Wiring Harness 1992 Diesel 140 (300SD) - Mercedes-Benz Forum
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post #1 of 19 (permalink) Old 06-08-2013, 11:44 PM Thread Starter
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Wiring Harness 1992 Diesel 140 (300SD)

I recently purchased a 1992 300SD.
I have been successful at starting the car but have a few other issues.
The dash gauges (oil pressure, engine temp) do not work.
matino
The glow indicator does not light up and does not work.
So far looks like rest of the interior stuff works (clock, seat controls, climate controls, mirror, windows, door locks, sun roof).
I have read a lot about the wiring issues.
I have discovered that the glow plug wiring is badly disintegrating. The more I tried to repair it the worse it desintegrated.

I have three related questions:

1. Does the wiring for the diesel mercedes suffer the same issues (is it the same harness) as the Non diesel 140 style cars (1992/1993)?

2. Does this model/style car have multiple harnesses and does one fail more than the other or can I expect all to fail?

3. Does anyone know of a source to purchase these harnesses or is it possible to repair.

If all the wiring suffers from the same issue as the glow plug wiring it could be ugly. I'm surprised the car starts if all the wiring has issues.
I don't mind getting my hands dirty and already plan on creating my own harness for the glow plug wiring but I am not sure I can do this for a major wiring harness for this car.
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post #2 of 19 (permalink) Old 06-09-2013, 02:27 AM
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I just replaced the harness in my S500 -96... Since yours is a Diesel, and an early one, I assume it's still the one with the mechanical injector pump?

If so, those engines really doesn't require that much working electricity to operate (if any, but then again, just assuming)

Hopefully, from what I can google it seems that the 300 SD have a big cable harness going from the engine, via the firewall, to the black coffinbox, if you just check, you might have a big nice connector there which can make the change/removal of the harness quite easy.

Eitherway, it is possible to rebuild the harness by yourself, and depending on how complex it is on the diesel (hopefully less than the V8) it may be quite doable!

I also belive it is possible to purchase the connectors needed seperatly from mercedes, that way you wont have problems with connecting a new wire, to the old wire still on the connector. Some plugs mightr be possible to dissasemble, and some not... So I guess you'll have at least a few hours of fun from this!

Make sure you have a good wireing diagram diagram though, it will come in handy!

If you do it by yourself, make sure you do all the wires. Even if some look good, they're about the same age, and been in the same environment as the ones gone bad. If removing the harness is anything like on the 500, you really dont want to do it to many times (take it from someone who knows :-))

Personally I went for a brand new harness. A bit insane due to the cost (€1800, probably different for the Diesel, and checking doesnt cost!), but should it fail, I have someone else to blame!

Also, from what I seen on forums the insulation sort of disintegrates on the harnesses, but that was not the case for me, it had fatigued were it goes from the firewall to the engine (i.e where it moves the most). That was actually the main reason I went for a new one.

Anyway, hop this proves at least slightly helpfull for you, and good luyck!
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post #3 of 19 (permalink) Old 06-09-2013, 04:58 AM Thread Starter
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Originally Posted by suktinis View Post
I just replaced the harness in my S500 -96... Since yours is a Diesel, and an early one, I assume it's still the one with the mechanical injector pump?

If so, those engines really doesn't require that much working electricity to operate (if any, but then again, just assuming)

Hopefully, from what I can google it seems that the 300 SD have a big cable harness going from the engine, via the firewall, to the black coffinbox, if you just check, you might have a big nice connector there which can make the change/removal of the harness quite easy.

Eitherway, it is possible to rebuild the harness by yourself, and depending on how complex it is on the diesel (hopefully less than the V8) it may be quite doable!

I also belive it is possible to purchase the connectors needed seperatly from mercedes, that way you wont have problems with connecting a new wire, to the old wire still on the connector. Some plugs mightr be possible to dissasemble, and some not... So I guess you'll have at least a few hours of fun from this!

Make sure you have a good wireing diagram diagram though, it will come in handy!

If you do it by yourself, make sure you do all the wires. Even if some look good, they're about the same age, and been in the same environment as the ones gone bad. If removing the harness is anything like on the 500, you really dont want to do it to many times (take it from someone who knows :-))

Personally I went for a brand new harness. A bit insane due to the cost (1800, probably different for the Diesel, and checking doesnt cost!), but should it fail, I have someone else to blame!

