Am trying to diagnose the A/C system on my '89 Ram 150 by applying +12V to the compressor's clutch coil. Since it is a coil, I'm guessing that I can apply +12V to either wire and ground the other one. Neither of the two wires have any color coding - both are black. Is this the correct procedure to test for clutch engagement? BTW I probed the connector and neither wire ever gets a +12V signal when I turn on the A/C - and have already confirmed that the system is pressurized.

Have you tried jumping the pressure switch?

More info - yes I have tried jumping the pressure switch by pulling the harness connector and jumping across the harness connector's contacts (and verified with voltmeter that there is continuity between the contacts). I also checked the pressure switch for continuity - there is none - totally open circuit at the switch. Lastly I had a buddy check the pressure (wasn't there when he did it) because my gauge set has a bad leak and needs replacing.

One more thing - I converted this truck from TBI to carb + electronic ign so most of the factory engine wiring is disconnected and taped off. I don't really use the A/C so am just dinking w/it for something to do. I was planning to use a relay w/a toggle switch in the dash to engage the clutch when desired.

Running the A/C through a relay in the future you need to keep the pressure switch in series with the control side of the relay circuit.... On older systems auto makers used a expansion valve to control refrigerant flow.. In the mid-late 70's the trend changed to using a fixed oriface & cycling the compressor clutch to control refrigerant flow.. So if you just run the compressor you'll push the high side till it blows the relief valve... Or if system pressure is low doe to a low state of charge you'll likely seize the compressor...

If I were to keep the pressure switch on the control side of the relay - would this be an ok way to cycle the compressor? Is this how the factory cycles it? Before I go that route I suppose that I need to verify that the pressure switch in question is a high-pressure switch and not a low-pressure switch. I only saw one and it is on the high side just before the orifice block at the firewall.

Typically with a CCOT system (Cycling Clutch Oriface Tube) the cycling switch is on the low side.. it prevents the low side from pulling a vacuum.. actually they typically kick out at around 24 psi & back in around 47 psi... They high side numbers are a function of a proper charge level... Cycling time vs ambient temperature can be used as a pretty good indication of state of charge as well... The hotter is is the longer the clutch will take to kick out & the quicker it will be back on...

Is there power at the low switch?You have to have it there first.Most clutches have a ground and hot.They are grounded and the low switch applies power to activate it.Rocky

Upon further inspection the pressure switch that I found is on the low side. This is the only pressure switch that I can find that is connected to any of the A/C lines under the hood. I'm assuming that it is either the switch that enables compressor operation when there is enough pressure (typical low-pressure switch) or something to do with clutch cycling like the previous post mentions. And no there was no voltage to either of the terminals in the pressure switch connector. After looking at a wiring diagram - it appears that the PCM is involved in A/C control. Since most of the underhood wiring is disconnected (converted to carb + elect ign + ext alt volt reg), I expect that I'll have to control the A/C clutch engagement by another means. Without a high-pressure switch, I'm not feeling too comfortable about that. Any recommendations?