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Old 09-15-2008, 11:38 PM
Registered User
Join Date: Jun 2006
Location: Ft Myers, FL
Posts: 26
AC & Tach fixed - 85 300TD

On my 85 300TD Wagon I was having a problem with an intermitten tach and the AC was not very good then the tach stopped completely. And the AC also stopped being cold.

I checked the compressor and it wasn't engaged. I came to the forum & started running the checks it recomened. In the end I jumped across two legs of the Kilma, and the compressor engaged and the air was cold. But the tach didn't work. I checked the OVP relay and the fuse was good. So I changed it with a friends OVP and WOW the tach worked. So I took the jumper off the Kilma and tried the AC & WOW it worked also......

It was a bad OVP relay all the time.
Maybe this short article will help someone else.


Phillip Babcock
Ft Myers Beach, Florida

2005 E-320 CDI Desert Silver
2005 E-320 CDI White
1992 300D 2.5 Turbo 2 tank WVO system - selling
1984 300TD Wagon 2 tank WVO system Sold

Last edited by phillipb; 09-15-2008 at 11:53 PM.
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Old 09-16-2008, 01:57 AM
JimmyL's Avatar
Rogue T Intolerant!!!
Join Date: May 2004
Location: Sunnyvale, Texas (DFW)
Posts: 9,673
Good fix Phil, and nice of you to post.
Took me almost 3 years to fix the tach on my '85. It was quite a bit more involved though.
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

'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
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Old 09-16-2008, 11:26 AM
NY300SD's Avatar
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Join Date: Feb 2007
Location: Westchester, NY
Posts: 133
This was the exact same experience I had with my 1995 E300 Diesel. Put in a new OVP relay and tach and a/c work beautifully.
"Poor Krusty, he's like a black velvet painting come to life." -Lisa Simpson

1995 E300D 216k black/mushroom

1982 300SD (sold)

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Old 09-16-2008, 09:08 PM
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Join Date: Apr 2005
Location: East Toadsuck Vermont
Posts: 61
what year did MB start putting this OVP rel;ay in the vehicles.
and where is it located??
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Old 09-16-2008, 09:34 PM
Jeremy5848's Avatar
Registered Biodiesel User
Join Date: Mar 2006
Location: Sonoma Wine Country
Posts: 8,394
I believe they began (in the E-class and predecessors) with the 1985 300D (W123), in which the OVP relay is behind the right front passenger kick panel, just below the glove box right side (near the right front door upper hinge).

In the 1987 300D (W124) the OVP is with some other electronics, in the compartment behind the battery.

The W210 models (1996 – ?) have a "K40" relay that does the same function, and a bit more. In is in an instrument case under the hood, on the right side, behind the air filter.

The OVP may have originated (first use about 1984-85) with the W201 (190, C-class) cars but that is only a guess on my part. Don't know about OVP use in S-class.


"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
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Old 02-10-2009, 11:10 AM
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Join Date: Nov 2006
Location: Boston
Posts: 2,040
Does anyone know if the OVP from W124's is a direct drop in replacement for my 1985 W123?

I am having the same issue as the OP.

sounds like it may be time for a junkyard run...
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car
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Old 02-11-2009, 03:05 AM
compress ignite's Avatar
Drone aspiring to Serfdom
Join Date: Feb 2004
Location: 32(degrees) North by 81(degrees) West
Posts: 5,554
OVP for '85 300 D



See below:
Attached Thumbnails
AC & Tach fixed - 85 300TD-screenhunter_04-feb.-11-02.02.gif   AC & Tach fixed - 85 300TD-screenhunter_05-feb.-11-02.03.gif  
'84 300SD sold
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Old 02-12-2009, 12:41 PM
dieseldan44's Avatar
Registered User
Join Date: Nov 2006
Location: Boston
Posts: 2,040
OK, so i need the OVP out of an '85.

I don't understand the significance of the picture. I now know what it looks like though so I can go grab one. This is right on the borderline of just getting one from a parts counter.


'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car
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