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Quick help, Ground for Electric Fan!

98XJSport

98XJSport

Destiny is the rising sun
Location
Western Maine
98 XJ 4.0

Im in the middle of trying to troubleshoot my electric fan. So far, I know it works fine via jumpering. I have power to the feed of the relay, and +12V to the coil. Does the ECU control the fan by sending it a ground signal?

I am running the XJ with the AC on, no fan. The sensor is good, the guage works fine. So Im thinking the wire between the computer and the relay, but I have no idea what color that one is? My haynes isn't right, or hasn't been yet.

Quick help would really be appreciated, burning daylight!

Thanks.
 
Well that thread has nothing to do with the electric fan, but the link you posted might be the problem. I found that the ground is blue w/pink stripe. I have perfect continuity between the relay socket and the pin socket at the computer.

I know that my AC doesn't work, but it's a 98 and may or may not need a working pressurized system to work. When I turn on the AC, the ground for the relay coil goes from infinite resistance to ~380 ohms. So it appears that the computer is trying to turn things on. Perhaps a bad/dirty ground, Ill have to chase that tomorrow.

Also note that the fan doesn't work when it overheats, either. I get a CEL for non-working fan after a good heatsoaked startup. :dunno:

Any other thoughts other than a bad computer? No other problems to note besides this one...
 
nm, I thought you were referring to the blue wire on the fan harness.

dark blue/pink is the wire listed in the Haynes schematic, running between the PCM and the fan relay. I think it's a + trigger, but I hope someone will correct me if I'm wrong.
 
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That is true for the fan. I was looking for the coil ground on the fan relay. It's blue with pink stripe for anyone wondering...
 
I think you'll need to bypass (jumper?) the pressure switch on the compressor before you can trick the PCM into triggering the fan relay.

Check 12-47 in the Haynes. AC High pressure switch. No pressure, no fan relay operation when turning the AC on, IINM.
 
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Ill try that, but that doesn't explain why it doesn't turn on when the coolant gets 220+
 
Nope, but I can't find a schematic for that side of things to offer any other ideas. If the fan works when wired directly to the battery, the only thing I can suggest is a simple thermoswitch.
 
That side of things should be as simple as the coolant temperature sensor reaches a certain resistance that equates to 220 (or 215, or...) relayed to the computer. The computer then should ground the coil of the relay, turning the fan on.

Now, the sensor would be suspect BUT my temperature guage reads normally. Only one sensor for both functions.

Im really hoping that the slight resistance I was reading is between the CPU and ground. It has to be, since there were no ohms (at least not measurable by my meter) between the plug and the relay.

Does the CPU have it's own ground, or is it with the grounds next to the window washer fill bottle?
 
No idea, but my thoughts would be that it is either something wrong in the PCM, or the wire that triggers the relay (presumably a break or short between the module and the relay). Could you let the jeep run up to temp, and put a meter on that relay contact to see if you're getting any voltage there when you should? Short of running it up to temp would be, I think, fooling/bypassing the pressure switch, and engaging the AC to get the relay to trigger?

I can't find anything else useful in the Haynes, but presumably someones got a FSM with better schematics.

Also, have you tried swapping the relay with one of the other 3? I pulled the PDC cover, and I've got the same relay in all 4 positions. I'd swap one out and try that if you haven't already, before I got any more involved in tracing a wire through the harnesses.
 
Sorry, didn't realize that you electric fan wasn't coming on at all...with or without AC.

Anyway, sorry if you had already mentioned it in this thread...but you try testing the relay itself? If you know which terminal in the relay goes to +12 and which is ground...you can use a set of jumper wires to your battery...relay out of the junction box socket...to test it...you should hear a click noise...you can connect a multimeter on the relay switched contacts to check for continuity...

good luck
 
Ive tried swapping the relay with one of the other 3 in several different combinations. And Ive checked both sides of the coil, one is +12V whenever the key is on, and one is either nothing when the fan isn't being told to run, and 380 ohms when it is being told to run. That's the switched wire, the ground for the coil.
 
No help, but I am having the exact same problem with my '91. The guy that does my electric work said that the AC and ECM both open ground to activate the relay so if both are not working then it's likely a problem with the reay ground. However I am supposed to get a check code and I'm not getting it. Another possibility is that the toasted AC would cause the computer to ignore the whole circuit.
 
Have you tested the connection between the relay junction box to the fan? You can use a insulated jumper wire (make sure it is of decent gauge or it will get hot..) to short/bypass the relay. The fan should kick in...this will test the connection between the junction box and fan.

Oh, not sure if this will work on yours, but I know this will work on my '00 XJ...discovered it by accident. While its running, if you disconnect (becareful) the temp sensor (near thermostat)...it should cause the fan kick on. I believe when you disconnect the temp sensor, the PCM doesn't know what the temp is, so by default, it turns on the electric fan. However, this will throw a code that you'll need to clear out later...
 
Yep, this still works...I just went out and tested it...

XJ been sitting overnight, so its cold...but cranked it...disconnected the temp sensor...about 3 seconds later...the electric fan kicks on and the check engine light comes on.

As soon as reconnected the temp sensor, the fan turned off...
 
Ive jumpered the NO contacts of the relay, fan runs. The A1 of the coil is hot as soon as I turn the jeep on. It's the ground that is the problem.

What does the computer use for a ground?
 
I had the same problem you all are talking about and after tracing down wires and testing everything i could find, i ended up replacing the a/c high pressure switch and the fan works just fine now
 
Anyone know if there is a way to bypass the AC switch? There must be, since XJs come without AC, and those have working fans...
 
Nobody? How do the non-a/c XJ's do it?
 
98XJSport said:
Nobody? How do the non-a/c XJ's do it?
Click to expand...

I know this is an older thread, but the A/C pressure switch can be bypassed by jumpering the 2 wires that run to it. If you never plan on running the A/C again, disconnect the harness to the switch, clip off the connector, strip the wires, twist them together and tape them.

Unfinished threads are like books with the last page ripped out...
 
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