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Old 02-26-2005, 05:21 PM
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OVP relay whys and wherefores

The overvoltage protection (OVP) relay provides system power to a buss that supplies the CIS-E electronic control unit, the ABS electronic control unit, and also the idle speed control (ISC) valve, throttle valve switch (TVS, located on the throttle valve shaft end) and microswitch (on the throttle linkage).

The OVP relay contains a zener diode that will trip the relay off if the zener breakdown voltage is exceeded. I haven't found a value, but would guess that it is in the range of 16-18 volts, which could be produced by certain voltage regulator failure modes. This will protect the microelectronics in the ABS and CIS-E control units from damage due to an overvoltage condition. There is also a 10 amp fuse in series with the relay contacts (mounted under a clear plastic over on top of the relay) that will open if buss current exceeds ten amps, which could be caused by a short to ground anywhere on the buss.

Prior discussions indicate that the relay itself may have a reliability problem due to cracks that can develop in solder joints that will prevent it from delivering power to the buss. These can be repaired by removing the relay cover and reflowing the solder, however, one should also be aware of other electrical problems that can cause the relay to trip and not just blame a faulty relay - either an actual overvoltage condition due to a charging system fault, or a buss short to ground.

Since all the cold start functions are provided by the "E" portion of the CIS-E system, a car with a faulty OVP or a problem that causes the OVP to trip off will be very difficult to start cold. If you do manage to get the engine started, the ABS warning light will be illuminated due to no power, and there will be no idle speed control either cold or hot.

Probably the easiest way to check if the OVP relay is providing power is to remove the connector from the microswitch and check it for voltage with ignition on. If none is present, the OVP could be tripped due to a system fault or not functioning due to an internal fault.

On W201s the OVP relay is mounted to a bracket on the outboard end of the CIS-E control unit and is easily recognizable due to the plastic cover over the fuse at the top of the relay. The Tempmatic climate control (KLIMA) relay is immediately outboard or the OVP relay, and the fuel pump relay is immediately ahead of the KLIMA relay.

This area is accessed by removing the soft plastic cover behind the battery (no fasteners - just tabs). Depending upon battery size, it helps to remove the battery hold down and move the battery forward for better access or you can just remove the battery.

The above explanation of what the OVP does is from my review of the shop manuals (including the electrical troubleshooting manual, which has all the system schematics) I downloaded from a recently posted link. If you own a W201 you would be very wise to download all this information. It's about 225MB of data, so it took me the better part of three days to download all the applicable manuals for my car and engine on my dial up line, but there were no glitches, and I just ran it in background while I was doing other work. By going to the home page, you will find shop manuals for other models.


Last edited by Duke2.6; 02-26-2005 at 05:33 PM.
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Old 02-27-2005, 01:44 PM
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Gd info on ovp and great link , thanks... d/l'ing as we speak
1995 E320 smoke silver / parchment
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Old 07-03-2011, 08:33 PM
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The link does not work. But great info on the OVP relay. My car has these exact same symptoms It stopped charging awhile back and shortly after that the hard start stuff started. I am not sure I understand how to test it, do I remove it to test?
1987 300 TDT
1990 300 SEL
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Old 07-03-2011, 08:37 PM
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Location: Metro Detroit, Michigan
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Old 07-03-2011, 09:24 PM
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Old post, but very good explanation to what the OVP ACTUALLY does.
1991 300E - 212K and rising fast...
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Old 07-04-2011, 09:31 AM
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Are OVP relays all the same or are they vehicle specific?
1987 300 TDT
1990 300 SEL
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Old 07-04-2011, 10:00 AM
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Generally not vehicle specific, but there have been several versions of OVPs. MBUSA recommends that early versions with a single fuse and 5- or 7-pin connectors be replaced with the current dual fuse, 9-pin model.

Thanks Duke for the detailed information for those of us who own OVP-controlled motors.
1988 California version 260E (W124)
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Old 07-04-2011, 02:08 PM
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So the 9 pin will fit the seven pin?
1987 300 TDT
1990 300 SEL
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Old 01-27-2013, 12:20 PM
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Location: Greater London, UK
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Smile A new OVP relay: car runs perfectly

I am very grateful to Duke for his accurate advice on the OVP relay. I own a 1992 mercedes 190E (W201) whose ABS light came on during the cold weather just after Christmas. The next time I tried to run the car I had terrible difficulty getting it started and nearly flattened the battery in doing so. When I did manage to start the engine, the ABS light remained on and there was no idle control - and then I knew the car wasn't safe to drive until the fault was fixed.

Following Duke's instructions, I removed the battery and found the OVP relay behind the soft removable pannel. It wasn't easy to spot as it was in a dark area behind the cradle that holds two 'ECU' systems, against the bodywork and towards the engine side of the cradle. I had to carefully cut some of the front black plastic cradle with a hacksaw in-situ in order to lift the two 'ECU' units out, whilst leaving them still connected. The old OVP had a black top with a single fuse; another reason why it was difficult to spot. The part number of the old OVP was 201 540 32 45 and had seven pins. The new OVP has 9 pins and a red top containing two fuses (I presume the red is to make it easily, visible). There were two spare pin holes in the socket which weren't used by the old OVP relay - the pins on the new OVP relay simply used up the spare two holes in the socket. The part number of the new OVP is 201 540 37 45. I bought this for 58 (including VAT) at Mercedes Parts at Brentford.

When I test ran the car, it fired up straight away without any problem at all. The ABS light has remained off all the time and the car has been running perfectly ever since.

I am really pleased to have the car working again. Without this forum I would have needed a specialist, as I have little understanding of the electronics on modern cars. There is no doubt that the accurate diagnosis of the OVP fault has saved me a great deal of trouble and money. Thank you.
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