Resolved Idle Stumbles When Radiator Fan Turns On

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
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

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.

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".

cjcoburn said:

Ok...so I lied, unintentionally...it was way longer than another 2 years. But I think it's fixed.
I replaced the fan assembly today with a TYC unit (p/n 620500) from RockAuto (~$100). I also replaced the 7-year old battery 4 months ago (may have been a factor). So now when the fan kicks on at idle the idle actually kicks-up a tiny bit for the briefest moment almost before the fan starts and then returns to normal.

Click to expand...

Can you confirm 3 years later my 99 Cobra is doing the same thing but worse, it will stumble as soon as the fan comes on and eventually stalls and die!!! Super frustrating issue, may need to just replace the fan, but it seems like yours was testing good even with it being bad?