Rear A/C Diagnosis
>> I have an 86 928s. The front AC was working fine, but the
>> rear AC was not working, the fan worked, but no cold air.
>> Are there some things I can check. I have heard something
>> about the rear thermal expansion valve, could this be it and
>> what could the cost be.
>> Which AC hoses under the hood are the most likely to be leaking
>> and where can I get hoses rebuilt?
I just walked the AC repair walk, so I'll step up to this. The rear air relies on a solenoid valve that lives under the passenger's seat. That solenoid starts the flow of freon to the rear evaporator. Solenoid --should-- open when the fan is started. With a meter or test light, you can verify that the valve is actually getting voltage. The only way to verify flow is to remove the valve, a process that requires a system evacuation and recharge. If the valve is partially plugged or only opening part way, the valve and some of the downstream tubing will be cold.
Once the liquid has made it through the solenoid, it travels to an expansion valve at the rear evaporator. The expansion valve is a fancy metering device that watches the differential pressure across the evaporator, taking into accont the ambient temp, and decides how far to open and allow liquid to flow. The metering orifice is small and may be partially blocked with stray crud. You can check for even the tiniest flow back there by pulling the cover off the rear unit, and feeling the expansion valve for cold. The cover is held on by a few screws that are accessible after the little vent grill assembly is removed. You shold also plan on removing the rear seat backs, according to the manual; I didn't.
Oh-- almost forgot to answer your other questions--
The thermal expansion valve for the rear is under the rear evaporator cover, and sits in line with the supply and return lines from the front system. It's about 1.25" square cross section and maybe three inches long, with a little disk on the end with a very short capillary tube curled up there. Four tube connections allow it to sit in line there right at the evaporator. From memory, the part cost was in the less-than-$30 range from 928 Specialists' David Roberts. It included the O-rings in the kit, by the way.
Changing the rear valve is a bit of a chore, with a little pulling and prying necessary to get all the tubes and the valve to come apart and out. If you get this far and have the system open, do yourself a big favor and replace the drier in front at the same time you have everything out and open in the rear. The drier is unbelievably inexpensive and is very cheap project insurance.
I had hoses rebuilt at my local AC specialist, where they installed barrier hose necessary for my R-134a conversion last week. Do a little telephone research and I'm sure you'll be able to find somebody. Be aware thet there are no hoses in the rear system, only at the front for the connections to the compressor. No need to replace or rebuild those hoses if you are going to stick with R-12; just new o-rings at the ends.
DR also put together the kit of o-rings necessary for my conversion, all the later better material for the new refrigerant and oil. Cost of the whole package of genuine parts was quite reasonable.
Well I got my rear AC working. I think the conector at the solenoid under the RR seat was the problem, Lots of crap crud and corruption built up under there. While wrestling with the plug both of the wires broke off, the black and red wires coming fron the solenoid. I checked the wiring diagram in the manual and it does show the switch and the connector, but it is very unclear. It seems to work being connected both ways Solenoid black to connector brown and solenoid red to connector blue and opposite. it seems that one of the wires cross over to the other side.
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