'77 Heater

[prx1ckt]

I have a '77 early model bronco and the heater recently stopped working. Thinking it was the blower motor, I replaced it today but still no action. When I turn the heater on, nothing happens and I don't hear anything. Could it be the control valve, heater switch?? Aside from changing out all parts, how do I tell what may the true problem?

Thanks. CT

 

[SaddleUp]

If you do not hear anything at all then it is probably the switch or wiring. Have you checked the fuses? The control valve itself can be ruled out. It will only efect where the air goes for the defrost valve and whether water circulates or not for the temp control. The fan itself will be electrical and you would at least hear the fan when it was turned on. If you have a cable on the dash to turn it on then the cable will lead to the switch itself. (The cable could be broken and then not operating the switch) If it was me I would check for power right at the blower motor when the switch is turned on. Then check it at the switch itself. Before doing either I would make sure the cable is actually operating the switch (If a 77 still uses cables) and see if there are any bad fuses. At the point the power is failing to get through you will then need to track back to find the actual problem. For instance power into the switch but not out when turned on would obviously be a bad switch. Power out of the switch but not at the blower motor would be more dificult since it would likely be in the wiring itself. This might in fact be impossible though since the switch might be part of the motor assembly if it is cable operated. (My lack of specifics here is due to not having seen the motor in mine and also not knowing if the 77 is different since mine was originally a cable pull that was replaced with a switch in the dash, the principles are general electrical ones though that apply to any older vehicle like this)

Rick

 

[Past_Miner]

As far as I know all 66-77 Broncos used a switch on the dash with a pull cable that ran through the switch housing to control the damper inside the heater box. The switch sends current to a set of resistors mounted on the back side of the heater box. The high speed side of the switch basically sends full current directly to the motor and the low speed side sends it through a large power resistor.

I second the idea of checking it at the motor and working your way back. Be warned that you may have to pull the heater box off of the firewall to get to the resistors.

If the switch is bad you can get a replacement from most of the EB parts vendors.

 

[prx1ckt]

I've checked power at the blower motor when the switch is on and I have current running to the blower motor - which tells me that the switch, wiring, resistors and fuses should all be in good working order - correct? If this is the case, then any other thoughts as to why the blower motor's not working (it's brand new)?

 

[maritime mudder]

You have power, but do you have ground?

 

[prx1ckt]

I have not checked to see if it's grounded? What's the best way to check this - I do have a voltage tester that I used to test that I had power going to the motor. Can I use the same voltage tested (alligator clip on one end and probe on other a light to indicate power) to check the ground?

 

[STLKIKN]

Run a wire from the metal case of the blower motor to the dash.

Use a test lead if you have one (alligator clips)

Turn the fan on, if it works now, then you need to ground the motor permanently.

As well as I remember, all EB heater motors have been externally grounded (two wires coming from the blower motor)

Good luck.