The defroster and heater in my 2002 chevy venture only work at highway speeds. Can you help? last winter everything worked just fine, but now that we need heating it takes forever to warm up the car.
For the fan speed problem it’s probably a defective blower motor resistor pack. This is located under the passenger side dash next to the blower motor.
For the long warm-up time, a stuck engine thermostat could cause this.
Do you have ESP? I don’t see any mention of a fan speed problem by the OP.
I think that “only work at highway speeds” indicates that the only the air movement the OP gets is from air being forced in thru the fresh air intake. My assumption was the same as Tester’s - that their fan isn’t working. I’ll bet even the reference to taking a long time to warm up is a reference to the inside of the van - not to the engine itself.
Perhaps the OP will clarify.
I’d be surprised though if it was just the resistor since there should still be plenty of air moving w/ the switch on high speed.
While the OP was not totally clear, it is a good bet that the problem being described is the cabin air blower. Not much else would likely be described the way the OP used.
About the only other thing I can think of would be a very worn water pump.
Now that I’ve reread the OP’s question, I think they mean there’s no heat from heating system unless the vehicles moving at a high rate of speed.
A stuck thermostat can still cause this. But so can low coolant level. Have you checked the coolant level in the cooling system?