OVP Woes (Dissection Pics)
 

So, Tach dead, ABS light on blah, blah, blah. Pulled the OVP. Fuse ok. It is the early 5-pin. I ohmed it using the handy diagram provided on the side of the housing:

http://i870.photobucket.com/albums/a...g?t=1316966899The diodes seemed neither blown open nor shorted, so I figured the relay itself was faulty. Upon measuring across the pins leading to the coil, I measured the value of the resistor indicating the coil or an associated connector/conductor was open. So, doubting the possibility of repair at this point, I figured I would use this specimen for a learning experience.

The outer cover is continuously crimped around the entire lower edge. I decided to make a small cut on each lower corner using a Dremel with an abrasive cutting wheel installed. This would allow me to fold the lower edges away from the body enough to release the outer cover and pull it off. (Apologies in advance for crappy pictures. All I have, right now, is a cell camera).

http://i870.photobucket.com/albums/a...g?t=1316966830


http://i870.photobucket.com/albums/a...g?t=1316966900

Once the cover was off, it was readily apparent what a PITA it was going to be to get to the inner workings.

http://i870.photobucket.com/albums/a...g?t=1316966900

It was clear, at this point, that many of these joints were going to have to be de-soldered for further disassembly.

http://i870.photobucket.com/albums/a...g?t=1316966898

http://i870.photobucket.com/albums/a...g?t=1316966905

http://i870.photobucket.com/albums/a...g?t=1316966903

Once unsoldered, the board is rather simple:

http://i870.photobucket.com/albums/a...g?t=1316966905

http://i870.photobucket.com/albums/a...g?t=1316966903

After getting the relay away from everything else, I was able to test it. Apparently the windings of the coil are, in fact, blown open somewhere as I have no continuity across it measuring directly to the leads. I'll keep this assembly as a spare in case I acquire another relay with a different problem that I can swap components between the two (Yes! I'm a cheap-a$$ :D ). I have another OVP on order and (hopefully) won't have to go tachless/ABSless etc. much longer.

Interesting post. Thanks for taking the time. Pics are OK.

Update: Yet Another Learning Experience, But Not A Fix.
 

So, the relay I ordered arrived today...only it wasn't the one i ordered. While it looked very similar, it only had four active pins. It also didn't have the clear fuse door that mine had. Looking at the number on the side answered why.
The original relay had "89 71 34" stamped on the housing (Mercedes part#2015400845). The new arrival had "89 71 24" stamped on its housing (Mercedes part#1265404745). A little research came up with the fact that this relay was meant for a W126 among others, but not a W124.
All was not completely lost, though. The diagram indicates that the circuitry in each is identical with the exception of one missing output.

So, I decided "what the heck" and installed it in the car. Upon firing up the engine, the tach began to work, and the idle was back to normal. The missing output apparently is for the ABS as the warm glow of the ABS warning light continued to brighten my day. I might have been able to get the same information from the manual, but I'm not so sure since both output pins are marked "87". I didn't look, though.

I have contacted the seller to (hopefully) get it exchanged to the proper one. Meanwhile, I hope this info is useful to someone.

http://i870.photobucket.com/albums/a...g?t=1317336632

Cheers! :vbac47679

I'd just splice the ABS wire output to the other 87 and be done with it.

I could've done that, but they sent me the correct relay and all is well. :beerchug:

First Post
 

Hey guys, I've been referencing peach parts forums so much lately for help with my Benz I figured it was only logical to make an account! First Post!

I apologize for any future breaches in forum protocol I may commit.

Anyway, I just bought myself a 1987 300d and I may be having a related issue. Anyone know the diagnosis for why my OVP blows the 10A fuse as soon as it's "plugged in"? My Tach and Clock are both dead in the water, I pulled the instrument cluster and verified that they're each getting 12V and checked the strip ground across the cluster. Is it likely the tach and clock are both internally fried? To the best of my knowledge all fuses and relays should be functional. Ideas?

ABS doesn't work either but that's fine with me I never liked ABS.

Could the OVP be the culprit?
As I said both clock and tach are getting 12V so I'm skeptical about it being the OVP but hey the fuse is and was blown so who knows.

Someone said the alternator my be faulty and providing too much current which "trips" the OVP. I will check voltage coming in with my multimeter in the morning. If it's getting that good 12V's should I just bight the bullet and buy another OVP or rip it to shreds any chance of fixing it?

Any help would be appreciated!

Also anyone have a trick for getting to the ground connections behind the instrument cluster save pulling the dash? The driver side vent causes an obstruction.