87 300TD tachometer not working
 

My tach isn't working, nor is the A/C or cruise control. I was thinking it could be the Overvoltage Relay, but that checks out OK. Then the Klima, which also seems to check out OK. Would this point to the RPM sensor? I haven't had a chance to disconnect it from the housing and clean it up, but if that doesn't work would you assume the sensor to be bad?

Thanks.

How about the fuses?

The fuse for the tach looked OK, but I changed it anyway.

I'm out of ideas. I know other 124 owners that have had this problem will chime in.

I had this problem and the RPM would slow down and it was intermitent what I found was a corroded plug behind the battery. You will have to remove the battery to fix. I removed the plug and hard wired and soldered and have not a problem since. Good luck.

It's either the OVP (which you said is good), or the RPM sensor. I've had this problem before. It connects with a 2-pin connector near the battery, if that connection is bad, clean it & try again. If that doesn't cure it, replace the sensor (it's located approximately below the starter and oil filter housing area). I've got a good used one I'll sell cheap if you want it, lol.

:cool:

IF someone would be so kind...
 

as to post a photo of the connector near the battery??

I had a similar issue on the interstate.
Running 65-70mph - sudden drop in power (had to floor the pedal to keep it at 55) associated with tach indicating about half what it should AND dark, medium thick smoke from the tailpipe. Hasn't happened since but I'm guessing the same thing??

Nope. Those symptoms have NOTHING to do with the RPM sensor. When the RPM sensor goes out, the AC compressor won't engage, the tach dies completely, and the electronic idle control (ELR) doesn't work so idle is lower then normal and shakes the car.

A sudden drop in power as you describe is typically loss of fuel delivery, but with smoke like that, I'm thinking something related to the turbo or EGR, if it cleared up and stopped. Weird.

New 1987 300D/TD owners should read this article, if you haven't yet seen it:
http://www.w124performance.com/docs/mb/articles/124.1x3_buying_tips.txt


:scholar:

While checking the OVP and Klima the other day, I noticed a 2 prong connector, right behind the battery, with a lot of corrosion. I tried to slide the female connectors off by hand and they wouldn't budge. At the time, I didn't know what that connector was for, but after reading the suggestions here have a good feeling my problem is related to that connection (rpm sensor lead). I'll post back when I've cleaned it up.

Thanks for the help.

That's the connector. Clean it and try again. If it's still not working, disconnect it and plug a voltmeter into the side that goes towards the middle of the firewall partition (it goes out with the engine harness, towards the intake manifold.) With the engine running, there should be a low AC voltage present, which increases with engine RPM. If not, the sensor is bad. My connector was all green and corroded too, but cleaning it didn't help - a new sensor fixed the problem. They're about $50 new. I've got a good used one from my '93 that I'll sell for half that, free shipping. (My tach was flaky, I thought it was the sensor, turned out to be the OVP relay instead - D'oh!)

:antlers:

http://img.eautopartscatalog.com/live/P401338301OES.JPG