Electrical Idle Stumbles When Radiator Fan Turns On


Aug 25, 2011
Denver, CO
No...I have not forgotten about this problem; been living with it everyday still. :O_o:

Just tonight I installed a reman'ed OEM Motorcraft alternator and the problem persists. Since the car was warmed-up I could get the cooling fan to kick-on pretty frequently by running the A/C. To rule-out the relays in the CCRM (to some degree) I unplugged the fan and ran the A/C again long enough to have the fan commanded on -the engine didn't stumble.

So in spite of previous testing conclusions I plan on replacing the fan. But first I'm going to make sure the wiring to the fan is all good.

More to come and quicker than in 2 years (sheesh).
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SN Certified Technician
Aug 14, 2009
Houston Texas
FWIIW, for the Mustang GT the low speed fan runs with the AC is on (not the high speed fan). The only time that the high speed fan is if the AC head pressure is above a certain level.

So turning the AC on and off is only testing the low speed fan.

If interested in testing the high speed fan then consider using an advanced ODB2 scanner with two way communications to "command" the high speed fan to run.

ForScan ODB2 scanner w ELM327 USB

Howto perform charging system voltage drop test

If this were my car I would be performing a voltage drop test between the fan ground and battery negative on low speed and high speed. Then repeat the test on the power lines and battery positive.

Repeat the voltage drop tests between the alternator case and battery negative as the fan is being cycled. What you are looking for is IF the voltage drop suddenly goes up when the fan draw kicks in. Repeat the measurements between alternator B+ and battery positive.

This should also be cross checked against the voltage at the alternator and battery to confirm that the charging system voltages are stable.

If all of the voltage drop measurements are low, then the only reasonable conclusion left is that the fan is drawing way too much power.
Last edited:


Aug 25, 2011
Denver, CO
Using FORScan I found out that the low-speed fan is not working. I can hear the relay energize but the fan does not run; zero volts at the fan connector's low-speed terminal.

Later I will try the voltage-drop tests you recommended on the high-speed and ground circuits.

As for the part where the voltage at the alternator and battery are checked for stable voltages, they do drop when the fan kicks on but come back up and are stable after a second.

More to come. Thanks again.


SN Certified Technician
Aug 14, 2009
Houston Texas
Many cooling fans use a ballast resister in series with the low speed fan to drop the voltage so that the fan runs slower. If this were my car I would rule out an "open" ballast resister.

If on the other hand there is zero volts at the low speed terminal with the connector disconnected, then in this case think "bad CCRM".


Aug 25, 2011
Denver, CO
Haven't done the voltage-drop tests yet but I figured-out why the low-speed fan was not coming on. Turns-out the high-current fuse box had been removed. The wires were just cut, folder back, wrapped in electrical tape, and then put back into the plastic harness which had its end wrapped in electrical tape. From what I can tell, this box just holds a 30A fuse that's between the CCRM and the fan motor.

What's on the left side of the picture on the front of the strut tower circled in blue is missing in my car.

So I wired-in a weather-proof fuse holder. The low-speed fan now works as it should. When it runs the engine idle does NOT stumble. High-speed fan still does stumble the idle.

I did end-up getting that ELM327 USB dongle and ForScan. Had some fun with that, especially the bi-directional controls. Wish it could do just a bit more like show live data while activating some of the bi-directional controls. But for the price I'm not complaining -still a geat deal. Thanks for the tip on that.

As always, more to come...as life permits.

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