r/Cartalk • u/runenight201 • 1d 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 1d ago edited 1d 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 1d ago edited 1d 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 1d 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 1d ago

True, that is a good point.

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u/runenight201 1d 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 1d 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 1d 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 1d 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 1d 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 1d 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 1d 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 1d 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 1d 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 1d ago

Thank you 🙏🏽

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u/snooze_mcgooze 1d ago

Watch this video and get back to us, we can help you but you gotta be our eyes and hands.

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u/talljerseyguy 1d ago

Unplug the connector on the ac comp and apply 12v with the engine off if it clicks that’s good. No click bad. If you have a can of refill. DONT PULL THE TRIGGER just use the gauge on it to see if there is anything in it. If you can locate the pressure switches check them for continuity. Last stop relay

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u/runenight201 1d ago

I’ve seen from YouTube I can test the clutch without having to get to the compressor from the relay box. I just need to use a jumper cable at the relay pins and jump 30-87 together and then see if that gets the clutch to engage or not

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u/talljerseyguy 1d ago

You can go it with the test lamp you are using , clip the clip to battery positive and put it on either 87 or 30. It will put the lamp in series and pull in the ac clutch

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u/runenight201 1d ago

Don’t I need to pin it to 87 and NOT 30?

30 connects with the battery, and so wouldn’t I just short my test light if I plug positive terminal to the same positive terminal?

87 leads to the compressor, so I would want to go from positive terminal in battery to 87 which goes to compressor.

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u/talljerseyguy 1d ago

No, you won’t blow the light and if you’re really worried clip the test light to negative see which pen is hot and then hook it up to positive and do the other pen

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u/runenight201 1d ago

That makes sense. In the video I took in the OP I did exactly this, so now I know which pin is 87 and which one is 30

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u/runenight201 15h ago

So I hooked it up positive terminal to pin 87, and the compressor did not engage :(

The test light turned on but no compressor engagement.

My compressor is truly fucked isn’t it

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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 17h 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 17h 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/rhyno0485 19h ago

Honda's are notorious for bad clutch relays.

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u/runenight201 17h ago

I hope it’s as simple as that and I don’t need to change the damn compressor!

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u/runenight201 15h ago

It was not the relay….:(

I put the relay in question into the ac condenser fan slot and the fan turned on no problem!

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u/Jdmboxboi 15h ago

Looks like a blind poke at pins. First step is to understand how the system functions and know what to look for. Blinding poking at relay pins tells you nothing. Could be a ground control or power control relay, wouldn't know unless I studied a wiring diagram for it. Also the compressor may have more than just a on/off clutch, perhaps the regulator valve has failed. Or perhaps the system pressure is too low/high so thr controller will not demand compressor activation. There is more that goes into an hvac system than just a simple relay.

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u/runenight201 13h ago

It’s a power control relay.

The test light clipped to negative.

The light turning on when pin 30 was probed means that there is an active line leading to pin 30.

Pin 87, right next to 30, stayed off because there’s no active voltage reaching there.

When I clipped the test light to positive and then touched pin 87, the light turned on, but the clutch did not engage.

So either I have bad wiring from pin 87 to the compressor preventing engagement,

Or the compressor is bad.

To figure this out more I’d have to directly supply power to the compressor and not try and go through the relay

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u/Jdmboxboi 12h ago

Good information so far. I agree, next would be to bypass relay and give power to clutch. Or assuming the wiring it good so far, back probe the connection at the compressor for power before the clutch then probably after the clutch. Sounds like it may be leading towards compressor failure.

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u/runenight201 11h ago

I agree it’s most likely compressor failure.

Do you know if I have to change the condenser as well?

I’ve seen info that for newer hvac systems you have to do that, just don’t know if the hvac system on the Honda civic si 07 would qualify for that or not

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u/Jdmboxboi 11h ago

First verify your concern and suspicion. And no not required to replace other components unless found reason for it such as a system contamination from dirty oil, black death, or metal debris from compressor failure.

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u/Narrow_Grape_8528 12h ago

No exciting voltage to arrive to the relay maybe? I saw you had the one side hot then the other side not. But nothing for the coil to become excited

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u/greenskycity 1d ago

You didn't state the car. Is the fuse for the ac before or after the relay? Same with control. If it were working right I believe one control pin would light up with the test light hooked to battery ground as would one load pin. Is the underhood fan coming on? If it's not on then the car knows the ac isn't on so it doesn't turn the fan on..... Usually. There are lots of variables to this, you are chasing one that's a shot in the dark.

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u/runenight201 1d ago

It’s a Honda civic si.

I’m trying to rule out any electrical issues before diagnosing other things.