Automotive Air Conditioning Information Forum

I found this forum when a Google search lead me to this thread (Dodge truck in the thread but I'm having the a very similar issue) https://autoacforum.com/messageview.cfm ... adid=19149

Vehicle specs: 1977 Chevy C30 dually. Gen 3 6.0/4l80e combo. Been running for a few months. Each head is grounded to the firewall. The transmission is grounded to the frame. The battery is grounded to the block and the frame (stock LS truck setup).

Bought a new Denso 10S17F A/C compressor back when I started building in late 2018. Finally finished up the A/C and went to get it charged this week and the compressor clutch is toast. Never been run. Had power applied to it once but immediately blew the fuse.
I chalk it up to the compressor sitting so long and eat it. Head to NAPA for another new Denso 10S17F. Bench test the resistance in the coil and it's 3.9 ohms. Install it. Go back to A/C shop. Same result. Check coil resistance 0.4 ohms (dead).

Double and triple check wiring, relay, change relays, everything checks. NAPA was good enought to exchange the compressor at no cost so this time I bench test it for Ohms and get 3.9. This time I decide to test the clutch also and it engages/disengages with 12v applied as it should.

Not wanting to risk another compressor, I jumper the Low Pressure Switch and rig up a test harness that plugs in where the compressor normally would. I set it up to run a small air pump that draws 7 amps. To test the load accuracy, I install a 5a fuse and turn the A/C "on" with the dash switch and it blows the fuse (as expected). I install a 7.5a fuse and it holds fine (factory LS truck fuse size for this circuit is 10a).

So, my wiring checks out. Dash switch operates the air pump that's substituting for the compressor every single time I try it.

I install the compressor. NO ELECTRICITY HAS EVER BEEN APPLIED TO THIS COMPRESSOR. Lines are connected, and the High Pressure switch is plugged in. Since I'm so gun-shy now about killing the clutch, I test it again with it installed and the clutch WILL NOT engage. Fearing the worst, I check the resistance and once again get 0.4 Ohms.

WHAT AM I MISSING???

Update to add info:
Checked the wiring again. Appears to be correct with the following "question marks"....
The harness plug is aftermarket and came with a diode in the back of the plug. From what I'm reading about diodes, this appears to be installed backward (black stripe should be on the ground wire). This raises some questions....
1. Is this the cause of my issues?
2. I believe that the factory compressor (which is what I'm using) has an internal diode, making this one unnecessary (and possibly the problem).

Do you have refrigerant in the system?

Tim wrote:Do you have refrigerant in the system?

Not at this time. It had one can in it each of the first two times we tried it.

Revisiting my previous reply regarding the diode stripe. The diode body is solid black with a silver stripe on the positive end.

You need refrigerant in the system to trip the LPCO switch. Should have power to one side prior.

Tim wrote:You need refrigerant in the system to trip the LPCO switch. Should have power to one side prior.

LPCO is jumped to activate the circuit. No need for refrigerant just to test the circuit.

Then you should have power to the clutch. Otherwise there is a wiring issue someplace.

Tim wrote:Then you should have power to the clutch. Otherwise there is a wiring issue someplace.

I DO have power to the clutch. From my original post...

Not wanting to risk another compressor, I jumper the Low Pressure Switch and rig up a test harness that plugs in where the compressor normally would. I set it up to run a small air pump that draws 7 amps. To test the load accuracy, I install a 5a fuse and turn the A/C "on" with the dash switch and it blows the fuse (as expected). I install a 7.5a fuse and it holds fine (factory LS truck fuse size for this circuit is 10a).

So, my wiring checks out. Dash switch operates the air pump that's substituting for the compressor every single time I try it.

The vehicle the compressor came from has a 2 wire plug.

Inside the clutch is a diode ACROSS the coil windings. Wired properly, it shunts the reverse current from the magnetic field in the clutch when power is removed.

If you ground the wrong wire from the clutch, the first time you energize it - you feed a dead short to ground through the diode. Most of those diodes are only rated 1.5 to 2 amps. When you nail the diode it often fails shorted. Then you have a built in short across the clutch coil.

Your aftermarket plug has a second diode from your description. Is it wired across the two terminals in the plug?