Need some diagnostic help folks - PeachParts Mercedes-Benz Forum

John300SDTurbo · #1 08-22-2004, 11:04 PM

A few months ago I purchased a 1983 300SD Turbo as my first non gasoline powered car and could not be happier with my choice. The old girl has over 350,000 miles on her and has been running like a dream. Recently my glow plug light stopped coming on and it is obvious that the glow plugs are not heating (takes forever to crank her up and I get a plume of raw diesel smoke when it does fire). I have searched here and on a couple of other UK mercedes forums for testing procedures for the GPs and the GP control relay. After performing the tests that I've found, here are the results:

1) Tested glowplugs by unplugging connector on the control relay and ohming them out. All plugs were between 0.7 and 0.8 ohms. I stuck one lead of my meter in the 1 - 5 positions of the connector and the other to the engine block and took the reading through the wires leading to the glowplugs.
2) Checked voltage on the 4 pin connector on the control relay and all seems to be up to snuff.
3) 80 amp strip fuse in the control relay is good according to my meter.

I can hear the control relay energize when I turn the switch on but it sounds like it is cutting off within a second or so of energizing and as far as I can tell I'm getting no voltage to the glowplugs. I also hear some sort of relay energizing inside the dash behind the temp control panel just under the radio and then cutting off within 2 - 3 seconds. I've got no idea what this second relay is. I've heard talk of a temp sensor of some kind that has something to do with this circuit. Is there such a beast? If so, where is it located and is there a way to test it? Could this temp sensor be bad and telling the GP control relay that it's too warm and not to energize the glowplugs? Although I am adept at working on gas engines I've got to admit to my ignorance of diesels and would greatly appreciate any suggestions.

sixto · #2 08-23-2004, 02:37 AM

Give the 80-amp fuse a second look. In fact buy a new one to be sure. A hairline fracture will allow the miniscule current of a continuity check but not the high amperage of glowing.

Sixto
95 S420
87 300SDL

John300SDTurbo · #3 08-23-2004, 10:55 PM

I actually hadn't thought about that but you make a valid point. Thanks. i'll let you know how it goes.

John300SDTurbo · #4 08-24-2004, 09:35 PM

Ok, replaced 80 fuse in GP control relay, still no luck. Could it be something to do with this temp sensor I've read about on one of these Benz forums? Any idea where it is and how to test it?

John300SDTurbo · #5 08-25-2004, 08:40 PM

Ok, here's where I probably get callled an idiot.
I had to jump the old girl off this morning. She just couldn't spin fast enough to get going. I stopped at Advance Auto on the way home from work, pulled the battery out for a quick test and charge, and to my surprise the battery was toast. Here comes the idiot part...... I'm assuming that if the battery didn't have enough umph to make the glow plug circuit work then I would get the symptoms I'm getting (no glow light and no heated up glow plugs). Somebody tell me I'm right so I can stop worrying about this. Or call me an idiot and I'll start over.

psfred · #6 08-25-2004, 09:15 PM

John:

If the battery would crank the engine, it would run the GP relay.

The temp senor is on the relay itself, not separate.

Try testing the resistance of each plug again at the plug, but if you have 0.6-0.8 opm, they should be fine.

From here it's up to you -- either spring for a new set of glow plugs and hope, or get a new relay, it sounds as if the circuit is bad and dropping it off line too fast. You can verify this by having someone switch the "ignition" on while you have the meter attached. If you get transient voltage, replace the relay, the temp sensing circuit is bad.

Peter

John300SDTurbo · #7 08-25-2004, 09:26 PM

Well, although that's not exactly what I wanted to hear it is what I was expecting. Thanks for the info, now at least I know where the temp sensor is. I don't think it is a glow plug problem because of the readings I got last time (I went through them a couple of times then just to make sure). I guess I'll have to go the MB dealer and get a control relay. I just need to remember to take the KY Jelly when I go . That way it won't hurt quite as bad

Once again, thanks for the info.

psfred · #8 08-25-2004, 09:45 PM

Order one of FastLane here at Mercedesshop, it will be cheaper, probably faster too unless you go to a big dealer and they actually stock one. I have to wait a week at the dealership here, they only order once a week....

Peter

manny · #9 08-25-2004, 09:46 PM

Quote:

Originally Posted by John300SDTurbo

I guess I'll have to go the MB dealer and get a control relay. I just need to remember to take the KY Jelly when I go . That way it won't hurt quite as bad

Once again, thanks for the info.

Have you seen this: http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1AM13H66E&year=1983&make=MB&model=300-SD-002&category=F&part=Glow+Plug+Kit

Gilly · #10 08-25-2004, 10:19 PM

The glow plug resistance test is an absolute waste of time. The only accurate test of the glow plugs is a current consumption test.

Gilly

John300SDTurbo · #11 08-25-2004, 10:20 PM

I was just looking at that. Excellent price for what they give you. Best price I've got on a relay by itself around here is about $175. Thanks for the info folks.

manny · #12 08-26-2004, 12:47 PM

Quote:

Originally Posted by John300SDTurbo

Thanks for the info folks.

No problem.
Btw. my favorite beverage is Miller Genuine Draft.

autozen · #13 08-26-2004, 06:21 PM

John,
I know I'm late on this one, but I concur with everything Peter said. It sounds like the relay.

Gilly,
I'm going to have to go head to head with you on this one. I believe the resistance test is valid 98% of the time. It always works for me. I agree that you do run into some weird situations where resistance is right on, but the system is still problematic. Thaen I would run a current check or better yet I remove the plug and hook it to a battery. I look for the glow. Some of these plugs can trick you, but I find that to be rare.

Gilly · #14 08-26-2004, 07:03 PM

Peter
If you test it and it tests bad, then it was a valid test. If you test it and it tests good, it may still be bad. To me that's about as worthless as it gets. The current consumption test results are the only "hard" test of the plugs.

Gilly