99 XJ A/C Clutch and Electric Fan Won't Engage

I was in the process of replacing my mechanical fan with an electric fan and wiring a control switch into the cabin when I fried something controlling the A/C clutch and auxiliary electric fan.

Backstory: My intention was for the new electric fan to engage whenever the aux fan was engaged, or when I turned on a cabin switch. I had two lines attached to the relay signal of the new fan; one spliced directly from the relay signal of the aux fan and a second leading from a two-way, on/off switch in the cabin. The problem occurred as I was trying to wire the cabin switch. The signal from the aux relay was ON-0V and OFF-12V. The cabin switch was originally wired such that 12V-ON and OFF-0V, so I swapped the ground and signal leads, and probably shorted the system somewhere. Now, even with the switch and relay splice removed, the A/C clutch and aux fan will not engage.

I've troubleshot the following:
Swapped relays and fuses.
Unplugged/plugged in the Coolant Temperature Sensor.
Jumped the A/C clutch switch.

Nothing I tried got the A/C clutch or aux fan going again. Now, the signal lead from the relay shows 0V when the A/C is turned on in the cabin and 3-4V (instead of 12V) when the A/C is turned off. Also, as I set testing things with a multimeter, I noticed the engine is running a bit more rough and struggling a little bit at idle. I recently had the entire A/C system overhauled, including new clutch, compressor and coolant, and it had been working 4.0 for the past few weeks, so I doubt there were any slow-burning issues that just coincidentally manifested when I swapped those pins.

Any suggestions on where to look now? Thank you in advance!

I'm having a little trouble deciphering your explanation, so I'll explain how the fan and a/c circuit works, and we can go from there.

All of the relays used a switched ground for control. i.e., the relay has 12v to (and through) the coil, and the PCM grounds the circuit it to turn the relay on. The relay then feeds 12v to the device it is controlling.


The a/c is activated as such:
When the controls are set to a/c mode, a ground signal is sent to the PCM. The pcm then activates the a/c and the aux fan relay by grounding the control circuit.

Both relay coil circuits, (A/C and Aux fan) are protected by fuse #10 (15a) in the junction block in the right kick panel.

If I read everything correctly, fuse 10 is blown.

I'll give that a shot. Fingers crossed it's just the fuse!

If not, I'll test again with the multimeter. I was in a hurry, and my multimeter shows ground as a "- 0V", easy to confuse with zero voltage, " 0V".

Follow on question, for the wiring, is there a problem connecting two signal wires to the same relay pin? I am worried that stray voltage, passing from one signal wire back through the other is what shorted something out. Do I need to connect a diode in between on both signal lines, or use two relays?

Lets not use the term "signal lines" There are no signals being sent or received to a relay. reserve that term for sensors.

A relay is an electric switch that uses a low amperage circuit to control a high amperage circuit.
The two circuits are;
1. the control; The low amp circuit that activates the relay,
and
2. the controlled; The high amperage circuit; the switch internal to the relay

You should not be trying to activate the OEM relay on the control (low amperage) side. That will set a CEL as the PCM monitors that circuit and will see your circuit as a short to ground on the control side.

You may activate the fan by applying 12v using a switch or a second relay to the hot lead to the fan. No diode will be necessary as the PCM does not monitor this side of the circuit in this case, nor will feed-back be an issue if the OEM relay is on or off.

SierraAlphaMike said:

I'll give that a shot. Fingers crossed it's just the fuse!

If not, I'll test again with the multimeter. I was in a hurry, and my multimeter shows ground as a "- 0V", easy to confuse with zero voltage, " 0V".

Follow on question, for the wiring, is there a problem connecting two signal wires to the same relay pin? I am worried that stray voltage, passing from one signal wire back through the other is what shorted something out. Do I need to connect a diode in between on both signal lines, or use two relays?

Click to expand...

Did you check fuse F10 in the JB? See post #2.

Yup, it was Fuse #10. Easy day. I must have missed it the first time I checked the kick panel.

I ended up using two relays. One activates on a signal from a switch mounted in the cabin. The second activates when power is supplied to the aux fan.

I may eventually add a temperature switch to automatically activate the fan, but we'll see how it goes. My only concern is the aux fan activation frequency. The engine temperature is held to a steady 205 degrees, but the aux fan seems to kick on every minute or so.

Sounds like it's operating as designed.