AC won't kick in...

[as400jockey]

I have a 1998 S70 Sport. No turbo. A/C had been acting strangely - compressor kicking in, then quickly off, then repeating. Felt like low refrigerant. When the car is cold, pressure in the system is about 80PSI. So it's not empty.

A week ago the ECC was dead, as was the sunroof and instrument panel lights. A 10A fuse blew. Replaced it. Everything else works except the a/c. Blend doors, heat, fans... all good except a/c compressor won't come on.

Low pressure switch is working. Currently reads closed (with 80PSI not running). Shorting the connector changes nothing. Replaced the high pressure switch. No changes. If I short the compressor pins on the a/c relay, the compressor comes on OK, sounds good, gets cold. Replaced a/c relay. No change.

As far as I understand it, there are no other sensors to worry about... it feels like the ECC is the culprit. I removed it. Opened it up. Nothing fried that I can see. My guess is that the cause of the fuse blowing was the ECC, and that the ECC is causing this current headache. But I don't have a spare to play with. I can get on on eBay for about $50, but that's an expensive guess.

Just now I connected a freon recharge kit to the low side. About 85PSI with the engine off, about 80F. Started the car. Shorted the a/c relay pins. Compressor came on. Low side pressure dropped into the 40's then continued into the 20s. THAT feels like low refrigerant, or maybe a blockage? So I let it charge. It started getting cool in the car. After a few minutes I got a hissing noise. Dunno if it was a leak or the clutch slipping. I pulled the jumper. I think it was the clutch slipping. Maybe now I'm overcharged?

I let the car sit a half hour or so. Jumping the clutch relay working OK. compressor sounds OK.

Wish Volvo had put high side ports on this car. I may see if my gauges will connect directly to the Schrader on the high side. Wanna see what the high pressure looks like.

Before I spend that $50 on a used ECC, does anyone have any other ideas? Maybe my replacement high pressure switch is no good, but seems unlikely. I'm stumped.

[as400jockey]

Figured it out...

I replaced the high pressure switch. Nope. The AC relay. Nope. Shorted the low pressure switch. Nope. Replaced the ECC module (in the dash). Nope.

I pulled the ECM, cleaned the contacts, let it sit for a few minutes for any memory to die, reinstalled, and voila! Compressor kicks in now.
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[Oly850]

There is such a thing as too low AND too high in these systems. Anything over about 70psi and the high switch will never click off and the compressor won’t run.