something to the relay that prevents me from taking it apart to see what is going on.

What else could it be?
Anyways, hope this helps.
Nick