I think this is pretty common on these trucks. Is the problem usually in the switch or the fan motor? Blows good and hard on high, but the other settings it barely blows at all.

 

Make sure the blower motor resistor didn't get unplugged, that's common. It's located in the heater plenum under the dashboard by the glove compartment. It has a three prong plug in the side of the plenum that faces the back of the truck and there is a corresponding three prong female plug from the wiring harness that it plugs into.

If that is plugged in, take an ohmmeter and confirm that the resistor is OK. There should be a low value of resistance between the three terminals.

If that's OK, it about has to be the switch. If the motor runs OK on high that pretty much says it's OK.

 

99.9% of the time if it blows on high only it's the resistor pack as suggested by Brian, and it's not just a thing on these trucks, it's common on all vehicles of all brands, used to see it a lot when I worked at a chev dealer, a lot of the time if it's out on all but high it's unplugged (probably 75% of the time) if it works on high and any other setting it's usually a bad resistor pack.
Heck I had that problem on my 2000 F250 went 3 yrs with only having high and finally decided to look and duh it was just unplugged appearently from the factory (and I even know better)

 

Thanks guys! I'll check it out. I put a new heater core in a few months back and probably left it unplugged.

 

Culprit.

 

Trust me you wouldn't be the first to do that, and yeah with recent work in that area it would definitely by my first thought too.

Let us know what you find.


And thanks Dennis for that helps to have a pic for those that don't understand the words.

 

I took a look and it is still plugged in. I will have to check the resistance next. If it is shot are these still available to replace or can it be fixed?

 

you should be able to still get them, heck they use the basic same setup to this day (my 03 expy has one that looks about like the ones used in the 70s I had replaced)

 

I used to leave mine on high and used the vent below to regulate the temperature. LOL

 

Mine has the opposite problem. On high it is blowing like it is on low. It was working but the switch on the dash had to be wiggled once in a while to get it work. Now it blows very little on the high setting. I would imagine this is a loose wire at the dash switch right? So how do I check it? Pull the heater contols out? How does it come out?

 

Try running a jumper wire with a full 12 volts to the fan motor directly.

I suspect your problem is the relay that supplies full power.

For topics like this

Please people.. tell us if you have A/C or not.... as it makes HUGE difference

 

As usual you guys were correct. One of the prongs had pushed into the heater box instead of into the wiring plug. I rectified the problem and it works like a charm!

Thanks for the help guys!

 

Thank you for taking the time to repost with the answer you found so others can find it and use it later.

 

Im hooking up a new harness from centech.

Ive got everything done excet the heating system.


The directions on how to hook up the heating system i are not the easiest.

I have a 79 f350, if any one has pictures or diagrams id greatly apreciate it.

--this is how i have it wired and no response from motor.

switch----->Blower motor resistor------>blower motor

there are four prongs coming off of the resistor, at this point only 3 are being used......one of the prongs has two wires,wire a going to the switch and wire b i think is going to the motor.

Can anybody make any sense of this?

Again Pictures and diagrams might be all i need to figure this out.

Thanks in advance.