heater/ air problems please help

[krakajacka]

i have a 99 jimmy, i just got, problem being when i switch on the air blows all the way to 3 on the knob then when you move it to 4 its stops, also when i switch from vent to vent it does not change any ideas on how to fix. and what i need to do

[GM-Tech]

There's a relay that controls the blower motor. In it's normal, inactive state, blower speeds are controlled via a resistor which feeds through one leg of the relay. When you switch to the highest blower speed, the relay is activated, supplying full power to the blower motor.

 

There's a separate fuse in the underhood fuse box for the high blower speed. Look for the HVAC fuse. If that's okay, it could be the relay or even the blower switch.

 

Not sure what you mean by, " when i switch from vent to vent it does not change".

[krakajacka]

Not sure what you mean by, " when i switch from vent to vent it does not change".

 

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when i switch from floor vent , to defrost vent etc. it only stays on the main one which blows at you, you cant change it, sorry about the terms dont really know what they are called

[krakajacka]

ne one??

[GM-Tech]

That's called the mode position. Vent/heat/defrost, etc. If it's a manual a/c system, all the doors except temperature are controlled by vacuum. Usually, when there is no vacuum, it defaults to the defrost position, not dash vents.

 

Leave the blower on its lowest setting and when you switch positions from floor to vent to defrost, do you hear any kind of hiss from behind the control or the behind the dash? There may be a momentary hiss when you switch positions, but it should not continue beyond a second or two.