A/C electrical problem

Hi guys,

I’m trying to get my A/C working, but I’m running into an electrical issue somewhere. The relays for the condensor fan and compressor are not activating, even though they are getting 12V when I switch on the A/C. I can run a jumper across where the relay would switch and both turn on and my A/C is cold, but it won’t do it without the jumper. I’ve traced back from my battery to the A/C and checked my dual pressure switch, I’m slowly trying to pick out wires which may have fried, but I’m hoping someone here has had a similar experience and could point me in the right direction.

Both relays work when connected to the battery alone. If I jump the 12V and ground on the harness for the compressor and the harness for the condensor fan, they kick on and A/C is cold. Fuses are good, dual pressure switch is good, any ideas? It’s almost like the relays aren’t getting enough current to activate, one of them the switching signal reads only 4 V which is too low, that’s for the fan. The compressor reads 12V though on the switch. When backprobing them with the relay connected I only read 0.25 V, I don’t know if that matters or not.

Anyone have any idea what’s wrong?

It's almost like the relays aren't getting enough current to activate...

Well, where does this current come from? An A/C control switch or module?

From your description of the trouble it seems to me that the relays are controlled by making a ground connection to the relay coil. Where the trouble is at depends on which side of the relay coil you measured the 4 volts. If it was on the power supply side of the coil then you have power connection problem. If the 4 volts was measured on the return side of the coil you should be able to manually jumper that connection to ground and turn on the relay. That would mean there is a connection problem either in that wire connection or in the switched device it ties to, perhaps the AC control module. High resistance is causing a voltage drop to occur where it shouldn’t be. That 4 volts should either be 12 volts, or zero volts (ground) if it is on the return side of the circuit and the system is supposed to be working.

I will have to try jumping the ground side to the ground and see if it kicks these on, as for the source of current I honestly do not know. I have this pink wire which provides the power and I have traced it back as far as I can without removing the dash board, but I did only check for voltage so perhaps I should check if for current?

What year, make and model?

94 geo tracker

The last problem that I tracked down like this…the ground signal was missing from the BCM (Body Control Module). You may just need a re-program or you may need to get a another one like I did from a salvage yard.

Perhaps one of the relays is malfunctioning at rest? All depends on how the relays are integrated into the system. Need to look into that…see what the relays are doing at rest and what one relay needs from the vehicle and or other relays. I was worried about your pressure switch at first glance but you said the switch is working.

Hmmm when you have a system that is as complex as a modern A/C system there are a lot of things to check. A lot of things rely on the other to work as well. Grounds and feeds are of course important. You need a schematic to see exactly how this system works and then you can slowly go thru each component to see if they are doing what is expected of them. What the relay is triggered by, where they are getting ground from…if the relay or relays require another relay to be doing xyz. Its all a fun game for sure…just go thru the system slowly and carefully and you will find the culprit methinks. Wish I could be more specific


It would be near impossible for me to offer any help without a wiring schematic. My suggestion would be to try and locate one. I took a look at the AutoZone repair info section and per the usual; seriously lacking in applicable information.

If you live in a major metro area you might consider checking with the local public library. The one here has a fair number of decent service manuals, electrical manuals, and parts interchange manuals for public use with a printer right there to run copies off.

Another possibility would be to subscribe to ALLDATA. A 1 year subscription for one car is about 25 bucks and 45 for a 5 year subscription.

Some Chiltons or Haynes manuals may have some wiring diagrams but it’s a coin flip both as to the info being shown or the accuracy of the info if it is shown.

here are drawings