Heater fan doesn't work, on heat but it does on others.
 

92 d250, The blower fan will work normally on A/c and defrost, but turn to heat to the floor the fan doesn't work. any ideas.

Could be that door in your heater box maybe:humm:

Not sure what you mean, you can hear the doors shutting and closing. But the fan actually cuts off in floor heat mode. blows in the A/c defrost mode.

yea mine does the same thing im thinking that ima have to replace the compresser all together that might just be the issue

Compressor??????????? What compressor has anything to do with heat?

But to answer the original question, there are basically two possibilities. Either the electrical contacts inside the dash control (where the buttons and fan speed switch r) are bad or there is something wrong with the resistor unit for the fan speeds.