I have a 1985 300D Turbo (405K+ miles) and recently had my tachometer drop dead. It's an '85, so it's not the little amplifier that dies on the '84s and older.

Looking under the hood today, I noticed this cracked plug on top: http://img169.imageshack.us/img169/5912/macroview.jpg
closeup: http://img526.imageshack.us/img526/15/closeupa.jpg

Is this my RPM sensor? (if so it would explain why it isn't working) If not, whatever it is is broken, what is it and can I get a new part?

Edit: I know the tachometer reading comes from the EGM computer, and I think the part is on this page: http://partscatalog.ru/mercedes/1/car/230/474/65P/113.992/07F/54/250 labeled "168-545-07-28 PLUG HOUSING ENGINE CONTROL MODULE". The numbers match what I can make out on my broken plug (5-07-28 for sure). And engine control module sounds suspiciously like EGM computer so I think this is my problem. Can anyone confirm this?

Edit #2: It's definitely the problem. When I touched it the plug disintegrated. I took the female pieces, which were still intact and hooked them to the pins and the tach worked! I still need to find a new part though because I'm sure they'll just rattle off eventually. Thanks anyway everybody

Thank you!

Lauren

No. Your RPM sensor is down on the transmission bellhousing.

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Bob
'82 300D Petrol B-G Metallic

That is a microswitch that turns off the a/c compressor when you floor the accelerator. Turning off the compressor removes a little bit of load and allows you to go a bit faster. Why Mercedes decided that the OM617 needed this is beyond me but whatever. As long as the pins and wires are still making contact and not shorting to the valve cover you are OK. A dealer might be able to get you a new plug but otherwise glue or duct tape will work.

Your tach gets its signal from a transducer back near the flywheel. The signal runs through the EGR control module and requires that the over-voltage protection relay (OVP) be good and not blown its internal fuse. In your car, both the OVP and the EGR module are cunningly hidden behind the right kick panel of the front passenger footwell area. Who would ever think to look there? Pull the panel and check the 10 Amp fuse in the OVP and report back.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Thank you both for your replies.

I'm a little bit confused, since I was able to solve the problem by making the pins connect and an a/c cutoff switch should be completely unrelated. I'll check for the OVP relay anyway. I think I've had that kick panel open before. If I remember correctly there was a small black box and a covered plug.

Is it possible that the plug was shorting out somehow and managed to trip the OVP relay?

There was a thread about a yr ago, where water had leaked down onto the black box behind the kick panel and corroded the pin connectors.
I want to say it was JIMMYL but not sure. he had to replace the connector and splice into the harness. was quite envolved.

The tack gets its pick up off the flywheel from a sensor that is bolted on the adaptor plate on the drivers side. there is a 1/4 inch black cable that comes up the left side of the engine into the harness that goes across the firewall towards the battery as I remember.

Charlie

WOW, I hit the 2000 post

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

And the tach has dropped dead again. It worked driving to work this morning, but was very bouncy when I was on the gas. I figured the connection on the plug I was playing with was wonky and getting bounced around, but I'm not sure.

I opened up the small door on the kick panel, and can see an outlet with a cover and a black box. Do I have to get the kickpanel off completely to access the OVP relay?