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Old 05-20-2019, 11:47 AM
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Join Date: Jan 2018
Posts: 32
OVP relay, don't leave home without - a spare

New to me '91 300SL. I was under the impression that a faiing OVP relay would lead to no or poor starting and running, but I have not heard of one failing while driving. Last week, my car started quickly and was running well - initially. I noticed some hesitation after a mile or two. Stopped to drop some stuff off, left it idling for a few minutes. It barely ran when I got back in.

Acted like it was out of gas (but gauge said 1/4 tank) so I limped into a gas station barely as it shut down. Put a couple of gallons in - motor would spin but no start and some random backfire.

About the only thing I could do beside get a tow truck was to uber to my house and pull a known good relay out of my W124. Ubered back, put it in, car fired right up and has been running even better since.

I now consider an OVP a mandatory spare when driving a Benz of that era.
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Old 05-20-2019, 12:27 PM
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Join Date: Sep 2013
Location: Alhambra California
Posts: 2,077
Thanks for the great advice.
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Old 05-20-2019, 07:19 PM
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Join Date: Jan 2018
Posts: 32
PS, I should add that this is the 3rd M104 engined car that has had issues with the OVP that I have been associated with. Pretty good record - 3 for 3.
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Old 05-20-2019, 11:29 PM
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Join Date: Sep 2013
Location: Alhambra California
Posts: 2,077
It sounds like the OVP did the job it was designed for - protect electrical components from voltage spikes.
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Old 05-21-2019, 02:30 AM
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Rogue T Tolerant
Join Date: Jul 2009
Location: North America
Posts: 1,582
Those little relays are given a hurculane amount of work protecting the ECU and later the abs and other electrical goodies.

My 380sl had a bad relay which resulted in the system defaulting to mechanical only fuel injection which is nothing more than boring old higher fuel economy.

They should always be replaced as a matter of preventative maintenance.
Originally Posted by Yak View Post
Going back to the original post: "Can you get the vac to blow instead?" No. Vacuums are low pressure so they by nature "suck" and nature abhors them.
1984 380SL
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