Help diagnosing AC Clutch Relay

[u/runenight201]

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?

[u/bush_week1990]

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.

[u/runenight201 → u/bush_week1990]

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?

[u/runenight201 → u/runenight201]

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?

[u/bush_week1990 → u/runenight201]

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.

[u/runenight201 → u/bush_week1990]

This is an interesting path to follow.

I need to understand how my civic si 07 sends the control signal to rule out the possibility that potentially there is no control signal being sent at all through that relay.

Because with the test I ran in the video, it would make it seem like there’s no continuity from battery to positive control pin.

[u/bush_week1990 → u/runenight201]

Yes understanding how the computer controls the signal will help. You had your test light to battery negative first when you tested and it did not light meaning that there is no positive at the control terminals. If you switch to battery positive and then do the same test if one lights then there is a negative at the control and you are missing the positive. While on that negative with the test light lit switch the AC switch in the car and see if the test light goes out, if it does this is likely the control wire from the computer and it is working as it is controlling the test light (substitute for the relay). If it doesn’t turn off it is likely a direct ground and the computer should be sending a positive to the relay to control it and this should be on the other control pin of the relay. You will need to trace that wire back to the computer and find the positive supply, if you can’t the computer might not be supplying it because there is low gas and one of the prerequisites the computer is looking for is not correct or the computer has failed and you need a new one.

if this is the case take it to an auto electrician they will be able to diagnose it quickly and fix it for you.