Also, from what I seen on forums the insulation sort of disintegrates on the harnesses, but that was not the case for me, it had fatigued were it goes from the firewall to the engine (i.e where it moves the most). That was actually the main reason I went for a new one.

Anyway, hop this proves at least slightly helpfull for you, and good luyck!
Ill check out what you mentioned.
I have had the car started and took it for a ride of about 3 miles. I don't want to go far because I need to replace the air filter/intake. I suspect your correct that the diesel version does not need a lot of the wiring that is associated with a as 140 as there is not the typical ignition. It starts and runs. I just was shocked at the condition of the glow plug harness, and no longer surprised that the glow plug circuit was not working.
I also suspect I have some other grounding issues. The car started pretty consistently before I disconnected the glow plug wiring. Now it won't start. It may be the battery now as I have been cranking it a lot. But I suspect disconnecting the glow plug wiring may have removed an existing ground that was helping the car start.
I keep searching. If you come up with any more information let me know.
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post #4 of 19 (permalink) Old 06-09-2013, 05:36 AM
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I'll see if I can dig up a wireing shematic somewhere. This is what I found so far;

Bear in mind that this is to a W124, with loads of different versions and so on, but if I'm not mistaken you should have a OM603, so the general buildup should be the same

Mercedes Benz Model 124 - OM602, OM603 Maintenance Manuals
http://w124-zone.com/downloads/MB%20.../07.1-0400.pdf

As many of the newer incarnations of this engine seem to have a crankshaft sensor you might have as well, which would mean that it is some kind of hybrid electric/mechanical fuel injection you have. but then again, these are just theories.

does the engine crank at all now when you try to start it or is it totaly dead?
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post #5 of 19 (permalink) Old 06-09-2013, 06:21 AM
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I also have a diesel 1994 though and my engine harness looks bad although have no symptoms yet.

if memory serves me correct it as about 400 bucks at the dealer for a engine harness and it looked like an easy job to replace.

one big plug on the passenger side

please let us know what you do

I will inspect my glow plug harness at some point but will probably make that one if needed

Steve
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post #6 of 19 (permalink) Old 06-10-2013, 09:21 AM
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This should keep you busy for a few minutes:-)
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post #7 of 19 (permalink) Old 06-10-2013, 09:23 AM
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and some more...
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post #8 of 19 (permalink) Old 06-10-2013, 09:25 AM
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In the zip file you have diagnostic information and so on, so you'll be able to determine which cables that are bad.

Hope this helps!
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post #9 of 19 (permalink) Old 06-10-2013, 09:27 AM
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Saw the quality was a bit bad due to resizeing, I have it all in a big zip if you'd like me to mail it to you?
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post #10 of 19 (permalink) Old 06-10-2013, 10:10 AM
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Mercedes continued to use mechanical (IDI) diesel engines up through the W210 model. The OM603 and OM606 are both mechanical injection.

Basically, the only wiring necessary to make this engine start is a 12v connection to the injection pump to open the stop solenoid...and of course power to the starter. The system is relatively simple for mechanical injection vehicles. Although the W140 has a diesel module (EDS) it does few things and mostly just makes trouble.

The glow harness is a relatively simple harness, at least from the glow relay outbound to the glow plugs themselves. I'm rather surprised that it's deteriorating, I've heard of no problems from the diesel cars regarding harness deterioration like that found on the gasoline powered cars of this era. My harnesses are original and there's no deterioration at all, so perhaps in your case something else came into contact with the harness that's causing the problem...a chemical of some kind?

Some of the problems with dash lights may be due to other issues too, if someone has been in and cobbled the harness in some way for some reason, i.e. aftermarket alarm installers can be brutal sometimes and the same with aftermarket radio people if left to their own judgement.

Generally the glow light will not illuminate when there is a problem with the glow system. Usually that's the result of a faulty glow plug but also can be the harness too. If it were my car I would rebuild the harness or find a used one. Also then test each GP on the bench to make sure that it works. I don't think the W126 OM603 glow harness will work although I might be wrong. If it did work it would have to come from a very late W126 diesel to have the right connection for the gp controller.

Fyi, there are separate harnesses or pieces used in the W140. Depending on which car you have (which engine) the appropriate harness is connected to the firewall coupling in the appropriate position in the box.

Electrical issues are not all that fun but when you figure out and get it fixed it can be very rewarding.

Good luck

steve

97 VW Jetta TDI
90 Passat wagon TDI conversion
92 W140 300SD

Last edited by glenlloyd; 06-10-2013 at 10:15 AM.
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