I just bought a 1979 300D. It is in excelent condition in and out and I think that it will be a good runner when I get it started. I drove it for a day and a half after I purchased it and then It wouldn't start. The seller sad that he recently had the glow plug replaced and a new battery instaled. So I don't know why I am having this problem. Here it is. I checked the uses and relay and they all are reading 12 + volts. I also did a grounding test on all the glow plugs while the ignition was in the glow plug heatting position, to see if they were all getting current. Only the 1st plug closest to the fire wall sparked the rest did nothing. Could there be a short between that plug and the rest? Is there something else that I should be considering? I really want to get this car on the road? Help!

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Jude JudeJude

the Glow Plugs in your car should be wired in series, if its the original configuration with the waffle wire (touch one when starting and you'll think it can toaast waffles on them!)
Each GP in a series configuration takes only about 2.2 volts to heat properly. If you grounded the second plug from the rear of the engine you put a full 12 Volts across the rear plug and it most likely opened up from excessive current.
Do not use a spark test any more. Use a small incandescent light, or even a headlight if that is all you have, the best thnig would be a volt meter of course, as it doesn't draw any current.
Using an Ohm meter you can check GP's with everything OFF and the battery disconnected. I'll bet your in need of one plug. The old series connected plugs last a long time unless shorted by testing or something.
You may be able to start the engine by spraying a little WD40 into the air intake and cranking her, unless its really cold there.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

Jude, the series plugs are indicated by having what appears to be bent clothes hanger wire between some of them. If by chance you have these post that you do. With the series plugs if one of them has gone bad none of the others will work as they are like old fashioned christmas tree lights. My old haynes manual states before 1980 they are indeed the filament or series type unless they have been converted. A simple 12 volt test light should indicate the bad one when you go from ground to each wire in the string. When you get to the wire that does not light your test light you have just passed the bad plug. Make sure you start at point from where you got the spark as that is the hot end of the string with an open glow plug further along. Strong possibility from your post. Also no more grounding the system out to test as can damage the relay and even possibly destroy a plug in the process.

That information helped.

The person that changed the pulgs, I suspect that he either didn't change the ceramic insolatore or that he tighted down to tight causing the insolator to get hairline fractures. So here is my question. If there is a fracture in the ceramic insolater (or a couple fractures) could this cause the curent to be disrupted and the indicator light on the dash not to work, thus casuing the engine not to start? Cause I had to change out the two pugs again and that wass the only thing that I noticed. Or could it be something that I am overlooking that would cause plugs to go out prematurely? Thanks again for all your help crash corse in repair and annalisys.

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Jude JudeJude

If the two plugs you changed were #s 4&5 and they were also changed reciently there might be a problem. Plug #3 may have an internal short to ground taking out the other two plugs. Taking the wires off and reading from the centre terminal to ground should be open. A little more information would help like the plug numbers that were changed before and this time. But if the plugs this time are different than before you should make sure the voltage is going off with car running. Relay points could be welded together is one thought. Anyways more information if you are still having problems. Do not expect a little hairline cracking of the insulators is a problem unless the insulators have become conductive and thats unlikely in my opinion.

Starting form the front of the car, I have the order of the plugs as followed: 1,2,4,5,3. I donn't know what pulgs the other person changed. But I changed #'s 5 and 3. I replaced them with Autolite plugs (it was all I could find at the last minute) and the others were Bosch. Should I change out the other plugs too? The engine also is a vary cold starter. Anything I can do to improve starting in the morning? It does start fine after running for a while. (I'm loacated in the Pacific NW is it just the climate here?) I just replaced the battery. The battery that came with the car was the wrong size and amps it did not have enough amps to do the job. Thanks. Your comments have been a great help!

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Jude JudeJude

FYI, the cylinders and plugs are numbered 1,2,3,4,5 from front to rear.

Best Regards,
Jim

Hope you get to read this. Looks like you replaced the # 4&5 plugs that were bad. Almost conclusive that number 3 the one in the middle has some kind of short to ground. Especially if previous owner changed the two same plugs reciently. Now time to borrow or buy a cheap digital meter $10-$20 should do it or borrow one. Remove wires from # 3 (plug in the middle) and you will probably find it goes to ground from the connection on the glow plug. It is not supposed to and is most likely why your #4&5 plugs burnt out. They will burn out again if that #3 plug is shorted and you do not change it. Another quick test instead of the meter is to take a 12 volt test light from the battery positive terminal to the connection on the #3 plug with the wires off. If the light comes on the plug is shorted. Post how this works out or if any other concerns or clarification required about this problem. Your engine is trying to start on only three glow plugs at best right now I believe and thats pretty hard when engine is cold. The numbers you posted where the firing order of the engine not the cylinder number . No harm as was able to establish what plugs you replaced anyways.

Thanks again for the information! I will change out plug 3. I also discovered that the battery that came with the car was not the right one when I checked that amperage spesifications. I changed that out and that has helped the problem with the cold starting.

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Jude JudeJude