I have owned this car new since 1991 and have had no serious problems with it. 172K miles.

I went to start it today,it was already warmed up from driving prior, I accidently let go of the key before it quite started. I then restarted as normal and the ABS light came on, the tachometer was not working and it acts like it's missing on one cylinder at idle.

It seems to run ok when driving down the street with tthe ABS light on and tach not working.

I suspect some kind of electrical problem but I'm at a loss on where to look. The fuses all look OK.

EDIT: OK. After sitting for about a half an hour I restarted it and everything is back to normal including the idle. Something is obviously about to fail here.

The alternator was replaced with a rebuilt Bosch unit about 18 months ago and I have had no other indications of any electrical trouble. The battery is the same age as the alternator.

All ideas appreciated.

Thank You!

Other than the rough idle - you probably caught the ignition switch in what I call the twilight zone - that in-between spot that becomes more frequent as our cars get older. BECAREFUL this was one of my warning signs that I ignored before my ignition locked. When this happens just lightly tapping the key usually restores instrumentation.

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Hi TX,

Thanks for the response. What you say makes sense. My key is getting a little bit sloppy which I guess is to be expected after 172K miles.

Were you able to replace your key/start switch assembly yourself? If so where did you get the parts? Sounds like a good place to start to resolve my problem.

Without a doubt, OVP (overvoltage protection relay) located on the firewall.. it is a silver can with a red top and a fuse.

Rough idle makes sense because with no power to the OVP there's no power to the EDS and thus no electronic control of the IP to stablize idle.

170K miles is a good time to replace all the fuses. Don't inspect them. Drop each one into a trash can as you remove it.

The key cylinder is fairly simple to replace but it's one of those jobs that's very difficult to describe in words. Basically you unscrew the black collar over the key cylinder. It takes a special tool or some creativity. Then you set the key to #1 position (ACC?) and slide a stiff wire into one or two slots (depending on year) to disengage the locks. Kinda like the locks that hold the stereo in place. Use the key to pull out the cylinder. Details are in the archives. A replacement key cylinder cut to the VIN is probably $150 from the dealer. Was a little over $100 3-4 years ago.

Sixto
95 S420
87 300SDL

sixto,

Do you agree that I should replace the OVP relay. The problem tach/ABS problem I had has not reoccured for two days.

Thank you.

Good question. I would be convinced of a bad key switch if other systems such as the ACC are unavailable when the tach is inoperative and the ABS warning light is on. The factory stereo is not a good indicator because it runs on a timer when the key is out.

First thing I would do is replace all the fuses. Not check, replace. Then have the OVP relay resoldered. If you don't have the skills or access to someone who can resolder the OVP then replace it. But I'd rule out the key swtich first.

Previously I described the process to replace the key cylinder. That was a rush to judgment. You should probably look into a new switch which is a device at the other end of the key cylinder. You have to pull the gauge cluster then the connector into the key switch and backhand a screwdriver to remove the key switch. I believe the detents of the key switch mechanism are in the replaceable switch, not the lock cylinder.

Sixto
95 S420
87 300SDL

easy rider, as the origional owner of the 350sdl, just curious if the mb company ever suggested that the engine be replaced with a upgraded version(non rod bender)/?
larry perkins

I just replaced the Over Voltage Protection (OVP) relay in my 1991 350SD. The broken one had a 2-fewer pins and only one 10A fuse instead of two. It was marked with "87" as if someone had put an old one in.

The symptoms were that the ABS light came on intermittently the and the Tach would not work at the same time. When it got warm I found that the AC didn't work either.

My Tachometer now works and the ABS light is out and the Air Conditioning works again. I found the new replacement unit for $62.10 online.

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

OVP relay replacement solved the problem.

My engine has 181K miles on it and has no bent rod or any other chronic symptoms.

Where did you find it for $62? My 190D is having problems similar to ezriders.
Thanks, John

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