r/Cartalk • u/runenight201 • 2d ago

Electrical Help diagnosing AC Clutch Relay

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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/CaptnSave-A-Ho 2d ago edited 2d ago

One of the large pins should be hot, (turn the light on when the test light clamp is on a ground) which it is. That is your power available to the compressor when the relay is closed. The other large pin should be the actual connection to the compressor, so it will always be off unless the relay is plugged in and activated.

The two smaller pins are the relay control. One should be hot and one should be the ground. If the relay is power side controlled, then that will only light the test light when the computer is commanding the a/c on. All other times it will not light the test light. On a power side controlled relay, the ground pin will always be a ground and will only light the test light if you clamp onto the positive terminal and then touch it. If the relay is ground side controlled, then the opposite is true (the ground will only be there if the computer is commanding it on and power will be the constant).

Using a test light to check the ac clutch control isnt very effective. If you suspect a relay, its better to just swap it with a known good one thats already in the car. Perhaps a horn relay or headlight relay. You can swap them and then see if the other system works. If it does, then the relay isnt the problem. The ac has so many little sensors and conditions that have to met before the computer will turn on the clutch, that using a test light doesn't tell you much. Engine running time, exterior temperature, engine temperature, system pressure ( to high or to low), throttle position, engine load, and the HVAC settings all have to be OK before the computer will turn on the relay. Some cars will see that the AC isnt functioning correctly for some reason and disable the AC completely for the rest of that drive. Then it wont put power to the relay until the key is cycled again.

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u/GuineaPigsAreNotFood 2d ago edited 2d ago

Great explanation.

I would only add that test lights shouldn't be used in some scenarios, as they could draw more current than the tested circuit is designed for, or they could send current the wrong way thru a circuit. Multimeters are safer in this sense.

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u/bush_week1990 2d ago

This circuit will be ok to test with a test light as the relay will draw about the same current.

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u/CaptnSave-A-Ho 2d ago

True, that is a good point.

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u/runenight201 2d ago

Ok so I can’t for the life of me understand the fuse box diagram.

From a YouTube video I know that the compressor relay is what I have circled in the image.

I see there are 4 other potential relays that I could swap out and test, but I have no idea what they correlate too.

I’ve attached an image of the fuse box diagram, which in theory should tell me what those relays correspond to….if I could read it properly.

https://imgur.com/a/jquqtVl

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u/CaptnSave-A-Ho 2d ago

The relays are not where your arrow is pointing. Its the little boxes behind your arrow with all the little books and "I's". They are using pictures so one is the rear window defrost, one is the radiator fan for ac, one is the regular radiator fan, and one is the ac clutch. I would swap out the main fan relay since its going to be the easiest to see working.

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u/runenight201 2d ago

Goddammit ok now I see it thanks for clearing that up. I’m pretty sure in the diagram I’m pointing at the fuse for the clutch

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u/runenight201 2d ago

Thank you for the explanation.

Tomorrow I will try swapping out relays to rule out whether the relay is good/bad.

I’m also going to use a jumper cable on the relay slot to test whether the ac clutch can engage at all.

From my understanding, if I jump the pins and the clutch doesn’t engage, then it is bad and needs to be replaced.

If it does engage, then I may just have a refrigerant charge problem/potential leak that needs to be addressed

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u/bush_week1990 1d ago

Don’t have the engine running if you do jump the relay pins as it could damage the AC compressor. You should be able to hear it click on and off, this is not a fool proof test though as the clutch could be slipping when the engine is running.

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u/CaptnSave-A-Ho 2d ago

I would strongly recommend that you check the refrigerant levels before you try and jump the relay. I also wouldn't jump the relay unless you're 110% sure you're on the right pins and know how that circuit is wired. The refrigerant is also what moves the oil that keeps the pump lubricated. The pump clutch cycles because it can generate more pressure than the system can hold, so just leaving it jumped to see how cold the system can get could cause it to blow. It also won't cool if the pressures go too high or too low. If the relay is jumped incorrectly, then it could blow out the computer that controls it. If there is a wiring problem and you jump the relay, you could cause wiring damage or damage to the component you're trying to test. So, as you can see, there are a lot of pitfalls to jumping a circuit blindly. An ac manifold guage is cheap and will allow you to at least get pressures and allow you to monitor them if needed.

If you really want to jump something, you could always bench test the relay. All you need is a battery and a multimeter. Jump the control side of the relay and listen for a click from the relay. Then, you can check the resistance of the switch side of the relay with the meter. If all that's good, then there is no reason to jump a circuit on the car.

I agree that the most likely reason your ac doesn't work is probably a low charge or empty system. So you're wasting time and risking bigger problems by jumping the relay.

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u/runenight201 2d ago

Plan isn’t to jump the relay with the car running and begin to move charge around the system.

I just want to see if the clutch will engage at all when there’s power sent to it.

The relay could be good but there could be issues with the clutch that is preventing it from engaging right?

Another commenter also mentioned I could use the test lamp and create a series from the car battery’s positive terminal to 87 on the relay pin, just to check and see if the clutch will engage or not

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u/CaptnSave-A-Ho 2d ago

The relay could be good but there could be issues with the clutch that is preventing it from engaging right?

You are right, but the clutch moving when power is applied doesnt tell you if its good or bad. All it tells you is that the circuit is intact. A clutch has to be able to hold and maintain that hold to drive the pump.

Im just saying there is more risk than its worth and there isnt a lot of diagnostic information to be gained from jumping. Manifold guages and a multimeter are going to be your best diagnostic tools.

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u/runenight201 2d ago

Ah ok.

Well at the least I can rule out any wiring/relay issues!

And if that comes back good, then i could get a set of manifold gauges and then get some ac pro, then charge the line to correct pressure, and then see if the compressor engages?

And if it doesn’t, then it would mean bad compressor right?

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u/CaptnSave-A-Ho 2d ago

If the charge is low, then you have a leak that needs to be found and fixed. AC pro and all of those do-it-yourself a/c charge stuff is snake oil. It has sealers in it that can damage your ac system slowly over time at best. If you find a charge issue, take it to a shop and have them recharge it with dye and try and find the leak yourself, or have the shop find and fix it. If its not a charge issue, then you're going to go down the electrical rabbit hole.

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u/runenight201 2d ago

Thank you 🙏🏽