Decreasing output from a/c or heater during long drive

cavalier
chevrolet

#1

My 2000 Chevy Cavalier LS has approximately 216,000 miles on it. I’ve recently noticed on long drives that the air conditioner will run fine for the first few hours, but then the amount of air begins to decrease, until the output from the “high” setting acts like I have it set on “low”, and then almost quits altogether. What little air is coming out is still cool, so it’s not a problem with the freon. If I shut off the a/c for awhile, everything works fine again upon restart, but then the same problem occurs. I have the same problem when using the heater, so I think it’s related to the blower motor, resistor, fuse, etc., but don’t know where to start troubleshooting first. From a limited Google search, this sounds like a problem I could fix myself, but just don’t know where to start. Any suggestions would be very much appreciated!


#2

Fuses and resistors either work or they don’t, they’re not variable.
I do think it’s the blower motor. That is somthing affected by time and heat. and at that age is likely.


#3

One possibility is that the air conditioner is working too well. Normally, when air is cooled by the evaporator core (the radiator-like heat exchanger behind the dashboard that gets cold when the refrigerant inside it evaporates) then some of the water in the air condenses onto the evaporator, and drains out the bottom. But if the evaporator gets too cold, then ice forms on the condenser, and eventually plugs it up so that no more air can flow through it.

Most modern cars also run the air conditioning pump to a limited extent while heating, in order to dehumidify the air and keep the windows from fogging up. So the evaporator could also freeze up when you’re heating, and block the airflow to the heater core. When you turn everything off, then the ice starts melting, and after a while air can flow through again.

You could start by checking the temperature of the refrigerant line that comes through the firewall back to the A/C compressor when it has been running for a while. If it is much below freezing and ice is forming on it (where it is not insulated), then it could be that your condenser is freezing up.


#4

Thanks, Ken. The fuse was last on my list, but I wasn’t sure about the resistor. I’m thinking it would be smart to replace the blower motor and resistor at the same time to save me one less body contortion getting under the dash!


#5

If you’re keeping the car, “that sounds like a plan.”


#6

The variable resistor would be my suspect 1. Cheap and usually easy to replace.


#7

Another couple of likely causes:
1- Compressor is not cycling off to let accumulated moisture thaw and drain off.
2- Plugged drains.


#8

It could be the blower motor, or that there is debris in the fan cage preventing it from spinning freely. After checking for debris, and before replacing the blower motor, I’d be inclined to probe the voltage at the motor’s power input connector. With it remaining connected. If the voltage drops at the same time the amount of air being blown drops off, the blower motor might be ok, and the problem is probably somewhere before, a bad electrical connection probably. You’d just follow the circuit backwards on the schematic until you found the source of the voltage drop.

Edit: You might ask why this kind of problem develops in cars, but rarely in home appliances, like hair dryers, refridgerator fans, room fans, etc. It’s b/c home appliances run on a higher voltage, and therefore use less current. High current is frequently what causes electrical connections to misbehave. The fuel pump in cars for example is a high current device, and bad connections to the fuel pump circuit are a common problem. Starter motor solenoid contacts another example.