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1999 300M ATC Fan Speed Controller Fan won't blow

PostPosted:June 11th, 2019, 12:59 pm
by ss427isaudio1
First the symptoms leading up to my problem.

When I originally got the car about 6 months ago, in the dead of winter, the rotary fan speed knob would sometimes take a "twist or two" to get the fan to run at the set speed. Summer came and same problem with A/C.

Went to work with A/C on and fan running. Came out and no blow. Twisted the knob many times, banged on the dash, all the other tricks....no blow. Took head out of dash, blew little fan clean, inspected same and no apparent corrosion or other bugs. Ran diagnostic self test and no error codes. Took out Blower Resistor Module, looked kinda wonky and dirty, corroded and old. Applied 12 volts to fan...fan ran fine. Replaced BRM and still no fan. My thought is the little black module that the Fan Speed Control Knob attaches to on the PCB in the "Head Unit" is bad. When I had the Head Unit out, I noticed that the knob is what I would call a "limitless range" control. It will go and go and go in any direction limitlessly. Is this normal? Is this part available? I have many years of PCB work and am not afraid to replace it. I have not done a "reset" since replacing the BRM. Any suggestions please. Thanks...Dale

Re: 1999 300M ATC Fan Speed Controller Fan won't blow

PostPosted:June 12th, 2019, 1:13 pm
by ss427isaudio1
I have now unconnected the battery and let it sit for 30 minutes. Reattached Neg cable and still no action. Still no codes showing on self diagnostics either.

Re: 1999 300M ATC Fan Speed Controller Fan won't blow

PostPosted:June 12th, 2019, 1:15 pm
by M-Pressive
My guess is the ATC. I don't think the knob is supposed to spin all the way around.

You can pick them up on eBay or sometimes members have them for sale here.

Re: 1999 300M ATC Fan Speed Controller Fan won't blow

PostPosted:June 12th, 2019, 1:43 pm
by remi
M-Pressive wrote: June 12th, 2019, 1:15 pm My guess is the ATC. I don't think the knob is supposed to spin all the way around.
Actually it does ;) At least mine does, with the knob removed you can spin the shaft all the way around. I guess this is a rotary encoder.

Dale, I'm backing Keith on the ATC...Maybe try another one from the yard or get one from someone here.

Re: 1999 300M ATC Fan Speed Controller Fan won't blow

PostPosted:June 12th, 2019, 3:47 pm
by In-trepid
I have a couple Remi. Send me an email.

Re: 1999 300M ATC Fan Speed Controller Fan won't blow

PostPosted:June 13th, 2019, 1:37 am
by ss427isaudio1
If the ATC was bad, Wouldn't the self diagnostic check show an error code? I did the "set temp to 75, push and hold the 3 buttons"...and get no error code.

Re: 1999 300M ATC Fan Speed Controller Fan won't blow

PostPosted:June 13th, 2019, 2:49 am
by remi
Not sure... The ATC drives the blower fan by the blower motor resistor (which is not a resistor on automatic AC by the way, but whatever ;)), blower motor resistor is driven by the BCM which itself is being driven by the ATC... Send a PM to John, try the most simple thing then go from there.

Re: 1999 300M ATC Fan Speed Controller Fan won't blow

PostPosted:June 27th, 2019, 11:45 am
by ss427isaudio1
OK...so far I have replaced the Blower Motor Resistor with a new one, just replaced the ATC with a tested and working ATC and the dam fan still won't blow. No error codes on the self diagnostic with both ATC's. Got a new OBDII test unit and ran it...no codes anywhere on anything. Put 12 volts straight to the blower motor and it runs and with the ATC set to Recirc/Lo Temp I get cold air, so I know it's not low on freon....I'm about at my wit's end. Any suggestions? I'm about ready to just hot wire 12V to the blower motor with an inline switch...it's hot here.... Thanks!!

Re: 1999 300M ATC Fan Speed Controller Fan won't blow

PostPosted:June 27th, 2019, 2:44 pm
by hrmwrm
the atc does not provide power. it provides a varying ground to control speed through the bcm.
make sure you have power and ground at the appropriate terminals at the resistor plug.
black ground, dark green power.

Re: 1999 300M ATC Fan Speed Controller Fan won't blow

PostPosted:June 27th, 2019, 11:20 pm
by ss427isaudio1
Thanks!! Appreciate the info....I've got the service manuals and am about ready to get elbow deep in this thing...it's wearin' me out...it's hot...