alec_b

Hi all-

Friend of mine has a '97 F150, super cab lariat 4.6. I brought it in to the shop to check why the A/C wasn't working, and found that the compressor wasn't engaging. After tracing wires, I found that the A/C diode isn't directing voltage properly.

I have good ground and running voltage at the compressor, and the compressor coil ohm'd out to 3.5.

When I place a wrench next to the compressor clutch when it's on A/C mode, there is a very slight magnetic pull, so there's some current getting through, but not enough to engage the clutch. The clutch is not seized.

I believe I need to replace the diode, but I can't find one! None of the trucks in the JY had one in the same place on the harness as his, and they had different connectors for the clutch. Anyone got some thoughts?

I really appreciate it guys! Great forum.

exitwound

Radio Shack maybe.

Good old Bill

With your ohm meter, test the diode. As you know, should pass current one way but not the other. Good thought, what side of the coil is the diode? Power side or ground side? Diodes are used both ways in different installations. Most likely its used to block another circuit from powering up the clutch. Don't forget to check the cycling switch that is attached to the receiver dryer.

alec_b

I checked the diode with my DVOM. It's allowing current both ways. The diode is in place to isolate the coils EM voltage from hurting other parts of the system when the field collapses. It's wired in parallel with BOTH the power and ground of the coil. I checked both pressure switches, they are operating correctly.

exitwound

Is the Diode burnt? If it isn't you should be able to remove it to find the correct one and when you remove it note which side the stripe is on and replace with one that is comparable. Radio Shack should have it, if they don't you will probably need to call these guys ---> http://www.diodes.com/. They should have it for sure. We use them where I work to get all of our electrical stuff.

When you say it is wired in parrallel with the power and ground what do you mean. It should just be in line in the ground side (most likely) or the positive side. If it goes across the positive and negative at the plug or wherever then it is wired in series. This would prevent the field from regenerating current and feeding back through the positive and blowing a fuse or circuit breaker.

alec_b

That's how it would normally be, yes, but it's not in this application. Thank you very much for the link, I may drop them a line. I have the diode number. It was hidden way down in the harness.

Here's the wiring diagram for the system: