My normally impeccably running machine has developed an odd problem. The normal idle is ~550-600 RPM in neutral or in gear with foot on the brake. It has started acting as if a switch to reduce the idle has been thrown by a gremlin, so the idle will reduce to something significantly less. I can't say how much because the other symptom is the tach quits when this happens.

I put a voltmeter on the car and did not observe any fluctuations when this was occurring. All other gauges are fine, headlights are fine, voltage level is good (13.8 - 14V while running with lights on, etc.) It's as if a button is pushed to reduce the idle to barely stable level (not misfiring, but quite low) and the tach quits. Then it will snap out of it and be perfectly normal. Once it did this twice in the space of about 10 seconds. Normal to slow, wait 5 sec on slow, then back to normal, then back to slow.

I can rev the car while this is happening, it revs normally but tach stays off. Seems to drive normally while this occurs. Normal idle eventually seems to return and stay (with tach operation normal), until next time I shut the car off, and then the next start seems to show the problem again.

Any ideas appreciated.

Rgds,
Chris W.
'95 E300D, 433K

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Objects in closer are mirror than they appear.

OVP relay would be my first guess.

I once had an intermittent issue with my tach, plus there was something else going on but I can't remember specifically what. (It was a long time ago.)

I don't have a wiring diagram in front of me, so I'm can't say authoritatively if the electronic idle control system would be affected by a bad OVP relay - but I'd bet that it would.

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Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.

I would check the security of the connector on top of the EDS controller module first.

Then the OVP relay.

Then the EDS module itself.

The OVP powers the EDS module, which controls the idle adjust solenoid on the IP and creates the tach signal from the ring gear sensor.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Good point on the connector jay-bob. And thanks for verifying my hypothesis on the EDS going through the OVP.

Chris - hopefully you won't find the wiring insulation decaying...

__________________
Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.

Then check wiring harness and ring sensor and wiring.

You could also read the codes after checking what Already been suggested.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

The wiring harness is definitely suspect as it is original. I'll check the OVP relay and then the EDS module.

Rgds,
Chris W.

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Objects in closer are mirror than they appear.

there is no EDS connector to speak on on the E300D, the wiring to the ELR cube is power, ground, tach, ELR control, coolant sensor.

The ELR unit is powered from the overvoltage relay output, if you see your tacho quitting and the rpm going to a shaky 500 rpm then your ELR cube is switching off (bad OVP)

At this same moment your A/C compressor will also quit working.

__________________
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)

Yep, that was it! My compressor clutch was intermittent too. Zulfigar nailed it! (And thanks for helping me find that MIA neuron...)

__________________
Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.

I ordered a new OVP relay today. I will report back after it's installed.

Rgds,
Chris

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Objects in closer are mirror than they appear.

Seems like we have some closure. My new OVP relay arrived yesterday and I put it in in the driveway after driving home from work, while braving the 17F air. Thank goodness I could warm my hands on the warm/hot engine. The new relay had 2 fuses and additional male pins, so I was concerned that it was not the right one, but after looking at the wiring diagram on the old and the new, and also seeing that the second circuit extra pins were plugging into dead female connection locations on the relay plug, I could see that having an extra circuit didn't matter. (for the record the fuse was good on the old OVP relay)

Anyway, to cut to the chase, the idle was steady today, and the tach kept working, so alle is gut, as they say. My next challenge is that the squirter nozzle heaters don't seem to be working.... gotta check those this weekend when I can get in the garage.

Rgds,
Chris W.
'95 E300D, 434K

__________________
Objects in closer are mirror than they appear.

My windshield washer nozzle heaters don't work either, the wires are cut. Don't need them where I live but would have been nice a couple weeks ago in Utah.

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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

I guess it was too easy. Symptoms are back again. Idle slows, tach quits. It happens irregularly. Ugh. I'd much rather deal with mechanical items than electrical.

Rgds,
Chris W.
'95 E300D, 434K

__________________
Objects in closer are mirror than they appear.

Check the electrical connections at the IP. If the idle-adjust thingy has a loose/dirty connection, you'll get the symptoms you are seeing.

__________________

"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

Possible resolution. A couple weeks ago my alternator gave up. I believe it may have been failing over the weeks before it finally quit, causing the drop out of the tach and lowered idle. Since I put in the new alternator, knock wood, things have been behaving well.

Rgds,
Chris W.
'95 E300D, 437K

__________________
Objects in closer are mirror than they appear.

the OVP does "switch off" with low voltage. Its also a good idea to check the socket female pins for dirt etc or broken/chafed wiring - quite common in that battery area in W124.

__________________
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)