heating problems

[jonp]

Ok so I have a 1979 gmc k15 when I put my heater/ac/vent to defrost the blower motor turns off and nothing happens doesn't matter if the truck running or not

and on a side note
I have ac
my fan only works on high
and I've made sure the rod on the box on the cab side moves

 

[chengny]

Unless you've changed the front clip that isn't a 1979. I'm not being picky but the model year makes a difference when offering advice on the HVAC system.

Anyway - regardless of the model year - a fan that only works on one speed usually means that the blower resistor is shot. Try replacing that first.

Located here with an A/C equipped truck:

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