Running hot. Fan only comes on with ac on? - Page 2 - Ford Mustang Forum
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Originally Posted by Bullitt95 View Post
Ok, at least you've probably ruled out an air pocket.
Since the fan does work when you switch on the AC, the relay must be OK so it's more likely that the ECT sensor's at fault. To confirm that, disconnect the wiring harness from the ECT sensor while the engine's running (AC off). If the fan comes on immediately, replace the sensor.
there are two relays one for high speed, one for low speed. when the a/c is on the high speed fan is on. so you cant rule out the low speed relay.

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In that case he could switch around the low and high speed fan relays and see if the fan works without the AC, or if the fan doesn't work with the AC.



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Quote:
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In that case he could switch around the low and high speed fan relays and see if the fan works without the AC, or if the fan doesn't work with the AC.
yes. it would be easier to check the low speed fan like that. the low speed fan works at base idle @ the temp settings

from what i remember the high speed fan parameters :

Your at 20% load or higher(which the a/c compressor does)
Have reached the Hi speed fan on temp set point (228)
Are below 47MPH (there is enough fresh air movement above that)
Above 1400 RPM

those are for a factory tune though, Im sure they can all be altered by a custom tune.
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My error.
I meant to say a bad resistor is another possible cause in my previous post, and not a bad fuse. The resistor is located in the fan shroud, which allows the fan to run at a low speed. (See item 5 in the attached picture.)

FWIW, the cooling fan has 2 fuses and 2 relays. A 15A fuse(F40) for the coils on the low speed and high speed relays, and a 40A fuse(F3) for the contacts on the low speed and high speed relays. And the cooling fan would not run at all if either fuse failed, even when the A/C is turned on.
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Quote:
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My error.
I meant to say a bad resistor is another possible cause in my previous post, and not a bad fuse. The resistor is located in the fan shroud, which allows the fan to run at a low speed. (See item 5 in the attached picture.)

FWIW, the cooling fan has 2 fuses and 2 relays. A 15A fuse(F40) for the coils on the low speed and high speed relays, and a 40A fuse(F3) for the contacts on the low speed and high speed relays. And the cooling fan would not run at all if either fuse failed, even when the A/C is turned on.
thats solid info too, thanks for sharing, I think we are all headed in the right direction to get this guy fixed up. I really believe its electrical in nature.
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Quote:
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My error.
I meant to say a bad resistor is another possible cause in my previous post, and not a bad fuse. The resistor is located in the fan shroud, which allows the fan to run at a low speed. (See item 5 in the attached picture.)

FWIW, the cooling fan has 2 fuses and 2 relays. A 15A fuse(F40) for the coils on the low speed and high speed relays, and a 40A fuse(F3) for the contacts on the low speed and high speed relays. And the cooling fan would not run at all if either fuse failed, even when the A/C is turned on.
Good info. as a point of reference. Since the OP's fan does run when the AC is on, it's very unlikely that either of the fuses or the fan resistor could be bad though.
I think we've pretty much narrowed down the problem to either a faulty ECT sensor or low speed fan relay.


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If you go that route, be very careful with the ECT wires, they are delicate.

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Quote:
Originally Posted by Bullitt95 View Post
Good info. as a point of reference. Since the OP's fan does run when the AC is on, it's very unlikely that either of the fuses or the fan resistor could be bad though.
I think we've pretty much narrowed down the problem to either a faulty ECT sensor or low speed fan relay.
Thanks.
I agree that neither fuse can be the cause, since the fan would not run if either one was blown. I also agree that either the temperature sensor or the low speed relay are possible causes.
However, the low speed resistor is also a possible cause, because it's located only on the low speed relay's circuit, which will allow the high speed relay's circuit to run the fan when the A/C is turned on.
The high speed relay's circuit will also run the fan with the A/C off if the temperature is allowed to increase to the point that energizes the high speed relay assuming the the temperature sensor is not bad. However, it appears the OP did not want the temperature to get that high, since the engine might sustain damage. Can't say that I blame him.
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post #24 of 30 (permalink) Old 08-24-2015 Thread Starter
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Swapping relays won't be possible. They are different. After looking around I've found quite a few people say it's the cooling fan resistor on the fan. Sounds very plausible as I see some of you great folks have said that. I still will test the relay to rule it out. Now I need to find where I can get the resistor.
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Here's a wiring diagram for the cooling fan. The low speed resistor is not shown, but it's located on circuit 1825 between node 87 on the the low speed relay and the cooling fan motor. I believe the wire color is light green and yellow.
(Note: It appears to be a dodgy website. Sometimes it works, and sometimes not.)
Ford Service Manuals - Wiring Diagrams
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post #26 of 30 (permalink) Old 08-24-2015 Thread Starter
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Well I took the chance of buying the resistor that cannot be returned once plugged in. PROBLEM SOLVED. 70 bucks and dealer only part. Kind of a pain to replace. Reservoir tank must be moved. Un clip both harnesses. Remove 1 screw. Pretty simple, just in a tight spot. The fan now revolves between low and hi speed, and no overheating.

Thanks for the input folks!!
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Quote:
Originally Posted by kingkoopa01 View Post
Well I took the chance of buying the resistor that cannot be returned once plugged in. PROBLEM SOLVED. 70 bucks and dealer only part. Kind of a pain to replace. Reservoir tank must be moved. Un clip both harnesses. Remove 1 screw. Pretty simple, just in a tight spot. The fan now revolves between low and hi speed, and no overheating.

Thanks for the input folks!!
awesome!
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Quote:
Originally Posted by kingkoopa01 View Post
Well I took the chance of buying the resistor that cannot be returned once plugged in. PROBLEM SOLVED. 70 bucks and dealer only part. Kind of a pain to replace. Reservoir tank must be moved. Un clip both harnesses. Remove 1 screw. Pretty simple, just in a tight spot. The fan now revolves between low and hi speed, and no overheating.
Excellent. Glad that solved your problem.

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Originally Posted by ski View Post
However, the low speed resistor is also a possible cause, because it's located only on the low speed relay's circuit, which will allow the high speed relay's circuit to run the fan when the A/C is turned on.
Didn't know that. I thought the resistor was for both circuits so thanks for correcting me. It's a different set-up from the SN95 that I was used to previously.


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Quote:
Originally Posted by kingkoopa01 View Post
Well I took the chance of buying the resistor that cannot be returned once plugged in. PROBLEM SOLVED. 70 bucks and dealer only part. Kind of a pain to replace. Reservoir tank must be moved. Un clip both harnesses. Remove 1 screw. Pretty simple, just in a tight spot. The fan now revolves between low and hi speed, and no overheating.

Thanks for the input folks!!
You're welcome. Glad you were able to fix the problem.
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Quote:
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Didn't know that. I thought the resistor was for both circuits so thanks for correcting me. It's a different set-up from the SN95 that I was used to previously.
Nie ma za co (You're welcome in Polish). The immense number of circuits and sensors in modern cars makes my head spin. Give me the good old days when the radiator cooling fan constantly spun when the engine was running, and stopped only when the engine was off.

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