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2004 chevy trailblazer_lt_ext
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Discussion Starter · #1 ·
I've been looking around for quite a while and cannot find the issue that i have. the issue is that my rear heat takes forever to get warm. example it's 30 degrees outside and after 5 minutes of driving i can turn on the front heat and it's blowing warm air. so i'll turn on the rear heat and it's blowing cold air, so I leave it on and 5 min later it's still "cool" not warm like up front. I don't understand why it would take so long for the rear heat to get hot due to the fact that if the motor is warm and the front is blowing hot air why wouldn't the rear. I have the digital hvac and i've done it both ways ie: setting the teimp up front at 90 and making sure it's only blowing out the top vents and then i tryed using the digital control in the rear. same issue.. i'll still get heat but it's not as hot as up front and it takes forever.. Thanks a bunch to anyone that can help me fix this issue i have a 2 yr old boy that is gonna freeze this winter if dad cant figure out this problem!
2004 ext,digital hvac,90,000 miles:thx
 

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2004 gmc
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26,181 Posts
There's two actuators in the rear - one for airflow mode and one for temp. Assuming you get good hot coolant flow to the heater core back there, you have a bad or failing temp actuator.

I've posted the exploded parts view of this area before, which you could find in a search. You have to remove the passenger side rear interior panel, poke about and feel for hot coolant hoses going to the heater core, and then observe the actuator move from end to end as you fiddle with the rear temp control.

If you have access to a trusted GM-experienced mechanic with a high end scan tool, they should also be able to talk to the HVAC control module and see if the actuator is following its commands.
 

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2002 gmc envoy_slt
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349 Posts
Service manual suggests feeling the inlet and outlet hoses of the coolant bypass valve (below the accumulator, #6 in the attached picture) after the engine has gotten up to temperature with the rear heat set at its warmest setting. If there is a noticeable difference in temperature between the two hoses, there is probably an issue with the valve or its controls.

There is also a service bulletin for poor rear heat performance on 03-05 extended Trailvoys: #05-01-37-002A. Here's the brief description:
Condition
Some customers may comment on poor 2nd and/or 3rd row seating heater performance when the vehicle is at idle.
Cause
This condition may be due to lack of coolant circulation to the rear heater core.
Correction
Install auxiliary heater pump package, P/N 89024885 (L6 Engine RPO LL8) or P/N 89024886 (V8 Engine RPO LM4 or LH6). This pump will improve coolant flow to the rear heater core.
 

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2003 chevy trailblazer_lt_ext
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625 Posts
:iagree: I have the same Problem after 10 - 15 minutes of driving mine starts to get heat back there and after 30 - 45 minutes you can cook the people in the back seats out.
 

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2009 chevy
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:iagree: I have the same Problem after 10 - 15 minutes of driving mine starts to get heat back there and after 30 - 45 minutes you can cook the people in the back seats out.
Is this really a problem, or normal operation :undecided
The front HVAC sits on top (figuratively) of the motor, the rear has a lot of plumbing (passing through a lot of cold air under the truck) before getting the rear unit hot.
 

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2003 chevy trailblazer_lt_ext
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625 Posts
I don't think mine is really a problem it's just normal operation. The bulletin vball61 posted is more for the person that wants there rear heat to work more efficiently.
 

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2004 chevy trailblazer_lt_ext
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Discussion Starter · #8 ·
first off thanks a bunch you guys for taking the time to pull up the diagrams and taking the time to answer my question. I will print this out and have a look tomorrow and will update as to what I find. However this may sound dumb or not what exactly is a service bulletin? Does that mean the dealer covers the cost or does that just help the dealer with fixing the problem and doesn't save me any money? as for cooking people in the back it takes atleast an hour of driving before i could cook anyone in the back. I agree thoe that it would take a little longer to get warm air out the back vents due to the lines having to run the lenth of the truck. thanks again fellas
 

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2002 gmc envoy_slt
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349 Posts
A technical service bulletin is only intended to help the dealer diagnose and correct a known issue. The work still has to be done at your cost, or under warranty, or as a gesture of the dealer's good will.

In this case, due to the nature of the problem and labor & parts cost to execute the bulletin, it'd likely be on your nickel.
 

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2005 chevy trailblazer_ls_ext
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330 Posts
I just got this pump package installed. It works great and was covered under my bumper to bumper warranty. It calls for 2.5 hrs labor but the shop had never installed one before and it took 8 hours. cost would have been $430
 
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