91 Aux fan wiring

[Black Squirrel]

Hello, I have a 1991 Cherokee and the electric fan is not working.

So far I have tested:

A/C on - no fan
Disconnect temp sensor at t-stat housing - no fan
Swap relays - no fan
Jump pin 30 of the relay to 87 - fan comes on
Pin 30 - B+
Pin 85 - B+
Pin 86 - shows connectivity to ground when the fan should be on
Relay clicks then installing
If I add a small gauge jumper wire from pin 86 to ground with the relay installed - fan comes on

I am kind of lost, the only thing that I can think of that I didn't have time to check yet is a partial open in the wire from the ECM to pin 86 of the relay, but not enough for the meter to see.

Any advise would be greatly appreciated!

 

[joejeep92]

 

[joejeep92]

Ok, so we are both looking at the same thing I enclosed the diagram above. You should have constant power at 2 and power switched to 4 when the relay operates. When you say 86 do you mean 87A? Disconnecting the temp switch shouldn't get the fan to turn on. The temp switch is a two wire thermistor and a good way to fool it into thinking it is hot is to short across the two terminals on the connector, unplugged, with a paper clip.

 

[Hellbent]

on every ho I've ever messed with, unplugging the cts does cause the aux fan to come on within 3-5 seconds. some kind of overheat failsafe.
it's a good quick test of the fan, relay, power flow, etc.

 

[Black Squirrel]

joejeep:

I dont think that diagram is exactly correct for my jeep, the relay is not in front of the washer bottle, its in the PDC on the passengers side of the jeep.

I've never seen a relay labeled as 1, 2, 3, 4, and 5, but judging by that picture:

1 is B+ of the magnetic switch - pin 85
5 is the ground of the magnetic switch (ECU controlled) - pin 86
2 is B+ of the load side of the relay - pin 30
3 is the normally closed of the load side (not used) - pin 87a
4 is the normally open of the load side - pin 87

So pin 1 and 2 (85 and 30) are B+

When the ECU supplies ground to pin 5 (86) the magnetic switch circuit is completed and the load switch changes position from 30 --> 87a (an open) to 30 --> 87 and supplies power to the fan.

If I manually ground pin 5 (86) the fan comes on, so either the computer is not supplying the ground, or a good enough ground is not making it to the relay.

 

[joejeep92]

Ah...I misread your original post about the jumper wire to ground instead of jumping the relay. Your diagnosis would be along my lines of thinking. Hook voltmeter from positive post of battery to 86 (5) and see if it reads twelve volts

 

[Black Squirrel]

I will check that tonight, thats a good way of verifying the ground.... good call.

 

[Black Squirrel]

Well, I gave up for now and bought a taurus fan. I will mess with it again when I put it in.