'91 4.0 (R12) a/c clutch won't engage unless...

[F1Luva92]

I use a jumper wire from the positive terminal on the battery to the clutch. I used my handy dandy test light to check for power from the wires that go to the clutch. there is no power from either side. I repeated that process but checking for a ground on the wires, and found a ground. this was all done while the truck was running and the a/c was on "max" with the blower motor turned all the way up. There seems to be at least some freon in the system, since when i push on the valve stem on the low pressure side, i get a hiss. Is it possible that a relay is bad somewhere? if so, where might i find this relay on the truck? i found three relays under the power distribution box and when i took one out, nothing happened, truck started right up and everything was the same. Next one, it would just crank and not start. last one, it would try to run, but die shortly after starting. Is this first relay for the a/c? any help would be much appreciated!

TIA

 

[ranger7ltr]

It is time to break out the gauges...

Your accumulator has a low pressure switch that closes the circuit to the clutch when there is enough refrigerant in the system...Did you try to jumper the connector to see if the a/c clutch kicked in? This bypasses the low pressure safety switch but it will test the refrigerant system even though there might be low levels of refrigerant in the system... Even with the hiss at the port I doubt there is enough refriegrant left in the system to engage the clutch...Unless, of course, there is too much refrigerant in the system and the high pressure cutoff switch is open...

You need to know what is in the system and the pressures...Since you have jumpered the clutch, see what the low and high sides are running and you will get a better handle on what is happening with the system...

But some background on the system would be nice...Did the a/c work and then suddenly stop working or has it not worked and you are trying to fix it? Yes there are relays in the system for the a/c clutch and fuses as well....

 

[F1Luva92]

well i just purchased the truck a few weeks ago, and the a/c according to the PO hasnt worked in a while. i'll try jumping the low pressure switch and see what happens. i know the a/c clutch fuse under the dash is good, i checked it with my test light.

EDIT: jumped the low pressure switch connector and it kicked on. Are there still some shops out there that will recharge R12 systems? or is my only option to ebay some freon?

EDIT #2: Just talked to a local mechanic about recharging with R12, he said that it wouldn't be much more to just convert it over to R134a (~$150) and that it most likely has a small leak somewhere anyway in which case I would have to recharge it again or find the leak anyway. So i'll be getting it converted next week.