Help with '86 HVAC Control Unit

[Willybill32]

The Forum users have been helpful with other questions on my '86 Mustang, so here's another request for help.

My '86 GT is up and running, and now we're working on the A/C system. The evaporator coil, accumulator and all lines have been replaced, leaving only the compressor and condenser coil from the original car. The system holds pressure, but we can't get the compressor to cycle automatically. Tracing thru all of the components, the dash-mounted control for the blower motor, temperature and function (heat, a/c, defrost, etc.) does not consistently make contact when in the A/C mode. Does anyone know if I have to replace the whole control unit, or is there a switch component that can be replaced?

 

[jrichker]

What sort of pressure do you see on the charging manifold when the engine is off and the power is off?

If the system is low on refrigerant, the clutch will not engage (safety feature, keeps you from burning up the compressor due to no oil). There is an aluminum canister on the firewall with a large hose leading to the A/C compressor. There is a pressure switch on it with a 2 wire connector: pull the connector off and jumper the wires. The compressor circuit will power up, and the test light will light if the electrical circuit is good. This test is also a good way to check the compressor & clutch if the system is low on refrigerant. You can jumper the wires and the compressor will turn if the clutch is good.

 

[Willybill32]

Thanks, jrichker! Right now, there is no charge in the system. We’ve got everything assembled and charged with nitrogen at 175 psi to test the system for leaks. It’s held constant pressure now for almost a week. We can jumper to get the compressor to run, but the power from the function switch (A/C, Max A/C, heat, vent, etc.) does not consistently operate when in either A/C mode, hence the question about the switch itself. We’ve been spraying contact cleaner into the switch mechanism and working the selection lever, and it’s better, but not 100% yet. Is the switch replaceable, or would I have to buy the whole control unit?

 

[General karthief]

Have you looked at rock auto or a parts store?

 

[slow84lx]

This is not an uncommon problem on the earlier fox dash controls. I've changed the whole control unit a couple of times but never tried to fix just the switch. Repairing the switch might be worth looking into.

If you change the control unit you'll need a unit from a 84-86 car and there are units with rear defrost switch built in or not as you require. These still pop up occasionally in the classifieds on the early fox forums.

A control unit from a 79-83 car will work but the backlighting is the greenish color, not red.

 

[Willybill32]

Have you looked at rock auto or a parts store?

Yeah, I've checked Rock Auto, eBay, LMR and did a general Google search, with no success. I've found the whole control unit off an earlier model that would work, but at $150-$180 and no guarantee they're any better than mine, and with green lighting to boot. That was behind my post on this site, as I figured I wasn't the only one with this problem...

 

[Willybill32]

This is not an uncommon problem on the earlier fox dash controls. I've changed the whole control unit a couple of times but never tried to fix just the switch. Repairing the switch might be worth looking into.

If you change the control unit you'll need a unit from a 84-86 car and there are units with rear defrost switch built in or not as you require. These still pop up occasionally in the classifieds on the early fox forums.

A control unit from a 79-83 car will work but the backlighting is the greenish color, not red.

Thanks, slow84lx! I've found a couple on eBay, but earlier models and no guarantee they're better than what I've got. I'll keep looking. In the meantime, a combination of contact cleaner and lots of operations is improving mine.