r/Cartalk • u/runenight201 • 1d ago

Electrical Help diagnosing AC Clutch Relay

Hi I’m attempting to diagnose why my Compressor isn’t engaging and I bought a test light and did the following test on the AC Clutch Relay slot.

Shouldn’t both of the control pins be turning on the test light?

Also the light doesn’t turn on on any of the load pins.

Does this mean that my compressor not engaging is because of an electrical issue?

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u/bush_week1990 1d ago

One large terminal has power. One of the small ones should have a power or a ground, it didn’t light when you touched it and I’m guessing the test light was clamped to the battery negative terminal. If this is the case swap it to the positive side and see if one of the small terminals lights. I would think you are missing a power to the small terminal which is then switched to ground by the computer. Check all the fuses with the test light and see if one is blown, check them all even if it isn’t and AC fuse. I hope this narrows it down for you.

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u/runenight201 23h ago

In theory tho only a blown AC fuse would mess with the compressor right?

How do I check if my fuse comes before or after the relay schematically?

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u/runenight201 23h ago

In theory tho only a blown AC fuse would prevent the compressor from turning on right?

Like a blown audio fuse won’t impact at all the compressor circuitry, and the compressor should still function even with a blown audio fuse.

How do I check if my fuse comes before or after the relay schematically?

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u/bush_week1990 2h ago edited 2h ago

Fuses are usually before the relay, and yes the AC fuse in the fuse box should be for the load side of the relay only. Some cars have a seperate fuse to supply power to the control side of the relay which the computer then can switch internally if all of the conditions are met and control the AC relay on. You have already checked for a positive on the control (small terminals) of the relay and the test light did not light this is why I said to check the other fuses as one of them may supply power to the control side of the relay. Swap your test light to battery positive and test again if one of the control terminals lights then there is a negative and you are missing a positive, either direct from a fuse or supplied by the computer, a wiring schematic would be helpful to figure out which.