The OVP Relay and you!

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 W124s 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.

I can't remember who wrote this information up, if anybody knows the individual or can find the original write up, let me know so I can credit the author.