Taking apart an OVP relay - PeachParts Mercedes-Benz Forum

tinypanzer · #1 04-14-2009, 11:08 PM

Since I replaced mine (probably unnecessarily) I am taking apart my old one to see what makes 'em tick. Pictures coming soon.

It looks like a single contact relay, two diodes, and a resistor. If this is the case, fixing one shouldn't be too hard.

I'll bet that half of the issue is simply burnishing the contacts on the relay.

BTW - the rattling appears to be normal. It's a plastic insulator card that's put in there to keep the circuit board off the aluminum box. The relay itself also rattles a bit, and that is normal for relays.

More soon.... Anyone else who has taken one apart care to share?

babymog · #2 04-14-2009, 11:51 PM

I've got one apart, but didn't bother trying to repair it since I already had a new one.

However, three OVPs on three cars in a year, I should probably start.

It has been my experience that bad relay contacts (and switch contacts) will quickly be bad again, very difficult to clean and lap them properly. If the relay could be replaced, that would be the way to go IMO.

04 Diesel · #3 04-15-2009, 12:11 AM

Yes, pictures please.

Ivanerrol · #4 04-15-2009, 12:59 AM

The Zener diodes go out of spec with age.

They will be just as unreliable if you paly with them.

86560SEL · #5 04-15-2009, 02:10 AM

Tiny... where did you buy a new OVP and how much was it? I dont see them listed here on the site. I hope thats my issue with my car and not that hose you mentioned in the other threads.

Cal Learner · #6 04-15-2009, 08:44 AM

Phil must surely be able to supply you, even if it's not listed on the site.
P/N 201-540-08-45 about $80 (if you're talking about a 1986 560SEL).

Kestas · #7 04-15-2009, 05:58 PM

I've noticed that burned relay (and other) contacts seem to be the common denominator on many problems with Mercedes vehicles. They were the root cause of the following problems I repaired recently:

- heated seat module
- seat belt presenter module
- high beam switch

If you can repair electrical contacts, you stand to save a lot of money.

tinypanzer · #8 04-15-2009, 06:26 PM

Man, I need glasses. I could swear I looked at that photo and saw him cutting the thing open. Sorry, ds190..... My bad.

tinypanzer · #9 04-15-2009, 10:22 PM

Pics as promised.....

The second picture shows the insulator card, which rattles around and creates the impression that something is broken.

The third picture shows the circuit board traces. Any cracked or loose solder joints here should be touched up.....

The fourth picture shows the actual relay.....

The fifth picture shows the zener diode as it lives - pressed into the aluminum heatsink/contact. Good luck replacing that part - if you can even find it.

tinypanzer · #10 04-15-2009, 10:23 PM

The first pic here shows the relay itself, and if you look carefully you should see the contacts. Does anybody know if this relay is normally open or closed when the car is running? I don't see much wear on these contacts at all leading me to speculate that the relay only closes when there is a "fault".

The last pic shows the pins. Silver plated!! That's quite a nice touch. For those not in the know, silver is an excellent conductor and even its oxide is more conductive than copper. Silver plating contacts is a premium feature.

tinypanzer · #11 04-15-2009, 11:23 PM

"Tiny... where did you buy a new OVP and how much was it? I dont see them listed here on the site. I hope thats my issue with my car and not that hose you mentioned in the other threads."

I got mine from Phil here on Fastlane. Search under "Overload Relay"

The hose is a much cheaper part than that relay! It's just much harder to install. Really, it's not so bad - just annoying that you have to pull the fuel distributor.

I hate to say it, but with a 19 year old MB, you either need to get familiar with some basic repairs or find a cheap indie mechanic who you trust. These are great cars, and quite reliable, but fixing the age related issues that the previous owner neglected can be time consuming and costly if you have to farm out the repairs........

Good luck!!

babymog · #12 04-16-2009, 08:14 PM

Might also be to protect against "load dump" when the field collapses in the alternator and other devices after being shut off, can cause a high-voltage reverse-polarity pulse known to us who design automotive electronics as "load dump".