91 Ls400 A/c Too Cold, Freezing The Evaporator

[Motorgnome]

Hi there. My 91 LS400 air cond is too cold. I have had this car for a year & love it, but have had this problem all along. The air conditioning when set on cold, just gets colder & colder (centre vent temps below freezing) & then the air flow stops as the evaporator freezes into a big chunk of ice. I have to manually turn the A/C off & on so this doesn't happen. If I bring the temp up, I get hot air out of the vents, but the evaporator still freezes up, as this is before the heater matrix. Is as if the thermostat is not working, as the compressor never shuts off.

Does anyone here know how to diagnose these systems? I am a mechanic & understand how air conditioning works, but have no idea on the climate control /electronic side of things.

Thanks heaps.. Jason

[Legender]

Not sure if it works the same on the car... but I think it does. This happened at my home last year with the AC system... cause was the unit was low on freon... once freon was added the unit operated fine. You may also be low on freon.

[Motorgnome]

No, the A/C is charged with the correct amount of refrigerant. I understand (correct me if I'm wrong), that the compressor is a "Variable displacement" type, that changes how much refrigerant it pumps around according to the needs. So when it's stinking hot, it pushes large amounts around to be most efficient, but when it has cooled everything off, it only moves a small amount around. The idea behind this is to minimise power loss on the engine & to not have the system tuning off & on with a noticable drag on the engine..sort of like the variable displacement V8 engines that turn off cylinders when not needed to save fuel.

Well,mine seems to be on full tilt all the time & I don't know where to start to diagnose the problem.It must have a dead sensor somewhere telling the air cond computer lies & to operate at maximum efficiency.

Has any one been down this road before??

Cheers! Jason

[Motorgnome]

I have just found how to enter the self diagnosis mode of the A/C computer...I'll check it out & report back

Cheers

[sha4000]

Did you figure it out yet?