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BigBob
BigBob, Shop Manager/Technician
Category: Car
Satisfied Customers: 4838
Experience:  Shop Manager with 25 years experience
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Heater blows..The water temp fluctuates..blow cold..blow hot

Customer Question

heater blows hot then cold. There seems to be no reason. The water temp fluctuates but the heat will also blow cold regardless of the fluctuation than blow hot again.
Submitted: 8 years ago.
Category: Car
Expert:  BigBob replied 8 years ago.
Greetings,Customer

what is the vehicle year,make, model and engine size.

Thank you,
Bob
Customer: replied 8 years ago.

heater blows hot then cold. There seems to be no reason. The water temp fluctuates but the heat will also blow cold regardless of the fluctuation than blow hot again.

 

Optional Information:
2001 Pontiac Grand Am 3.4

Already Tried:
Is not the blower or loose belt. Recently had the intake gasket and thermostat replaced. The heater will go back and forth between cold and hot air. It doesn't matter if the engine is idling or driving. It has no apparent connectin to the water temp. It will at times get over-heated and overflow but the effects on the air are still random. (It can be blowing hot air then cold at 220 degrees or 160 degrees)I have used the bleeder valve to bleed out any air but it makes no difference.
I have left the top off the expansion chamber while the car was idling to circulate out any trapped air.
It seems like the only explanation is trapped air but shouldn't it eventually make its way to the highest point (the expansion chamber)and stay there.

Expert:  BigBob replied 8 years ago.
Hello again,

You need to first check both heater hoses and see if they are both VERY hot. They should be within 8 degrees of each other. If they are both hot let me know, then you will have to check the temp blend door.

Thank You,
Bob
Customer: replied 8 years ago.

I checked this last night and one hose was hot and the other was warm as it should be.

Expert:  BigBob replied 8 years ago.
Actually they should both be pretty hot not just warm, 8 degrees difference, in this case the heater core could need to be back flushed. If you are satisfied they are hot enough you are going to have to check the temp blend door.

The purpose of the heater is to supply heat to the interior of the vehicle. The vehicle operator can determine the level of heat by turning the temperature switch, located on the HVAC control assembly, to any setting. The temperature switch can change the vehicle's air temperature regardless of the HVAC mode setting, heater, A/C or OFF.

The underhood fuse block provides power to the air temperature actuator through the ignition 3 voltage circuit. Power and ground are provided to the HVAC control assembly by the ignition 3 voltage, battery positive voltage and ground circuits.

When a desired temperature setting is selected, a variable resistor is used to determine the air temperature door control signals value. A resistor inside the HVAC control assembly provides a varying ground to the air temperature actuator through the air temperature door control circuit. This changes the 12-volt signal coming into the actuator and varies the voltage so the actuator is moved into the proper position. The motor opens the air temperature actuator to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. Ground is provided by the ground circuit.

 

graphic



graphic

You will have to monitor these voltage as the problem occurs, if you lose voltage to
the actuator you will have to probably replace control head or check the wiring between.

Good Luck,
Bob
Customer: replied 8 years ago.
That doesn't explain the fluctating water temp.
Expert:  BigBob replied 8 years ago.
How do you know it is fluctuating? Are you relying on the guage or do you have a heat gun? Didn't you say:The water temp fluctuates but the heat will also blow cold regardless of the fluctuation than blow hot again. You may have 2 different problems.

Bob