I am looking for some advice.
Details: 1999 Chevy Blazer 4.3 4x4.
Problem: Heat only works when cranked all the way up. Most any movement (interior themostat setting) causes a switch to cool/cold air. My guess is the AC is kicking on.

Other info: New radiator, new coolant, new water pump. Hot water is flowing to the heater core (gauging by feeling the inlet hose). Coolant temperature guage runs about 170-180, seems normal.

I apologize for interrupting, I just value the information you guys have. Is this likely to be a "control switch" type problem?

Thank you for any help or ideas.

Bocomo

 

i have the same problem with my 2001 trailblazer, heat setting has to be past the 27 degree point for heat to work.....

 

If you think that the A/C is coming on, you can unplug the compressor clutch and try it out that way. I'm betting it is the flow valve for the heater. IIRC, there is a diverter valve that will open and allow hot coolant to flow through the heater core.

Do both of you have auto-climate control?

 

Yes, I do on my 1999. Set the inside temperature to 74 or 78 and it blows cool air. Outside it is 16F. Put it all the way to 80 and it does blow warm air. Move down to 74 and cool air comes in.

Is the flow valve vacuum controlled? Sensor? Fairly easy to correct?

Bocomo

 

The fact that you have autoclimate control changes everything. It is mostlikely that you don't have a problem with the diverter valve, but with the interior temperature sensor. I have seen some get built up with dust so much that they don't properly sense the temperature inside. It is also possible that it has conked out and isn't reading properly. I'll have to do some more research and find out where the sensor is.

 

I'm hoping it is something I can do myself, if I know what to do.

Bocomo

 

Ok, now I'm going on the fact that your '99 should be the same Auto-HVAC as my '00 Bravada. There are 3 sensors on the inside. One is the interior temp sensor, and the other two are the upper and lower discharge temperature sensors. I'm not sure where any of them are. I'll have to look around on mine, but I do know that the lower one requires the Airbag on the passenger side to be disabled. The interior temp sensor should have a little 1" circle grid of small holes that should draw air into a duct that leads to the sensor. It is possible that the duct is not pulling air in properly and/or the sensor has crapped out. Tomorrow I can snap some pictures if I can find what you need.

Below is a chart of the proper resistance values for each sensor if you do find them, you can verify that they are working...

[IMG]local://upfiles/667/90476E687DC04A66991A707AC001B5C0.gif[/IMG]

 

I do suppose that the outside sensor should be checked as well. It is located behind the grill in front of the radiator and should be easy enough to pull and check the resistance at different temps.

Good Luck.


**EDIT** - I did just found some more information. The diverter valve is not on the heater hoses diverting the coolant, it is in the airflow and blocks off air passing through the heater core. I do suppose it is possible that the door is not moving properly or has lost its adjustment.

 

This is great information. I may have some trouble checking this all out, but I have some time tomorrow.

I really appreciate your time and effort. I owe you a beer..........or more.

Bocomo

 

Then I owe him a case....