OVP Relay: what does it do, exactly? - Mercedes-Benz Forum
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#1 (permalink) Old 12-30-2010, 10:51 PM
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OVP Relay: what does it do, exactly?

Sorry for visiting a topic that seems to have already gotten a lot of airtime, but even after digging through many forum threads, I cannot find a definitive description on what the OVP relay does and why Benz owners are always suspecting them as THE culprit for many engine woes, and subsequently replacing them only to find that the problem still exists??? So, two questions:
1) How can you tell if the OVP is "bad?"
2) Does it really cause different types of engine sysmptoms, or are they very well defined, i.e. can you tell when you really need to replace the bugger?

Why I'm asking: my '89 300SE starts up a bit roughly after sitting for awhile, kinda stumbles and coughs and then begins to purr. If it only sits for say 1/2 hour, it seems to start more smoothly. An earlier thread of mine resulted in someone suggesting that I replace the OVP relay. Others have done it and smoothed out the starting problem. I don't get it?

Also, is it recommended to buy a new one rather than a used $20 one on eBay that's guaranteed to be good?

Happy New Year and thanks for all of the great work you guys and gals do on this forum!
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#2 (permalink) Old 12-30-2010, 10:56 PM
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OVP Over Voltage Protection.

ERIC.

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#3 (permalink) Old 12-31-2010, 11:24 AM
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OVP: Over Voltage Protection

Ah, the short answer. Yes, of course that's what it does; that's it's name. It's a good start, but there were a few more parts to the question. Maybe someone with more familiarity with the forum than I could link me to the long answer. The manual doesn't say anything much...
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#4 (permalink) Old 12-31-2010, 12:27 PM
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Borrowed text from the Benzbin vendor info pages:


------------------------------------------------------------------------

"All of the sudden my Mercedes 300E is very hard to start, and runs poorly. This seemed to happen right after I jump-started my battery. What's wrong?

The primary culprit is usually the over voltage protection relay. The relay might have blown due to excessive voltage generated in your vehicles electrical system when you hooked up the jumper cables. However, the relays do go bad all by themselves.
On W124 and W201 chassis, the relay is located in back of the plastic cover behind the battery. On W126 chassis it is located between the 2 firewalls underneath the hood. On W107 chassis it's located behind the right kick panel.

Check the 10 amp fuse on top of it first. If it is blown, replace it. If it is good, check the socket that that the fuse sits in for corrosion. Clean and/or replace as necessary.


The relays were also known to have weak solder joints from factory and go prematurely bad, but chances are the relay was doing its job protecting your car's control units, and give thanks that it did. The components it was protecting are expensive.

What does the relay have to do with the way my car runs?
The Mercedes over voltage protection relay provides voltage to the ABS and Mercedes engine control units. In the case of hard start, when you cut off the voltage supply to the engine control unit, you loose all of your cold idle circuits. This includes the idle and cold fuel enrichment circuits. The fuel enrichment circuit is used to start you engine when it is cold. The idle circuit is used for increased rpm when the engine is cold, and curb idle when the engine is warmed up. When these circuits do not operate the engine can still run, but very poorly. Plain and simple - the control units can not operate the systems of your car without voltage. No voltage equals all kinds of drivability problems.

Tech Tip: When checking voltage at the pins of the overload protection relay pin 30 is hot with the key on and pins 87 are the output voltage to the control units. If the relay is blown there will not be voltage present at pins 87. "
-------------------------------------------------------------------------

I believe what Eric was doing was giving you the proper name for the part so you could do a search with much better results. Just a guess.

Welcome to the forum and good luck with finding the culprit for your starting issues. Good bunch of guys here with huge amounts of experience chasing down problems on their own w126's and most are willing to share that experience.
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#5 (permalink) Old 12-31-2010, 12:47 PM
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Very prof answer kodiakj2!
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#6 (permalink) Old 12-31-2010, 04:35 PM
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Quote:
Originally Posted by the_bends View Post
Ah, the short answer. Yes, of course that's what it does; that's it's name. It's a good start, but there were a few more parts to the question. Maybe someone with more familiarity with the forum than I could link me to the long answer. The manual doesn't say anything much...

ERIC.

___________________________________________
"I am enclosing two tickets to the first night of my new play; bring a friend, if you have one."
- George Bernard Shaw to Winston Churchill.

"Cannot possibly attend first night, will attend second ... if there is one."
- Winston Churchill, in response.
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#7 (permalink) Old 12-31-2010, 07:02 PM
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Great stuff, this Forum!

Thanks Kodiak. That helps a lot. Just want to add that Eric's post made me chuckle a bit. Me imagining him imagining me with the light bulb on over my head exclaiming "ah-ha!" But it wasn't so easy. Kudos, Eric! Many thanks to Eric as his job is a lot of work with not a lot of glory (or maybe I'm wrong). This forum has saved me untold hours of diagnostic torture, and hopefully I will be able to repay the "debt" with some acute mechanical insight or MacGyver fix that will save others some trouble. I was a Volvo guy and home mechanic for almost 40 years, but have changed brands recently. (Actually still have an '87 240 Wagon). Next car is an SL, and plan on keeping the 300SE for many years. HNY!
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#8 (permalink) Old 12-31-2010, 07:16 PM
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Seriously, the search function is your friend and the Google search function is your mother

http://www.benzworld.org/forums/w201...ovp-relay.html
http://www.benzworld.org/forums/w124...uel-relay.html
http://www.benzworld.org/forums/w124...ovp-relay.html
http://www.benzworld.org/forums/w124...ovp-relay.html
http://www.benzworld.org/forums/w202...ovp-relay.html

And this is just the tip of the Iceberg.

ERIC.

___________________________________________
"I am enclosing two tickets to the first night of my new play; bring a friend, if you have one."
- George Bernard Shaw to Winston Churchill.

"Cannot possibly attend first night, will attend second ... if there is one."
- Winston Churchill, in response.
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#9 (permalink) Old 09-04-2012, 03:21 AM
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Two months after purchasing my W126 I deduced (with help from the good people on this forum) that my car indeed needed an OVP relay so I ordered one from the States and it arrived today. To say that the car was transformed in many ways is an understatement. It not only idles better cold and hot,it has more power with less accelerator pedal and the annoying ABS light no longer stays on. All for the princely sum of $30.

My Mercedes is driving me around the benz..
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#10 (permalink) Old 09-05-2012, 04:00 AM
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Quote:
Originally Posted by Seltsamen mann View Post
Two months after purchasing my W126 I deduced (with help from the good people on this forum) that my car indeed needed an OVP relay so I ordered one from the States and it arrived today. To say that the car was transformed in many ways is an understatement. It not only idles better cold and hot,it has more power with less accelerator pedal and the annoying ABS light no longer stays on. All for the princely sum of $30.
Can you give me a link to a website with those prices friend?

I'm in Melbourne and need one for my 300SEL. PM me
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