The tach suddenly stopped working on the drive home after I picked up a 1991 350sdl last night.

A friend who knows these cars well checked the box that has the 10 amp fuse on it- I can't remember what it's called right now. The fuse was okay.

What else could cause the tach to suddenly stop working? Do they pick up the signal from the alternator like other cars? As far as I can tell, the alt is working just fine still.

pull out the box that has the fuse on it, and reseat it. The box is called OVP relay, sometimes, just pulling the fuse out and reinserting it fixes the problems, sometimes you got to pull the entire relay, and other times you got to replace it.

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Just because the fuse is good on the O.V.P. does not mean the relay is o-kay. On my 85 300SD when I replaced the relay tach started working took me months to figure this out.

Thanks. We did pull the box out and reseat it. I think I'll try again, with some electrical cleaner this time.

Any way to test it with a multimeter?

A common engine speed sensor drives the tach, EDS, cruise control and Klima relay. If the AC compressor engages, cruise control holds or idle speed isn't lower than normal, the problem downstream of the OVP.

Sixto
87 300D

Abs light is on. Cruise is inop, and car idles low- stalled once.
AC is inop.

Think we found the problem!

Would a dead ovp relay be related to kickdown shifting anomalies, too?

That's diversity for you. If all those things fell into my lap at the same time, the last thing I'd be worried about as far as posting on this forum would be an inoperative tach. Maybe second to the last

Sixto
87 300D

I guess I just figured the AC needed charged, the cruise was just shot, the ABS had a bad sensor that I'd fix along with the pads today, and the low idle was a result of the car sitting for an extended period of time and needing fuel filters... Never imagined they all had a common link in the OVP relay!