I have a 1973 F150 and am having trouble with the heater blower. I would like to get some suggestions on how to trouble shoot the problem, mainly switch and then the blower itself.

 

Moved to 1997-2003.

 

His truck is a 73

 

Woops. That was my first moderation of the day, I think. It was originally in the General Discussion section.

 

There's 3 things to diagnose, the fan-speed switch, the baffles/diaphragms changing from feet/dash/defrost, and then blower itself. If you're not getting air anywhere, it could be the switch or the blower. You can test the blower independently of the switch by wiring it directly to power/ground. Then turn the AC/heater switch on and see if it turns on. I had to wire my 85 like this since the switch is no good but the AC/heater controls, baffles, and blower works fine.

IIRC, mine works on a ground switch. The blower is always connected to power, but not ground. Basically the switch in the cab has a resistor that resists the flow of power to a ground. This way they only have to run one ground wire into the cab rather than both ground and power connecting at the switch.

If you're getting air but not where you want it, then check the baffles. They run off engine vacuum. A vacuum line could be disconnected or the plastic baffle inside the air ducts could be broken off its hinge (common problem on old fords).

Good luck!

 

Thank you for the information. If you could help me with written instruction, diagram, or picture for testing the blower as you described. I am trying to help my son with this and he lives 150 miles from me. So any more detailed information would be helpful.

Thank you
Larry

 

Here's some photos I found online. Obviously your 78 will have a different setup than my 85/86. The theory is the same, though. You can see the blower motor relay mounted on the fender (yellow wires going into it). Follow those wires to the motor (mounted behind the squirrel cage). You can get some wire, hook it up to the battery, and touch it to the motor manually for a test.


Blower motor/heater core location (coolant hoses are giveaway)


The squirrel cage inside the housing. Arrows by someone else indicate cover and motor mounting points.

 

Here's a diagram for the same:

 

Looks like other versions could look like this:

 

A few questions.
1. Is the blower motor running at all ?
2. Does the fan speed switch work in all speeds ?
3. Is the Mode selector working ?
Meaning does it change from Defrost to Vent to Floor etc.
4. Is the truck equipped with Air Conditioning and
do you or your son know if it is factory or dealer add-on A/C ?
The HVAC system is a combination of both electrical and vacuum operations.
If this is an electrical issue, then it's possible that the heater only
"Heater Blower Motor Resistor" has failed causing the blower speed to operate only on low or not at all. From the picture, you can see that it is two coils which over time, burn through, causing a break in continuity.


If it is a "Blend Door" issue, then that is a vacuum issue or a door issue within the ducting.