A/c quits cooling in humid weather

[cadfan64]

I had the system evacuated and recharged in April (previous owner had converted to R-134a) and it has been cooling well until this hot humid weather started. Now it will cool (35-40F outlet temp set at 65) for about 15-20 minutes, then I notice the air flow getting warmer and reduced air volume. It is still in recirc mode and the blower noise gets much louder as the air flow warms/reduces.
I pulled off the road yesterday when it was acting up, the accumulator was coated with ice, the compresor was not engaged, but there was NO dripping from the evap drain, and I noticed the garage floor has been dry as well.
Obviously the evap is icing up and the clutch switch is shutting off the compressor. Is the plugged drain the culprit, (no water under the carpets, either) or is the core plugged with debris?
Best way to clean that out without pulling the core? Or did the shop overcharge it? (3.25# of R-134a, had 29F outlet temp at 60F ambient)
BTW if you let the car set for an hour, the cools like crazy gradually quits scenario starts all over.

Thanks for any help!

[gadget73]

Sounds like the evaporator is icing up. If the pressure switch is set to an abnormally low cutout pressure (like under 20 psi) or is just dead, that will do it. I had problems with mine that turned out to be nothing more than dirt in the evaporator. I pulled the blower motor out and hosed out the evaporator as best I could and the problem went away. If the drain is clogged, that won't help either. I'd actually consider sticking a rubber tube into the drain and running it down to a point below the firewall. In stock form, the evaporator drips onto the frame and it causes frame rot.

It might be overcharged. I forget the standard R12 charge, but R134 charge is 80-85% of that. Normally with an overcharge though, it won't ice up. It will just run excessively high pressures and blow hoses or compressor seals. I wish I could get air that cold out of mine. I manage mid 40s at best on the Towncar, and a pathetic 48 degrees on the Mark VII.

[cadfan64]

Well, I spent about 3 hours in 90 degree heat and didn't get anywhere. Pulled the blower motor, clean as a whistle in there, but flushed it with a garden hose anyway. Drain worked just fine, mental note to install the drain hose gadget discussed.

When it runs at idle while the compressor is engaged, the evap inlet and outlet tubes are equally cold, and sweat but do not ice up. I slid a thermometer in the oulet seal and the evap housing gets down to about 34-35 degrees so obviously it isn't icing. Everything working along fine, then click, compressor clutch disengages and stays off (it was so hot/humid it hadn't cycled at all). Popped the glovebox, 13 in of vacumn at the servo, cranked to full a/c and recirc door open. That means the sensor and servo are working correctly, doesn't it?

Pulled the connector at the clutch cycling switch, power from the control head, but not thru the switch to the compressor. Mad dash to the Zone, replaced switch, compressor engaged for about 15 minutes, 38 degree outlet air at center vent, interior temp at drivers seat 71 degrees, so that's good. Go for a test drive gloating I had fixed the mother F______, get about 3 miles from home, starts blowing warm air. Pulled in startled old ladies drive, pop hood, no compressor engagement. jamb test light at compressor connector no power. same old shit at switch, power from control head, none thru switch.

Took it back to the shop that recharged it for me in April today, couldn't make the damn thing not work (almost 20 degress cooler out tho). Removed 1/2 pound of R-134a, clutch short cycled, added back 1/4#, all pressures are perfect, cooling like a freezer. Ah ha got ya bitch test drive, 5 miles, warm air!!!!!! Consider depositing car in deep ditch, drove home in vent, pulled in drive, switch to panel, deep freeze time once again. Pulled out factory service manual, traced clutch circuit from fuse, through control panel, to clutch cycling switch, to compressor to ground. then I get out the EVTM and get confused. (BTW, did Ford recognise that they put SEFI in thier cars in 86? every illustration and most of the text refers to CFI cars, and Yes it IS an 86 manual and EVTM!) EVTM shows circuit going to a wide open throttle relay from the cycling switch which I cannot find for the life of me, so I assume the EVTM is wrong again. Go to ALLDATA, no updates for manual.

Gadget or anyone who can help, I'll reward ya with a case of beer of your choice!

