Originally Posted by Mxfrank

OVP=OverVoltage Protection. It's a surge protector. Behind the battery there is a plastic cover... the OVP is behind that, along with the EDS electronics. I don't remember how the cover comes off on a 124, on a 201 you have to pull up the weatherstripping.

The relay is a complicated thing, but what it does is shunt high voltage surges to ground. It has a fuse on top, but even if the fuse is good, it doesn't mean the OVP is good. The working component is an avalanche diode, I think the breakdown voltage is around 20V. The diode can be totally compromised without burning the fuse or relay. Because of this, there's really no effective test. So it's not a bad idea to replace it every once in a while, and certainly when you have symptoms.

The reason this function is needed is that when a coil is electrically powered, it retains a great deal of energy. When power to the coil is cut, that stored energy is dissipated in the form of an inductive surge. These surges can reach several hundred volts, depending on the size of the coil. If you're wondering where you have coils in your car, they are all over the place: every electric motor (windows, seats, wipers, washers, sunroof, starter), every relay, every solenoid, even the alternator itself have powered coils. Every one of these devices can produce a destructive power surge under the right circumstances.

In the days of very simple cars, a subsecond voltage spike could safely be snubbed by just the battery. But modern (even 1987) electronics is very sensitive. The OVP only protects a few key circuits, specifically the EDS which controls idle and feeds a speed signal to the Klima and the tach. It also protects the ABS system. So if your symptoms include tachometer, idle and ABS weirdness, then the place to start is with the OVP.

Originally Posted by iamtheosprey

UPDATE:

I replaced the OVP relay with no change.

I replaced the thermostat today with a Wahler brand.

The one I removed was a Vernet and it had a noticeably weaker spring on it.

It also looked like the spring had collapsed a bit so one coil was actually inside the largest coil.

However, in a test drive on a 104F day the %&$*#(@) still overheats.

It does fine when the a/c isn't on.

The minute I turn on the a/c it starts heating up.

Electric fan is working.

I haven't replaced the water pump or radiator but everything else has been replaced or found to be working properly.

Originally Posted by iamtheosprey

UPDATE:

I replaced the OVP relay with no change.

I replaced the thermostat today with a Wahler brand.

The one I removed was a Vernet and it had a noticeably weaker spring on it.

It also looked like the spring had collapsed a bit so one coil was actually inside the largest coil.

However, in a test drive on a 104F day the %&$*#(@) still overheats.

It does fine when the a/c isn't on.

The minute I turn on the a/c it starts heating up.

Electric fan is working.

I haven't replaced the water pump or radiator but everything else has been replaced or found to be working properly.

Originally Posted by vstech

ok. STILL hitting 120?
the car should NOT crest above the 100 line under normal circumstances.

pull the radiator, and verify the condenser is clean CLEAN, and free of debris.

TOTALLY and COMPLETELY clean the radiator fins and passages with aluminum cleaner. a light duty pressure wash from the engine side is ok, DO NOT USE A HIGH PRESSURE WASHER!!!

next is a complete block drain, and flush... you HAVE to pull the block drain.

if your fan is on, the clutch is new, and the blade is the correct blade for the clutch, the radiator and the condenser is clean, and the block is flushed and it still overheats with the a/c on... you have a cracked head gasket.

you MIGHT have too much 134 in the system, and now that I think about it, 134 in 104F areas would be MURDER on the cooling system... (134 at 350psi is 172°F without the heat of compression! at 300 with the heat of compression, the condenser temps can crest 180F...) something you can try is getting more air gap between the condenser and the radiator. additionally, adding a parallel flow condenser to the output of the stock condenser should increase the volume and aid cooling of the refrigerant, lowering the temps of the air entering the radiator...