Air Conditioning Relay power

I have a 1989 Cherokee. The air conditioner clutch stopped turning on and the electric fan won't turn on either. The fan works with 12 v directly to it. IT has worked in the past as well. It just stopped working right after I installed a new higher amp alternator.

Relay readouts are as follows
AC OFF AC ON
30 - 7.3mV 30 - 9.5mV
85 - 14.1 V 85 - 14.0 V
86 - 60mv 86 - 60mV
87 - 39.9mV 87 - 39.3mV
87a - 105mV 87a - 0.1mV

This is all stock wiring and parts. I've no clue why 30 isn't getting 14 volts, and why 87a is the only one changing in volts. If there was a fuse blown on the wire to plug 30 I wouldn't expect to get ANY volts.
The relay itself works and I've tried it with other 5 pin relays that also work.

Could the clutch not be engagine because the compressor is low on oil? The orange wire going to the Air Conditioner isn't getting 12 volts of power either.n

Pin 30 isn't going to get 12v. It needs ground in order to work. Since the coil of the relay is the only thing in that circuit with a resistance value, it drops all the voltage. 87a is only changing by 0.1v, which is insignificant.

Let me go look at an 89 wiring diagram and I'll get back to you.

Looks like the ECU controls the ground going to pin 30. If it worked before an alternator swap, I'd just go back and retrace your steps to see if you left a ground loose (or accidentally knocked one loose) somewhere. I don't really have a good electrical diagram for an 89 so I'm going off the Haynes manual.

Yeah I had the Haynes too. I don’t think I touched a ground but I’ll check what I can see. Thanks.
Fan works if I let the Jeep get hot (drive it) so that’s good to know.

See if you're getting a ground signal from the ECU when the AC is commanded on. Of it's coming from the ECU but not getting to the relay you have a break in wiring somewhere.

The AC relay is wired kinda weird compared to what most are probably used to seeing. Pin #30 is NOT a source of power for the load (AC clutch). The source is #87 is, and that current comes from your AC control panel through the pressure switch. Did you accidentally knock the connector off where it plugs in at the drier when fiddling with the alternator?



To expand a bit on how this works, pin #30 goes to the AC clutch, the diagnostic connector, and the 'fan diode'. The weird thing is that when powered off, the AC relay takes the lead from the AC clutch and sinks it to ground via pin #87a. I ***think*** this is to drain flyback voltage released when the compressor clutch is turned off.



I had a problem a few weeks ago that shed light on what I believe to be a defect in the schematic. My radiator fan diode decided it didn't want to be a diode any more, so the current from the key switch that goes to the radiator fan switch in the radiator went right ou the AC clutch wire. The AC was off, so the current went straight to ground via pin #87a, frying all the wiring as it went. It was a lot of work to fix that mess! I think it was precipitated by the new AC compressor I installed not having a flyback diode on it. It does now. I also installed a 5A fuse on the yellow wire coming from the ignition switch going to the radiator fan switch. The diodes I had around, and made up a new double diode 'thingy' to replace the one that vaporized on me. There was nothing left of it but some melted orange plastic and the skinny spade connectors.

Obvious question, does the A/C still have a charge? No voltage on pin 87 tells me that you don't have voltage coming from the low pressure switch. Measuring across the low pressure switch should have zero resistance and voltage if there is pressure.

lawsoncl said:

Obvious question, does the A/C still have a charge? No voltage on pin 87 tells me that you don't have voltage coming from the low pressure switch. Measuring across the low pressure switch should have zero resistance and voltage if there is pressure.

Click to expand...

Good question!!

Also check your fuses. The ac circuit gets power from the blower fuse (25A)

Sure you are not low on freon?

Unplug the pressure switch on the drier and jump the contacts. See if it works then.

cruiser54 said:

Sure you are not low on freon?

Unplug the pressure switch on the drier and jump the contacts. See if it works then.

Click to expand...

I think the Freon is low. I’ll try jumping the contacts.

Saudade said:

Good question!!

Also check your fuses. The ac circuit gets power from the blower fuse (25A)

Click to expand...

Pretty sure the blower fuse is fine because the fan comes on inside when I turn the air or heater on.

Are you done yet?

cruiser54 said:

Sure you are not low on freon?

Unplug the pressure switch on the drier and jump the contacts. See if it works then.

Click to expand...

Works when contacts are jumped. ..guess I need some r12 or a r134a conversion.