Climate Control

Freeze Switch Replacement

Karl Chapman wrote to the list:

>> Hello all.
>> I recently acquired my 87 S4 and with the excellent help
>> of Dave Roberts, changed out my T belt, water pump and
>> "stuff." Now my a/c compressor will not come on. I read
>> on Greg Nichols page the various remedies, and I am now
>> undertaking them. The dilemma is this: As I was checking
>> the inadequate relay below the windshield, I detached what
>> appears to be a temp sensor tube (metal) which goes through
>> the firewall. I think a PO may have broken it as it seemed
>> to have been held together by heat-shrink tubing. Is this indeed
>> what it is? If so, what are the recommendations for this
>> situation? The tube is broken off right at the entry point.
>> Will this have had adverse effects on the operation of the
>> a/c? Does this line need to hold a vacuum?
>> Any help would be appreciated.
>>
>> Karl Chapman
>> 87 S4 A/T Indischrot


Hi Karl, and welcome to the flock!

The part you refer to is actually called the "freeze switch". Its purpose is to detect when the evaporator temp drops below about 35f, and interrupt the current to the compressor clutch until the temp comes back up a little. If the compressor continues to cool the evaporator below freezing, moisture in the air will gather and freeze on the coils, preventing any cool air flow into the car.

That tube coming out is a capilary tube, a hollow tube filled with some silicone fluid that changes size with temperature. Since a broken tube releases all the fluid, the failure mode for the switch is circuit open, a condition that would keep the compressor from operating. You can prove this by just connecting the two wires together, bypassing the switch. If everything else is OK, the compressor should now function correctly. Driving with the switch bypassed is not a good long-term solution, since you'll have erratic freezing in there and poor overall performance. Plan on getting a new switch, and install it before it gets hot and humid. 928 International lists this switch on their website catalog, and the other vendors and your dealer will also have it available. The part number suggests that it is not 928-specific, so the price isn't too outrageous.

Installation has the end of the tube stuck through a tiny hole in the firewall, into the evaporator compartment. There's a bunch of extra tubing that needs to be carefully routed and stored under the switch, with no sharp bendss or kinks in the tubing. That yellow (on mine...) sheath over the tube is insulation, and covers from that little screw tab back to the switch itself.

Last but not least, the lead from the compressor clutch routes up through the top of the compressor bracket, then forward to a connector that's tied to the wire harness just above and in front of the air pump belt. This wire gets disconnected when you change the timing belt, and may not have been reconnected when the parts were reassembled. On mine, the two wires and the connector itself are tie-wrapped to the big wire bundle so there's no strain on the connections at all.

Hope this helps!

dr bob