A/C compressor not coming on. Easy way to fix control unit?

[ScottJackson]

So I went through the whole process in the FSM. It all checked good to the end. It says I need to replace the control unit (aka, Auto Amplifier). Everything else about it works well. The display and all buttons work. The self-test showed nothing wrong. All it doesn't do is send signal from the ECCS (I like to call it the ECU) to the A/C relay. The relay is good, fuse is good, and pressure sensor is good. Is there something small inside the Auto Amplifier which commonly goes out that I can replace with a bit of disassembly and soldering? If not, I guess I'll rig up a toggle switch for when I need the A/C to come on, but I'd rather not. I had my friend hook his gauges up to it and it's showing 95-100psi in the system (of course without the compressor going) so that should be good.

[Q45tech]

Why one buys spares [a junk Q or 2], or just used modules for every electonics module.

Usually not a HVAC module....clutch, relay, low pressure switch.

Without spare modules how will you keep the Q on the road in 2017?

[ScottJackson]

The clutch and A/C works when I jump terminals 3 and 5 at the relay with a wire. I've replaced the relay with the cruise control relay and no change. The low pressure switch is closed (I believe that's as it should be). About all I haven't done is had the refrigerant evac'd and weighed to make sure there's plenty in there. Is 95-100psi (on both high and low sides) at 70* air temp about right when the system is at rest (no compressor clutch going)? Shouldn't it at least try to kick on? When I jump the wires to make the compressor come on, it works well. As it's running the high pressure line gets hot and the cold line gets cold. I also can't see anything in the sight glass (no bubbles, no foam).

I took the case apart for the auto amplifier and saw nothing out of the ordinary. Of my two other Q's, neither has working a/c so I don't know that the amps in those are good.

[qsiguy]

Pressure readings with the compressor off aren't really useful. What are the pressure readings when you manually get the compressor running? There are two pressure switches one for high and one for low.

[Q451990]

Looks like the next logical step is to swap out the controller with one from your other cars unless I'm missing something. The typical failure mode is a bad display, but everything else functions - so even if your other displays don't work - they may still control the compressor correctly. Mine failed differently in that the display would intermittanly die and sometimes it would also not turn on the compressor.

Heath

[ScottJackson]

WELLLL... I was reading back through my FSM after logically thinking through how the A/C works and found that I had skipped the test for the light green with white stripe wire that is supposed to be energized by the main computer. My ECU (originally from a '91 Q) wasn't giving juice to pin 58 which it is supposed to do to tell the relay to turn on for the A/C compressor. Swapped the ECU and it works great. I don't know how I missed that test yesterday when going through all the tests. I took the covers off the ECU that wasn't working right (works correctly in every other way) and I didn't see any burn marks or anything afoul. Hmmm.

[Q451990]

That is very unusual. Usually, absent of water damage or damage from a wreck, the ECU is the last thing to suspect of not working properly. Glad you found it!

Heath