So I have been searching all night on the forums about the notorious non-opTachometer problems on these vehicles...It seems that the OVP relay is usually at fault when a problem like this occurs (especially when the AC goes out)....

I thought for sure that my problem would be the OVP relay, so I pulled the relay checked the fuse, then applied 12v to pins 15 and 31 and checked for continuity wala it worked and the relay opened and closed.... I put the relay back in and same problem, no tach/no AC....

I have ordered a new OVP Relay just in case, but is there anything else that could be at fault here? I am stumped on how the relay could be bad and cause these problems when it tests ok? Any other ideas on what it could be? Is there anything else that will not function properly if the OVP goes out?

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2005 SL65
1999 E300
1995 E300
1994 SL320
1988 560SL
1987 300TD
1982 300D
1955 300SL

Excellent thread timing. I pick mine up tomorrow and the tach in inop. But the AC compressor does come on........

I am subscribed!

BTW, where is the OVP relay located on this model?

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

My AC Intermittently worked when the TACH worked, but ever since I pulled the OVP it has not worked....

The OVP is located behind the battery (there is a plastic covering which houses the ECU and a bunch of relays. The OVP relay is a metal relay that is attached to the EGR relay.

I have noticed that mine is the "old" style and was manufactured in 1994.... From what I have read these usually last 100-150k miles. I also noticed that my trip odometer has been a bit flaky, I think these are classic symptoms of a bad OVP...First the tachometer is flaky, then the trip odometer goes out, after this the A/C goes out....I should get it in later this week and hopefully it works....

Also about your other question on the FSM for the W124, I have the DVD version but its a pain to have to be near a computer, I prefer paper manuals, so I bought the W124 owners bible. The relay and the "bible" should be coming at the same time...Ill let you know if its of any use.

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2005 SL65
1999 E300
1995 E300
1994 SL320
1988 560SL
1987 300TD
1982 300D
1955 300SL

Could be a bad engine rpm sensor. It attaches to the bell housing below the oil filter housing. Check continuity across the refrigerant low pressure switch behind the left headlight, the one with contacts on the sensor rather than connectors on a pigtail. With the AC switch on, one of the wires to the low pressure switch should supply +12V. If not, the problem is in the cabin.

EDS and ABS won't function if the OVP goes out. Maybe SRS won't function either.

Jimmy, the OVP relay is in a cubby aft of the battery. There's a plastic shield sealing the cubby from the engine bay.

Sixto
87 300D

AKA CPS: provides signal to tach, some emissions components and AC. On gassers also provides ref for spark and fuel injection.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

Thanks I will check this tonight, i hope its something simple! Could the CPS also effect the trip odometer>

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2005 SL65
1999 E300
1995 E300
1994 SL320
1988 560SL
1987 300TD
1982 300D
1955 300SL

Same thing happened on my car. It was the OVP. I tried fiddling with the relay too...

BTW - the relay blew right after I connected a tiny 120v inverter to the cigarette lighter port in order to charge a laptop. I threw it away.

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1977 Mercedes 300D

Any ideas guys?

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CM

The Kilma relay also controls the Tach on the E300D. If no tach and no AC then the chance is very high with a faulty Kilma relay. Expensive item, $250+ and it is a dealer item.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

When reparing the idle speed fault in my E300D I found that the ELR (idle control unit) is the first junction from the rpm sensor, it has a pin called TD - which then goes out to other bits like the klima and the gauge.

The A/C relies on rpm information to reference it to the rpm sensor on the compressor (detecting belt slippage if any).

If the ELR is inoperative (no idle speed control) then the rpm wont work and neither would anything related to rpm sense. The usual culprit is the OVP as it switches on everything, but sometimes its also the fuse no. 6 in the fuse box (old flaky fuses cause such)

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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)

OVP was the problem with my tach. and AC.

I think replacement OVP is different design then original too.

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-- Chris

'95 E300, 216k miles, Silver Surfer
'05 E320 CDI, 138k miles
'07 S550 4matic, 69k miles

Gone but not forgotten:

'76 300D, 350k miles?, SOLD in 1995
'75 240D, 300k miles, SOLD in 1991

The one in my car works good. But every now and then (one out of five or so)I turn the car on it stays off. That is until I turn any electrical component on like the parking lights. The air etc etc. or I close the door, then it works fine.

Idle seems to be dead on.

Any help on this guys would be greatly appreciated.

The OVP was changed recently by the newer style two fuse type.

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CM