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Old 10-18-2001, 12:09 PM
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Join Date: Aug 2001
Location: Santa Maria, CA
Posts: 49
Overvoltage Protection Relay Failure Modes

There has been a lot of talk regarding the infamous OVP and how it contributes to or causes any number of failures in our Mercedes cars.

My question to all of you smart Mercedes mechanics (MBDoc, Benzmatic, etc.) is:

What are the failure modes within this relay that cause these problems?

The unit has a zener diode and a relay as its main constituents. Do the contacts burn or get dirty? Does the zener diode short out or open up? Does the relay functionally die? Does the zener threshold voltage change, perhaps as a function of driving conditions??

I pulled mine a few days ago and checked the relay operation, the contact resistance and the diode function (except for the threshold voltage). Everything checked out.

If all of these tests are ok, can I assume that the unit is fully functional, and not a contributor to my warm start problem? Or is there some other failure mode (perhaps intermittant) that is not easily checked?

If we had a set of tests we could apply to verify OVP operation we could all save wasted money troubleshooting by replacement.
The OVP tests in the manual only check for catastrophic faults.

Thanks guys

Tom Elerding

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Old 10-19-2001, 12:17 AM
Senior Member
Join Date: May 2001
Posts: 104
the ones that I have seen fail get solder cracks from heat - cold cycles. Look with magnifying glass and resolder any that are suspect. Maybe if somebody else has seen internal component failure, Iwould be interested in knowing.

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