I need some help diagnosing a cooling problem

[DRRummel]

1995 Mustang 5.0L with AC
I don’t think my two speed fan is running on the low speed. When I am in traffic my car is getting to hot. I have checked a couple of times and the fan is not running. If I turn on the AC the fan will run and the car runs cool. Both the temperature gauge and Speedo work, so both senders should be good.

It is my understanding that the low speed fan should be running when the engine is over 220 degrees and the speed is less than 45 MPH. How do I diagnose this?

Bad Fan?
Bad Fan Controller on the radiator?
Bad block to chassis ground?
Bad PCM?
Blown fuse?

[yeahloh95]

the fan is under recall , you can ground pin 17 on the fan controler or get a chip to turn the fan on sooner.i had the same problem, i just ran the ac more when going slow.

[Killercanary]

First thing I'd do is to "hot wire" the fan. Connect a large gauge wire (the fan draws 60amps at startup) to the ground and one to the low speed fan setting and see if it works, also check the high speed while you are in there. I have wire with an alligator clip on one end that I connect to the battery and I have female speaker style ends on the other to connect to the fan.

If they both work, which I suspect they might, the very next thing I'd look at is the coolant sensor that is in the hard coolant line next to the alternator. I have had three of these go bad on me over the years and the problem always shows up with fans that will not turn on. If you have to replace this sensor make sure that you get a quality one with a brass/metal base and not the plastic ones which don't work as well and seem prone to more rapid failure.

[DRRummel]

i just ran the ac more when going slow. AMEN. 30 degress out and I am running the AC with the tempiture control on heat. That has been working for me, for now.

[DRRummel]

First thing I'd do is to "hot wire" the fan. Connect a large gauge wire (the fan draws 60amps at startup) to the ground and one to the low speed fan setting and see if it works, also check the high speed while you are in there. I have wire with an alligator clip on one end that I connect to the battery and I have female speaker style ends on the other to connect to the fan.

If they both work, which I suspect they might, the very next thing I'd look at is the coolant sensor that is in the hard coolant line next to the alternator. I have had three of these go bad on me over the years and the problem always shows up with fans that will not turn on. If you have to replace this sensor make sure that you get a quality one with a brass/metal base and not the plastic ones which don't work as well and seem prone to more rapid failure. So even tho the temperature gauge on the dash seems to be working, you think the sender unit might be bad? I had ASSUMED that since the gauge seem to react normally, I could rule out the sender. I will try the jumper wires on the fan tonight if I can get out to the garage.

I have about six Fox/Explorer temp sender unit laying around. Does any one know if they are interchangeable?

I think the high speed is good because the fan does run when the AC is on.

[Killercanary]

The gauge uses a different sensor than the EEC. The EEC uses the sensor in the hard coolant line, the gauge uese the one in front of the intake that goes into the coolant crossover passage. I don't know if the fox/explorer sensors are the same unfortunately. The AC "should" kick on the fan even if the sensor is bad.

[DRRummel]

The gauge uses a different sensor than the EEC. The EEC uses the sensor in the hard coolant line, the gauge uese the one in front of the intake that goes into the coolant crossover passage. I don't know if the fox/explorer sensors are the same unfortunately. The AC "should" kick on the fan even if the sensor is bad.
Different sensors, good to know.

[DRRummel]

the very next thing I'd look at is the coolant sensor that is in the hard coolant line next to the alternator.

Can I can jump the harness to ground to bypass the temp switch?

[DRRummel]

The fan sensor is a thermal resister. As the temperature goes up the resistance goes down. So I jumped the wires on the connector and the fan did not come on. I only have some 16 gauge speaker wire around, so I have not tried to test the fan yet. I will pickup some bigger wire this weekend to test the fan with direct power.

Where are the high speed and low speed relays?