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Old 05-05-2001, 12:22 PM
Registered User
Join Date: May 2001
Posts: 356

How can I tell whether the OVP is in good working condition or should be replaced ?
The fuse (on Top) is intact. this is on my 1987 300 SE.

I Have been enjoying reading this informative forum for a few weeks now and have naturally joined the party...
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Old 05-05-2001, 12:58 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Since the OVP is a surge protecting power providing relay for a number of systems its overall operation can be verified by the fact that any of the systems are working.

A number of simple observations can tell a lot. Had a 500SEL on Tuesday that had the ABS light on and no power and was idling at 1500. My service writer handed me an OVP and the keys. Took all the fun out of figuring it out. In his case the car also had poor power because of a system yours doesn't have but the ABS light will be on (if the bulb isn't burned - view in the bulb check key on engine off). Your cars idle valve and KE controller are also powered by the OVP. The KE controller is not noticed much when warm. It is necessary for electronic fuel correction when cold. To verify operation of either the idle or the fuel control (EHA electro hydraulic actuator) one should use an ammeter reading ma (milliamps). The idle will range about 600-900ma. The EHA will have readings from -60 to +60ma or more. (See the DIY article on evaluating engine controls). A reading of zero should be followed by a new OVP (overvoltage protection relay).

The biggest problem with OVPs is the intermittant problems. I always replace an OVP with original date code if I am chasing an intermittant in any of those systems. Every original one, through three different versions in many models, has an updated version. Most have gone to two fuses and thus separate control for separate systems.

[Edited by stevebfl on 05-05-2001 at 01:01 PM]
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 05-05-2001, 03:18 PM
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Join Date: May 2001
Posts: 356
thanks, as first measure I will check the orginal date code on it and continue from there.
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Old 05-18-2001, 06:27 AM
Registered User
Join Date: Dec 2000
Location: Montgomery, Alabama USA
Posts: 61
Overvoltage Relay

I put a new overvoltage relay in my 1990 300E (140K)about 5K miles ago. Fuse looked fine. Symptoms were intermintant rough running engine (and not just at idle guys), and ABS light on. I looked at alot of threads here, made voltage measurements (ok), etc., but just was not sure that my problem was the relay. I was really thinking a vacuum problem related to the ABS system ($$$$$$). Finally, I found a thread where one of the real MB mechanics mentioned "thumping" the relay as a check. As soon as the ABS light came on again (with the corresponding rough running engine), I pulled off the road, popped the hood, and tapped the relay with the handle of a screwdriver. The problem cleared right up! Evidently the 10 year old relay had corroded contacts. Since replacement, the ABS light hasn't come on once.

Ken Fuller
1990 300E
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Old 05-18-2001, 08:44 AM
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Join Date: May 2001
Posts: 356
thanks for the info.
I had posted this subject originally because I have an intermittant rough idle ( when the engine is warm , slow speed , or at a stop light..) - The threads I read, give an indication that OVP is a little wonder drug that does it all.... - so I was looking for a quick fix ....I haven't gotten a new one yet- so I'll try the screwdriver on the head method.
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Old 05-18-2001, 09:53 AM
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Join Date: Jun 1999
Posts: 74

I have a 89 300SEL and I am also plagued by the rough idle.
Please let me know if you find a fix.
89 300SEL
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Old 05-26-2001, 09:12 AM
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Join Date: May 2001
Posts: 356
I was away for a few days - so only last night did I get to "let's whack the OVP on the side of the head with an open ended wrench method...)
I took the car out for a spin and no rough Idle..So now I'm getting serious about cleaning the contacts to see if this is for real.
Does the OVP just pull out of it's socket(which is sort of flimsy) like other relays in the car ?

1987 300SE euro
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Old 05-26-2001, 11:56 AM
Big Bowser
Posts: n/a
Yep, It does just "pull" straight up, just get a good grip, pull and wiggle, they sometimes take a real gorilla grip to get them out but they will come if you are persistant!
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