I have an 85 300d and the tach is not working. I checked the wiring under the hood (for the sensor on the bell housing) and it is plugged in. I removed the sensor and cleaned it. What is the best way to troubleshoot the tach? Is there anything that commonly goes bad? Thanks.

Mike

Check the fuse in your OVP relay... that's what my '85 300D needed to get the tach working. It's up underneath the dashboard on the passenger side.



Of course, it's not necessary to strip your car down this far to get at it... removing the plastic panel underneath the right side of the dash (3 screws behind plastic covers on the top/front edge) should be sufficient.

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1985 300DT, Classic White, California, 159K
1982 240D, Metallic Silver-Blue, Manual Everything, 376K
1981 240D, Orient Red, Parts Car/Engine Donor, 145K

"When nine hundred years old you reach, look as good you will not. Hmm?" - Yoda

X2 Its really more on the sidewall in the area circled in the pic tho....
Good luck!

Mine did the same thing when I first got it. I replaced the OVP relay
and it fixed it right up. I used a junkyard relay

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85 300TD FED-Daily
84 300SD-Wife's
86 XJS-Sunday
66 GMC-Work- Given to my stepson
83 BMW Airhead- Given to my stepson

In the '85 300D the tach signal goes through the EGR electronics, which is behind an almost invisible door in the right front passenger kick panel (right side of footwell, below the area where the OVP relay lives). The EGR electronics rarely fails but you need to be aware it's part of the circuit.

Does your a/c work? The Klima relay under the hood, left side near the power brake booster, needs the tach signal or it won't allow the compressor to operate. If you have a/c then the tach signal is getting through the EGR electronics and to Klima. The tach problem would then have to be in the gauge itself (just an analog voltmeter, really) or the wiring. If a/c does not work then the tach signal is not getting to Klima or there is another problem with a/c. That might help you to figure out where the problem lies. As others have said, OVP and its fuse is first.

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

How about the tach amplifier shimming trick?...
Might be easier to do then checking the fuse. Also more likely...

__________________
1982 240D auto 152k
1984 380SL 113k
2015 Ram 2500 6.7 Cummins
Previous:
1985 300DT
1984 190D 5sp Euro

No such component in this year of the model 123.

__________________

"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

^Agreed, MB changed the set up in 1984.

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83 SD

84 CD

Thanks for your help guys! I think it just had a bad connection at the relay. After removing the relay and cleaning the connections the tach works! I am missing two of the plastic screw covers for the panels by the drivers and passengers knees. What is a good source for these?

I noticed that there was water below the "egr" deal behind the little door below the tach relay. Any idea where the water could be leaking in from? The windshield should be sealed up well as it was just replaced.

Glad it was an easy fix... I would tend to suspect that the source of that water is the "pocket" directly below your passenger side hood hinge and spring... debris tends to collect in there and block the drains. Check under the battery tray as well, it can get pretty ugly down there!

__________________
1985 300DT, Classic White, California, 159K
1982 240D, Metallic Silver-Blue, Manual Everything, 376K
1981 240D, Orient Red, Parts Car/Engine Donor, 145K

"When nine hundred years old you reach, look as good you will not. Hmm?" - Yoda

so where is the ovp in the '83. same vicinity?

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Ben
1987 190d 2.5Turbo

IMHO '83 should not have one.... but if you do..it is at fuse box/firewall.


.

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w126 500SEC gen II euro, powered by OM617 turbo stolen from 84 300SD 2.88 diff,EGR blinded
next wish/project: w114 coupe OM603 powered

I don't think it has one either.

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83 SD

84 CD

so if the tach is not working on an '83 300d, where do you suggest I look first?

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Ben
1987 190d 2.5Turbo

An '83 tach amp on the left inner fender is the first suspect. Next is probably the sender on the flywheel, then a break in the wiring or a dirty connector. Finally, the tachometer (gauge) itself, which is just an electrical meter movement. It can be tested, carefully, with an ohmmeter.

Jeremy

__________________

"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