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Hi, I discovered a smell in my hvac and discovered some rodent nesting when i pulled the fan. The squirrel cage had some chewing on it so i replaced the fan with an aftermarket TYC fan.
The new fan works, but for some reason, it only operates with the ignition on, but engine not running. When I start the car, the fan shuts off. Unfortunately the old fan that was still working went out with the garbage so i can't put it back in to see if the issue lies with the new fan and something in the computer not wanting to operate it when the engine is running. the Any ideas of what could cause this? Is there something that needs to be reset?
What can cause this ??…probably your “aftermarket” fan !!
This was my first thought but I'm questioning this because it works fine if the engine isn't running with the ignition on. I know it has a controller in it that receives a pwm signal but why would it care if the engine is running or not? The only difference would be 12v vs 14v powering the systems. I'd hate to throw money at an oem fan only to find out something else failed.
Well as you can see the blower motor is powered from B+ through fuse 28…since the signal is PWM so if you have a graphing multimeter or a labscope you can check the signal..you can even use a DMM and check with the car running.
Thanks for the info C5 Diag.
I've confirmed battery+ and ground at the fan connector. Power is there all the time as it should be.
I have a multi meter with a frequency setting. i probed ground and the signal wire and it showed 0 when the ign is off, there was a signal with both the ign on and with engine running but the numbers were all over the place. it's a pretty cheap dmm so i'm not sure how much to trust it.
looking at the schematic, it appears the pwm signal is a pulsing ground. The explanation talks about a 5v signal. Is that 5v what goes to the bcm, and then the bcm sends a pulsing ground to the fan signal output? Did you have a particular way to test with a dmm in mind that wouldn't involve using the hz selection?
Below is a labscope capture of my blower fan from low speed to high speed...low speed on left half of screen and high speed on the right half...power (blue) stays at system voltage always and ground (red) stays near 0 volts always....the signal (green) is near 5 volts but with a DMM will be at 3.4 volts low speed and 1.10 volts high speed...at LOW speed Duty Cycle is 22%, PWM is 6.1 ms, and frequency is at 36.4 Hz...at HIGH speed Duty Cycle is 80.5 %, PWM is 22ms, and frequency remains constant at 36.4 Hz....your numbers should NOT be all over the place !!..when you see the green signal at 5 volts if duty cycle is say 50% you will only read around 2.5 volts with a DMM…if duty cycle were 100% (always on) you’d see 5 volts…that’s how that works.
Blower speed frequency in Hz is around 36 to 37Hz at all speeds…the PWM signal is pulsed to ground in the HVAC module if you looked at the wiring diagram.
Readings I got:
fan off, less than 0.1v
speeds 1 through 7
Engine running 13.08/13.35/13.55/13.72/13.89/14.01/14.13
on, not running 10.85/11.03/11.16/11.29/11.42/11.53/11.64
Last edited by Sean2009; Oct 12, 2025 at 08:40 PM.
Reason: add info
All my readings and the labscope waveform was with the connector plugged in…I had pierced all 3 wires…the frequency reading was the signal wire (gray/black) and was referenced to ground…I have no explanation why it does not run with the engine running vs not…I’d suspect it’s the aftermarket fan.
Dang my blower motor randomly stopped working and then started working again by itself after the car was off for awhile… Been okay for two weeks now so not sure what happened.
All my readings and the labscope waveform was with the connector plugged in…I had pierced all 3 wires…the frequency reading was the signal wire (gray/black) and was referenced to ground…I have no explanation why it does not run with the engine running vs not…I’d suspect it’s the aftermarket fan.
I have a feeling you are correct. I'm ordering oem. If it works, i should be able to return the A/M fan, if it's still not working, Looks like i'll be on the hunt for a control module and have a fan for sale...
What can cause this ??…probably your “aftermarket” fan !!
@C5 Diag well, after running through all the diagnosis, you were 100% correct! OEM fan just arrived. I installed it, fired up the engine and it works perfect!
For anyone that comes across this post with the same issues, be wary of the aftermarket pwm controlled blower motors. They may not work.
Blower fan issues
