heater fan speed control inop for 5 minutes??

[JStieger]

I just went through 17 pages through a TDR search and can't seem to find a clue to the following:



-Had truck block heater plugged in all night and through this morning.

-After starting the truck at 4:00 am when it was about 0 degrees I was expecting to get some luke warm air out of the heater vents. What I got was nothing blowing - no cold air, no hot air, nada... no heater fan noise whatsoever on any fan speed or HVAC vent position.

-I was thinking that a fuse may have blown so I went to scrape the windows. I also left the heater fan control speed on medium anyway. After doing the windows for about 5 minutes I get back into the cab and the heater fan decides to start by itself!

-Played around with the heater controls and speed and everything ran fine as if nothing ever happened.

-After work in the parking lot it was 15 degrees and the truck started up with no heater fan speed issues at all



When I got back home I searched on the TDR and found the TSB 24-004-03 for the broken defrost door, but it doesn't seem to apply to me because everyone else seemed to have heat and the blower speed albeit not in the right place.



Any thoughts?



The last time I drove the truck was a few days previous. It was snowing, but the air was pretty humid yet hovered around freezing. The days since then have been a "dry cold". Could it be that maybe the AC or blower "froze up" temporarily due to the sudden change in humidity coupled with running the AC that runs on most of the heater settings?

 

[TBrennan]

Sounds like an intermittent ground or open circuit somewhere. The dash selector is wired to ground and just changes the resistance seen by the blower, there is no positive connection involved in the switch. The blower is located right behind / under the glove compartment. It's a simple two wire connector that provides 12V through two 10 guage wires. If it happens again you could pull that plug and use a voltmeter to see if it's getting juice or not.

 

[PKnoerzer]

Maybe a stupid idea - but when the intake heater is on in my truck the fan is so slow I can't hear it with the radio on. Or it might be related otherwise to the low voltage when the intake heater is on?

 

[Jeff Clark]

My dad's 1/2 ton gasser does the same thing.

 

[JStieger]

Thanks for the advice and shop manual search time savings. I should have known better especially since I just finished rewiring my '45 jeep where I really concentrated on making good connections to power and ground. Then there was the Wiring Connection article in TDR #46 that I read after I posted this message.



I'll check the connection behind the dash this weekend for continuity as well as look at the blower connection behind the glove box for good power. Maybe I loosened a ground connection by accident this summer when I was hooking up my gauges.



This morning it was even colder, but the blower ran no problem at all speeds. So far I'm on a 1 electrical glitch per year rate - In 2003 the overhead computer read "------" for about 5 minutes and in 2004 this incident.