I have a 91 300D, 2.5L. Out of the blue the typical symptoms of a bad OVP occurred....rough idle, ABS light on, and no tach. the fuse on top of the OVP blew. Replaced fuse and blew again. Ordered new OVP with 2 fuses and 1 of the two blew on start up. ABS light is out but rough idle and no tach. After many hours and days of testing and checking components, I'm stumped. Here is what I have: key not in ignition, take neg off the battery then retouch the neg to battery terminal I hear the convenience relays click and there is spark at the the battery terminal...With battery connected, I turn the key to position 1 and still good OVP, but turn the key to position 2 (not starting or engaging the starter and the fuse on the OVP blows. I've disconnected the radio, the CCU to isolate them and the fuse still blows. I've replaced the convenience relays, but no luck. Can anyone offer some advice? I think something has gone to ground, but I don't know what component....

Hello Finster,
The overvoltage relay (K1/1) powers the following on your 91 2.5:
1. Anti-lock brake system
2. EGR valve vacuum transducer (Y31/1)
3. Electro-magnetic idle speed actuator (Y22)
4. Boost control valve vacuum transducer (Y31/2)
5. Boost pressure control vacuum transducer (Y31/4)
6. Electronic Diesel Control unit (N39)

It could be any of those components or the wiring between pin 6 of the OVP (K1/1) and the above listed items or pin 7 of the OVP and the anti-lock brake system.

Hope that helps.

__________________
1985 300TD - "Panzer Wagen" - undergoing full restoration (resto - mod)
2014 GLK250 Bluetec - Wife's car

Mercedes-Benz Master Certified Technician.

http://www.flickr.com/photos/joshuajeeper/

Thanks for the info. However, the convenience relays click when I connect the neg wire to the battery terminal even when the OVP is removed from the vehicle. My 87 300D doesn't do that. Any chance the glow plug relay is the culprit?

As of 1990, the convenience relays are powered by the convenience control unit which has constant battery power, hence why you can operate the power seats without having the key on. The 87 doesn't have a convenience control module.

The glow plug relay gets its power before the OVP. Both get fed power by fuse 7 in the fuse box. If that fuse was blowing, then yes, it could be the glow plug relay. The only way the 10amp fuse in the OVP could keep getting blown instantly would have to be a direct short to ground from either of the previously listed components or a chafed wire to any of the components.

__________________
1985 300TD - "Panzer Wagen" - undergoing full restoration (resto - mod)
2014 GLK250 Bluetec - Wife's car

Mercedes-Benz Master Certified Technician.

http://www.flickr.com/photos/joshuajeeper/

Again thank you for passing on your knowledge of these cars. I shall once again search for a chafed wire or bad component. At least now I know which ones to target. I'm very grateful this forum exists...

I isolated each component individually above by disconnecting the wire harness. No luck 1of the 2 fuses keeps blowing at key position 2. All exposed wires, before they enter a trunk do not have signs of chafing. Any other suggestions before I take it into a shop?

Your car has the biodegradable wiring harnesses. Maybe time To inspect those.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

all wiring harnesses? or just those to the components mentioned above. I also noticed for the first time, the door buzzer isn't working when the key is in the ignition to remind me "hey dummy take the key after you turned off the car".

Engine heat seems to accelerate the degrading process. On my 1991 500sl I have found one harness in need of repairs which runs on top of the intake manifold. On your car I would start with the harness running behind the engine. At least one other member had that problem on his 124.

__________________
92 e300d2.5t
01 e320
05 cdi
85 chev c10

dieselbenz, the wire harness under the cowling where the blower motor housing is?

One other note...
yesterday, I used my mutlimeter to chech voltage at the battery...normal...same with the engine runnning...so I pulled the new OVP and checked the voltage with the engine off and key in the # 2 position and with engine running at pin 30....no spikes in voltage either time.....seems odd if I have OV which blows the 10A fuse, shouldn't i see that on the multimeter?

I had researched this subject about 3 or 4 weeks ago as I was thinking I had a wiring harness issue and it turned out it wasn't. I recall reading the harness affected has a connection to the starter as well as 6 or 7 other connectors. Darned if I can find that article again.

Have you disconnected n39 and tried that would isolate and identify if the problem is in that harness which seems likely.

__________________
92 e300d2.5t
01 e320
05 cdi
85 chev c10

No I have not disconnected the n39. I'll try that today. I have a rare day off from work. Standby...

N39 is the culprit. Is that unit programmable?

Not to my knowledge. But it will be in one or more of the sensors/ actuators. Do you have a good wiring diagram you can use to troubleshoot each devices wiring separately?

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92 e300d2.5t
01 e320
05 cdi
85 chev c10