Heater is blowing cold air. What I have checked, coolant level, replaced water pump and thermostat, checked blend door, flushed heater core. When I checked the blend door via the heater treator method I was getting hot air coming off the core but also getting very cold air coming from the A/C evap in side the plenom box. I also pulled the fuse for the A/C clutch and I am still getting cold air. Does anybody else have any input into this issue? I am look at replacing the HVAC contol.

 

What sort of mystery vehicle are you working on?

Such info tends to be helpful in these instances.................

 

Sorry about not putting up the vehicle info. F150 2001 5.4 CrewCab 4x4. Manual heater/AC

 

You might post this in that particular truck forum but my guess is the temp control or the blend door drive motor are acting up.

I'm assuming your F-Series of the same era as my '97, '00' and '03 are all similar with their control systems.

 

Check the relative temperatures of the two heater hoses after the engine is fully warmed up. Are they both hot, one hot and one cold, or both cold? This will tell you which side of the firewall to look at for the problem.

 

Yeah, I checked they are both getting pretty warm

 

If they're both HOT (coolant temp), the the problem is inside the plenum, ie, blend door, etc..

 

Need to check fuse that controls blend door I am currently working on the same issue and I have a short somewhere, but when I put in a fuse I can control Hot/Cold temporary.

 

Does the fuse blow only when driving or shortly after inserting a new one while sitting still?

Your particular issue could be a failing Temp control or the blend door motor itself. A resistance check at the motor's connector while cycling the Temp control might show if that's working properly. If it consistently shows a change in resistance sympathetic to the Temp control position, never goes to infinity or shows a dead short it might be the motor itself.

If you can access the blend door shaft and check for it binding that could also be an issue.

 

The OP hasn't visited the site in almost 6 months..... Best to start an all new thread with complete information.