AC trouble - How do you check this circuit?

You can test the low pressure by jumping the plug with a paper clip. If it’s bad, jumping it will make the compressor engage. It’s to protect the compressor from running when Freon leaks out. Some people jump them when filling an empty system to speed things up. Bad practice. Best to put one can, 2 if needed to activate compressor when charging. It takes longer, but avoids damaging compressor. This is the switch I’m talking about. I still forget and get them confused. Lol. This one definitely controls low pressure, and high pressure, so running it too long jumped may blow a hose, if the high side of the switch is also bad. Pretty sure other one with pigtail is fan only. I think that’s how it works.

 

Best way to speed things up is to rev the motor.

You also get a better idea what your pressures are.

Sticking with idle isn't the greatest idea.

 

Sounds like you have Freon in it. So I doubt being low is causing it not to work. You’ll find out when changing switches, if it’s under pressure. Little poof as they come off. If the fans not operating and engines getting too hot, fans a separate problem, the switch with pigtail only kicks it on when pressure gets too high on a/c. When it kicks fan on, the other switch should kick compressor off. I think that’s right. One pressure switch with low and high pressure settings. One fan switch with high and low settings. Those are the simple most common problems. Worth just changing, since they don’t last long anyways. The chances that both switches, and solenoid went bad all at once seem unlikely, if everything was working fine. The other stuff Dino mentioned, Temp cool probe and PCM in the fuse panel would be my next suspect. Don’t want to guess much more past the parts you have. Inspect all the wires from switches to compressor, and solenoid wires, Often a chafed wire, or almost chafed wire somewhere. You should just throw the 2 switches on now. A 7/16 wrench, even pliers is all that’s needed.

 

Low pressure isn't how you determine charge level. It's only used for diagnostic.

High pressure is what you want.

 

you need a gauge set.. YES.

The can gauge gets you NO WHERE. Specially since it's not accurate and does more harm then good by allowing air in to the system.

 

I meant the fan switch and bianary switch. Never knew about the one in the dash. Learn something new every time, working on A/C. It’s good to know. No telling how many oz. of freons in it. Since it had pressure already, I wouldn’t add any more. It needs about half a can to engage it, initially. You had to have at least 5 or 6 oz. to begin with. Probably more, unless it has a leak, was already low, or you lost a lot changing parts. I’d say you have plenty in it, probably a bit too much. Too much will cause high pressure, and compressor clutch will cycle on and off a lot, fan will kick on while compressors off, till pressure drops. Then fan kicks off and compressor engages. As long as its not doing this short cycling while parked at high idle, I’d leave it alone. 30-40 on low side is good. Higher the outside temp and especially humidity, higher pressure will be. If it’s not short cycling, but still not cold enough,when it’s real hot outside, and/or compressors running constantly, never cycling off, It may be low. You can try adding a bit more. Not too much too fast. Can held upright only, not upside down, you’ll split the compressor open doing that. Add it slowly, till compressor cycles off, then stop. Watch the cycling process, compressor should stay on almost twice as long as it stays off, at least longer. Typically on for 50 seconds, off for 20 seconds is a good operation. Too much Freon, and it will stay off longer, and fan will run longer. It’s easy to overcharge it guessing this way. Sure fire way is to start from scratch, adding 5 cans, 60 oz. After putting hose on can, bleeding the air from the hose, just for a split second, hearing a bit as you’re hooking the hose up to the low side port. Check out Chris Fix on YouTube. He’s good at explaining vacuum, gauges, pressures, etc. in real clear simple terms. His videos are working on cars. Same procedure though using gauges. Glad it’s fixed.

 

You can get vacuum pump and gauges at Harbor Freight. Freon and nipple adapter at parts house like Napa or whatever. You'll be under $200 for everything.