The overvoltage protection relay is sort of like a surge protector for your PC. It provides protection to a number of different control units on different cars.

On yours I think it only powers the KE-Jet controller (and thus idle control) and the ABS controller.

The system was designed to fail in a way that still works well. The two controls that are activated by the KE-Jet controller are the EHA and the Idle valve. The EHA does all forms of fuel correction but it is adjusted properly to zero (median able correction) with all enrichment off. When the ECU is dead it also is at zero so everything works quite well. Since there can be no forms of enrichment like this, the 50-70ma of correction used to cold start and warm-up is affected severely at zero.

The idle valve also has a rest state that allows a higher than warm idle air amount. You could get the same effect by unplugging the idle valve without damaging the cat by also disconnecting fuel control.

The reduction of voltage at the controller drops the control current (should be about 600ma at idle with A/C off in neutral) to around 500ma which drives the valve closed. We have actually watched this. It seems hard to believe that zero current is the rest state of 8-900 rpm but the reduced current of 500ma is a more closed position.

Anyway, if your problem is because of the OVP relay this is what you will see: a reduction of current and a reduced OVP output voltage. It will not be a total failure as that would leave the idle valve in the rest state that you have seen to run the car at a greater speed.

There are many other considerations for the intermittant stalling condition but the above is what we have found with OVP relays when they are the cause.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician