I have a 1985 300TDT. It was running just fine until a few weeks ago when the pre glow light didn't come on. It started a few times, then went dead. I replaced the battery and it started a few times ( the pre glow light still didn't come on). The alternator checked out good. I went to start it the next morning and it didn't start. I checked the continuity in the glow plugs like it says in the manual. I've read some things on this site and they said they should read 0.5 to 0.9 ohms. I'm getting pretty close to zero! The Haynes manual doesn't state any values, rather they say that if you get an infinite reading then they are bad and must be replaced. Is it a possibility that the pre glow timer relay is bad? Can you test it to see if it's bad. The car turns over and sounds like it want to start.

With the meter set on volts and the black lead to ground, when you have someone turn the key to ON from the OFF position, all of the glow plugs should have about +12V on their metal tips where the wires connect. No voltage - its a clue.

Check to make sure you have +12V on both sides of the glow plug relay fuse. When you pull the cover off the glow plug relay, there is a heavy strip of flat wire between two screw terminals. That's the large-amperage fuse.

Check back in with us after those tests - we'll narrow this down.

Ken300D

Ken

I checked the voltage at the 80-amp fuse, and it was 12V (on both sides). I took the plug apart where the glow plugs come into the relay box and tested at the metal pins for voltage (when someone turned the ignition on). There is no voltage at all, but there is still 12V coming to the relay through the fuse. Someone said you should hear it click on, and then after a period of time, click. I hear neither. Does this mean it's the pre-glow timer relay? It seems to me it is, but the part is not cheap to be replacing if it's good. If you have any other suggestions let me know. Thanks.

You are right that it is an expensive part, on the order of $120 I think.

The troubleshooting you have accomplished so far shows why the car doesn't start - there's certainly no heating being done by the glow plugs.

Before you can isolate the problem entirely to the glow plug relay, you need to check the voltage signal that causes the relay to throw. This would be coming from or controlled by the ignition switch, and is what makes the relay throw when you turn the key.

There should be an additional plug or connections beyond what you already have tested that will be this signal. I think it might be a ground lead and a hot lead, but am not sure. But it will be what is left over (in terms of electrical connections) besides the connections going to the glow plugs and the big battery and fuse screws.

It should come down to one of two things:

Your glow plug relay is bad because it will not throw.

Or, your glow plug relay is not getting the signal to throw from the ignition switch. Could be an ignition switch contact going bad or a fuse blown.

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Hope this helps,

Ken300D

Pull out the 4-wire connector and turn the key into position "2". Line #1 should be +12v. The voltage comes from the fuse box, possibly fuse #12.

Line #4 is ground. You need to check this wire for good ground too.

Line #3 goes to the lamp (If you bridge line #1 and #3 with key into "2", the indicator lamp should be lighted up).

Line #2 is from the starter.

David

be - are the terminal numbers the same for a 1983 240D. I noticed that I was not getting the glow plug light on when I tried to start it prior to pulling the engine out for rebuild. Hopefully, I will get the engine in this weekend and will then start troubleshooting the glow plug issue.
thanks

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Jim

Hi Jim,

I hope now, you are putting together the new engine. I have never worked on a 240D but I think it must be fun doing it since the engine is small and there should be a lot of room under the hood.

I do not know anything about the 240D but I got a chance to look up my 123 CD service manual today. It seems to me your 240D has the glow plug system just like the late model W123 300D. If that is true, the 4-point connector should be the same. If you have the old glow plug system (wired in series), the diagnostic is different.

David

It is the same system as what is in my 300D. I am installing new glow plugs when I try it later this week so I will see what happens.

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Jim

Thanks be459 for that information you gave. I fixed all my problems with a new (used) glow plug relay for $75. My car runs great again.

I found what I am sure is my problem. Took the cover off of the GP relay and the contacts were gunked up. Took the emery paper to them and sprayed everything down with contact cleaner. I will see if it works later today.

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Jim