I bought my '86 190d 2.5 5spd with 227,000 showing on a DOA odometer. The tachometer was DOA too. I've since had the speedo and odometer rebuilt and they work fine. I then replaced a relay and a fuse (per an earlier post) to try and get the tach working, but nothing happened. I then bought a replacement speed sensor, but can't seem to find a speed sensor on my car to replace.

As I drove to work this morning, after having driven the car for 15k miles since I've owned it with no tachometer, and out of the corner of my eye I saw something moving on my cluster...it was the freakin tachometer, working fine and acting like it had worked all along. Thinking this was some kind of x-mas miracle, I fully expected it to be working when I hopped in to head off to lunch. Guess what, it was dead again........Outside of being haunted, is there a plausible explanation?

So, my tachometer had not worked since the other day when it randomly worked for about 30 minutes. Last weekend I had to take of the air intake to work on my glow plug wiring. When I took off the glow plug wiring harness, I noticed a break in a lime green colored wire that lead to a sensor on the side of the block, located about 6 inches below where the glow plugs are. I repaired this wire, and when my work was done and I cranked the engine, guess what--the tach was working.

The next day on the way to work I was pretty pleased to watch my tach happily bounce up and down between the rpm range--fixed it seemed. But alas, when I cranked her up to go home that day, no tach...and no tach since then.

Is there any connection between the green wire and sensor on the block and the tach, or is this just another random incidence of tachometer haunting?

Try replacing the Overload protection relay.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

The wire that goes from the engine rpm sensor behind the below the oil filter to the tachometer is green in the 124 and 126 so likely green in the 201 as well.

Function of tach, AC compressor, cruise control and idle control of 60x engines relies on an engine rpm signal. If only the tach is affected, it can't be just the wire nor OVP relay. If everything is affected it can be sensor wire or OVP relay or both... or any number of other things.

Sixto
87 300D

my 300sd did that. My mechanic said it was some sort of amplifier gone haywire. he reccommended i not replace it because it was too expensive, Of course with him if it wasnt from the dealer he would not buy it or install it on my car.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual

I noticed something interesting--the tach will work when it is raining outside, but not the rest of the time--as if the water is allowing a connection to be made that isn't made when dry.

The cruise control and ac compressor don't work - where is the rpm sensor? I've replaced the ovp relay, so I don't think it is the culprit.

I'm having the same problem. I think my cruise works only when my RPM gauge is functioning right. Am I correct?

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Regards,
Ian White

1995 E300 Diesel w124 OM606
2014 E550 w212 M278 biturbo

2001 BMW 740i E38 M62 (past)
1981 300SD w126 OM617 (past)

Along the transmission flange at the aft end of the engine block under the starter mount.

Sixto
87 300D

There's a coaxial connector on the non-facing side of the tach that feeds engine rpm to the cruise amp (and Klima relay?). More than once I've forgotten to reconnect the wire when refitting the gauge cluster.

Sixto
87 300D

Happened on my 81 300SD and yes it was the tachometer amplifier. Located on the drivers side wheel well. They are very expensive to get new, I ended up getting one from a guy on ebay for $25. It has worked like a charm for the past year now. I can dig up the guy's email address if you would like.

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W210 1996 E300 - Daily
W126 1981 300SD - Nice weather
W111 1966 230S - Weddingmobile
AVPCAR 1999 Honda Si - Drag/Showcar
W111 1968 230S - Parts car

Can't fault a guy for wanting OEM, especially if your the one paying.
As for tach amp. you can try shimming under the amp circuit board.
It's worked for me for about a month. Otherwise, try this thread -
OM617A Tach Amp
Bob's got it figured out, and is willing to sell his invention at a very reasonable cost.

Don't know which other models (ie 190D) his device will work with.

I don't know about the Tach amplifier but I do know several have taken out the cruise amplifier and reheated and soldered each connection on the board. Over time these old solder joints can get weak and loose contact. I would expect the same for the tach amp. You might try that.

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-E300d '99 350k
-Suburban '93 220k
-TDI Jetta '03 350k
Sold
-F250 '96 7.3
-Dodge Ram 12V
-E320 '95 200k
-E320 Wagon 1994 155k
-300d Turbo '87 187k miles
-E320 1994 200k
-300d Turbo '84 245k (sold to Dan62)
-300d Turbo '84 180k
-300sd '80 300k
-7.3 Powerstroke Diesel 15P Van 500k+ miles
-190d '89 Non Turbo 2.5 5cyl 240k (my first MB)
Tom's Imports of Columbia MO Ruined the IP in changing leaky delivery valve O-Rings - Refused to stand behind his work. Mid-MO MB drivers-AVOID Tom's.

x2 on the old solder joints, it's just very delicate soldering in there.

OEM all the way for me too! Beautiful thing was that the Tach amp was a used OEM. Guy said that if it didn't work I could send it back and he would send me one that did.

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W210 1996 E300 - Daily
W126 1981 300SD - Nice weather
W111 1966 230S - Weddingmobile
AVPCAR 1999 Honda Si - Drag/Showcar
W111 1968 230S - Parts car

I've got a good used spare, just waiting for the shim trick to quit working