[87gtVIC]

I have a 1986 manual as well and it is all CFI stuff too.

[gadget73]

yeah the 86 manuals suck. All mine are CFI BS.

The WOT relay is one of the three over near the washer bottle but be damned if I remember which one it is. There is an ECM relay, a fuel pump relay, and the AC relay. I want to say the AC is the green one but don't quote me on that. Unplug it, if the car dies that wasn't it.

It sounds like your evaporator is icing up to me. What pressure do you get on the low side service port when the system trips out? If its under 20 and staying there, the evaporator froze and it won't kick back on until it thaws. The low pressure switch is supposed to prevent that, but clearly it is not. You might try turning the adjustment screw on the switch in 1/4 turn. That should make the system cycle out sooner which in theory will help with the thing freezing up.

[cadfan64]

pressures: 24 when it kicks out, 48 when it reengages. + or - 2, that's where every reference says it should be. I only have 2 relays on the left fender, and they match the wire colors for the eec and fuel pump relays. David, could you scan the ATC and corresponding eec pages from your 87 manuals? Also the tripminder if you could, Date code under the dash pad was stamped July 10 86. Think I might have an 86/87 bastard car LOL

[Lincolnmania]

we had this problem with yucatechs wagon.........the low pressure switch needed to be adjusted iirc, and it had a grounding issue and the compressor was fast cycling

[cadfan64]

so just turn the screw,,,which direction...1/4 turn? Thanks as always!

[gadget73]

clockwise should increase pressure, ccw drops it. 1/4 turn is about 3 psi.

There ought to be four relays over there total, horn, ecm, eec, and ac. At least my 86 has four over there.

[87gtVIC]

David, could you scan the ATC and corresponding eec pages from your 87 manuals? Also the tripminder if you could, Date code under the dash pad was stamped July 10 86. Think I might have an 86/87 bastard car LOL

Craig, I only have an 86 manual for my 87 car. Sorry.

[cadfan64]

There ought to be four relays over there total, horn, ecm, eec, and ac. At least my 86 has four over there.

Well Gadget was correct, there were four relays on the fender just behind the washer reservoir. The a/c WOT relay was behind the eec relay. i was working on my friend's 68 Cadillac a/c yesterday and while it was pumping down I started workin on the GM. The a/c hasn't worked at all for over a week, but yesterday it was 92 with 60% humidity and it kicked right on.. The compressor stayed engaged for about 25 minutes , was getting 48 degree outlet air then "click" compressor kicked off and stayed off. I started moving the wires around on the relay after it had been off for a few minutes and it kicked back on and stayed on. Pulled the relay out of it's holder. wire colors match the EVTM. The wire from the relay to the ECA had bare insulation back about 1/2 inch from the connector, but it was not burned, it's like the insulation just shrunk? I left the realy out of it's holder and the a/s hasn't quit since. Should I just replace it (figuring it might be overheating. I believe the ECA provides a ground to close the relay except when at WOT...... correct?

Thanks as always for all you help.

PS anyone ever worked on late GM cars with a TXV and POAST system. I am getting extremely hi low side pressure, slightly low high side, and no cooling....

[gadget73]

wait, so you have working AC without the relay even in the socket? That shouldn't happen. The relay is normally not energized, and it completes the circuit through the normally closed contacts. Pulling the relay out ought to make sure the AC doesn't work at all. The relay is grounded at WOT, and not grounded under normal conditions.

[cadfan64]

No I just pulled the relay out of it's holder bracket/ thingy. It was still plugged in to the connector, but wiggling the wires made it come on. Replaced the relay today, no failure yet

[gadget73]

Ah, gotcha. I've had the crispy wire thing on my car. I used liquid electrical tape and painted the goop on the bare wires. I guess a better fix would be to depin the socket one wire at a time, put heatshrink on it, then re-assemble the connector. I didn't do that with mine because the sockets were pretty brittle. I actually have a broken wire lock tab on the fuel pump relay, so there is a zip tie holding one of the wires into the relay socket. I keep forgetting to harvest another socket from the junkyard.