How do I test a relay?

[peregrinfalcon]

I think my a/c relay is shot. How do I test it?

[Bob's944]

Easy way to check a relay is to swap out with someone elses. What is the a/c doin? Is it running for a few minutes then shutting down and blowing the #17 heater fuse? If that's what it's doin, most likely a capacitor in your a/c clutch has gone bad... new a/c clutch might be in order.

[cduke]

If it's useful to ya: I had a bad headlight motor relay when i got my car but i made sure it was just the relay by putting the A/C relay in the headlight motor relay...it only makes sense that you could go the other way and see if the relay works that way.
Charlie

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1985.5 944 N/A

[Lawrence Coppari]

I believe your problem is a bad AC clutch. If you have an ohm meter, unplug the leads to the AC clutch and measure the resistance across the coil and capacitor that is in parallel with the coil. The wire splits at the connector with one side going to the coil and the other to the capacitor (I am told it is a capacitor). The resistance should be about 3-5 ohms, meaning you would be drawing about 2.4 - 4 amps. The fuse is 7.5 amps.

What happens to the clutches is that the wires in the coil lose their insulation and shunt out some of the winding. When this happens, the length of the coil wire is drastically shortened and has lower resistance. The insulation on the coil winding is shellac or something of that nature and deteriorates with age/heat.

I think you need a new clutch. Check it out.

[This message has been edited by Lawrence Coppari (edited 11-01-2001).]

[peregrinfalcon]

I get nothing at all when I turn the a/c on. At the advice from another post I tested the voltage at the pressure switch. I have nothing at either pole. According to the guy that replyed to my last post that, is indicitive of a bad relay. Now, how do I test the relay to make sure. Before, I just buy a new one.

[Lawrence Coppari]

Relays usually have an electrical diagram on them in EE hierogliphics. If you understand that stuff, place wires in appropriate places to turn on the AC. They would plug into the relay holes.

[This message has been edited by Lawrence Coppari (edited 11-01-2001).]

[peregrinfalcon]

Lawrence, I would not suggest that to anyone. It's gonna take me a week to get the smell of burnt wire out of my car. The wires I inserted melted instantly when I turned on the a/c switch.

[Lawrence Coppari]

You wired it wrong or you have a dead short somewhere in the system. All the relay does is connect some of the prongs together. Some prongs are to power the relay; others, when the relay closes, power the AC. You have to determine which ones to jump to which ones. If you are not completely sure which way to jump it, don't do it. This was implicit in my suggestion. Checking the female holes with a voltmeter will let you know which ones are hot and might help figuring out the electrical diagram.

You just have to know what to attach to what because that is all the relay does. Done properly, you are doing what the relay does.

Sorry you burned wires.

[JonA]

A few simple suggestions:
When you turn the a/c on/off you should be able to feel the relay click in and out. This may help you tell if the the controls are telling the relay to turn on.
To test the pressure switch, disconnect both leads and read resistance (ohms) across the two terminals. If the switch is closed and telling the relay to turn on you should read 0 resistance.
Put everything back together and disconnect the clutch leads. Turn the ac on and read voltage across the two leads. if 12v is present and the fuse doesnt blow, you probably have a bad clutch.
Hope this helps, it is easier to do than to explain!!! Good luck.

Jon Aborn