Recently my tach was inoperative 95% of time so I replaced the OVP relay with the MB updated style with two 10A fuses.

It solved the intermittency issues; however, the tach is not reading correctly. At 60mph, it now indicates 1k RPM. A full throttle acceleration shows the tach maxing out about 3k RPM.

Any ideas as to why the tach is reading approx. 1/2 of expected RPMs?

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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)

Hi Ian,

I’ve never heard of that. Do AC and cruise control work? I’m tempted to suspect the ring gear sensor but The engine wouldn’t idle properly without it. Maybe the bad OVP relay fried the tach. If so, you might ask Jeremy (jeremy5848) what he thinks. Good luck!

Sixto

Yep, the A/C and cruise work.

__________________
Regards,
Ian White

1995 E300 Diesel w124 OM606
2014 E550 w212 M278 biturbo

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

I was thinking, occasionally in the morning it idles roughly. Could this be related to the electronic idle control and thus the tach?

__________________
Regards,
Ian White

1995 E300 Diesel w124 OM606
2014 E550 w212 M278 biturbo

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

The rpm sensor reads 144 teeth of the flywheel, I dont think you can even start the engine with half of them missing altogether.

I think your tach meter is gone bad in the gauge cluster, the idle control system in this car works just to smooth the idle to 650 rpm, without it the engine idles at about 500 rpm and bogs down if you apply a load to it, like AC and power steering.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

That sounds like a possibility. I suppose it could be the tach itself. I was just wary of it being the root cause.

__________________
Regards,
Ian White

1995 E300 Diesel w124 OM606
2014 E550 w212 M278 biturbo

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

I've got a similar problem now in my '95 E300. I haven't been driving it very much at all for the last year or so, I try to drive it a bit at least once a month. Yesterday was exercise day for the car, but when I started the engine the idle was too low for a minute or so, and the tach didn't work at all. After that first minute or so, the engine idle speed jumped up to normal (about 750 RPM) and the tach started working again, but it read low (around 400 RPM) and when I drove the car around the tachometer was consistently low.

I think I've got an OVP issue, so I'll be checking that first.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

The OVP relay is generally "good" or "dead." Given the age of the car and all, I'd be more inclined to suspect a wiring failure due to the "environmentally friendly" insulation used on wires in those cars, or a damaged/dirty/corroded connector. Although the pins in the connectors are solver-plated, they do oxidize and begin to fail The EDS (Electronic Diesel System) includes the EDS controller (behind the battery), the tach including the ring gear sensor, the idle control assembly on the fuel injection pump, all the wiring and connectors, and probably some other things I've forgotten.

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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

When the capacitors in the tach fail it can read incorrectly or die all together.

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CENSORED due to not family friendly words