Rear heater question, 2003 Envoy XL

[{tpc}]

Hi all, been reading the forums again this morning looking for a new issue that has seemed to pop up but I couldn't exactly find it. Anyways, here goes. So typically we don't have rear passengers. But the other day I was driving my kids home and after about 10 minutes or so of driving, (was like 0 out) I turned on the rear heat control. They said it was really cold, thought a window was open or something. So I shut it off. So it got me to thinking, and today after my drive into work (which is an hour) I thought I'd check it out. Front heat controls seem to work just fine as I have heat, lol. So I set the rear control to "r" and the rear comes right on, I can use the rear controls to change the fan speed, but even with the temp all the way to "h", it feels as if it is blowing cold air. I assume this is what the kids were talking about, though I didn't check the upper vents, I know they worked fine this summer (at least for AC), and I assume the window comment was that it was also blowing down on them.

So I'm not sure where to go with this. Maybe its just a calibration issue? Or a stuck thermostat? (though I would think a stuck t-stat would also affect the front). The only other thing I can say is that after finally fixing the temp gauge, it seems to hold steady just above the first major hash mark between the lowest mark and the middle (210 is the middle?), though I think its off slightly and maybe a little high because it starts off slightly above the lowest mark (100?). Had a hard time calibrating that one and the battery gauge because both were completely frozen so I wasn't sure where to make my original "mark" of the lowest point of those needles. I did notice yesterday (first run after replacement) that the needle went a little colder as a drove, at least at first. But it was like -5 or something. Today it was 8 out this morning and seemed to be more stabilized.

Are there any codes that might be thrown for something like this? My SES light is on solid (though last week it would clear itself randomly), and I have yet to pull the codes to see whats up. Thoughts?

 

[hockeyman]

I have the same rear heat setup as you in my '03 Envoy XL. My rear controls didn't work at all when I first purchased the truck, and I narrowed it down to a faulty rear HVAC unit.
Anyhow, after buying/installing a new HVAC unit, the rear heat worked, but it takes a while for the air to warm up, even when the engine is at operating temp. It literally takes the rear vents about 4-5 minutes to start blowing warm air out of them. If you're not getting any heat at all, then you might have a clogged line going to the rear (second) heater core. There is also an actuator of some sort that you can check too, and I think it's located under the console somewhere near the rear HVAC unit.

Just for reference, this is what my rear HVAC setup looks like:

As for your Check Engine Light;
You can have Autozone scan it for free. If it goes on/off randomly, it "could" be a faulty gas cap, but I would definitely have it scanned first to make sure. Bring a pen & paper with you to write down the code(s) as they're being checked.

 

[The_Roadie]

Recalibration of the rear actuators is done using the same fuse pull process that's been documented many times for the front unit. Typically if an actuator fails the recalibration process in the rear, its behavior will be that it clicks every time on a start-up for a minute, then the control module notices it's not moving properly and then it quits sending commands to the offending actuator and then it goes silent. The rear heating performance was so poor on early models like yours that GM added a liquid booster pump to send more hot coolant to the rear piping, located on the passenger side firewall. You may or may not want to spend the money retrofitting this pump kit.

The other fault you may have is a frozen valve, again on the firewall, that uses a vacuum hose from the intake manifold for powering the solenoid valve. If this doesn't open when the rear HVAC controls are calling for heat, then the heater core in the rear won't have any hot coolant to work with. You should be able to trace the supply and return lines for this HVAC unit under the vehicle in the right rear corner. See if one of them at least gets hot at all.

Here's my classic illustration about the parts involved, underneath the right rear cargo side panel cover.

 

[{tpc}]

Thank you both for the replys! I think I'll start by running it on high via the rear controls (if that matters) for the ride home, and check it when I pull in for any heat at all. After being on for an hour +, if I don't have anything I will have to go a bit further.

 

[{tpc}]

Update- I think everything is working as intended.

What I noticed was that it took a few minutes (maybe more than 5, but ok) to get actual heat out of the back vents. All the controls work properly, it was the time issue that I had overlooked. The upper vents seem pretty useless in the wintertime, because the best I got out of them was lukewarm air. I also noticed that when you set the controls to upper and lower in the back, the upper blows pretty good, but the lower suffers. If you set the controls to just one of the other both work fine. And if you set the controls via the front knob (1,2,3 instead of R) that only the floor works, which seems odd, but maybe thats just how it works.

Anyways, it appears everything is in good working order. Thanks for the info!!

 

[oh05ext]

i noticed that on our truck if the front is on defrost the rear heat doesnt work correctly but when you switch the front to dash the rear heat works fine.im going to buy the add on pump from gm and get it installed as soon as i get time.

 

[The_Roadie]

i noticed that on our truck if the front is on defrost the rear heat doesnt work correctly ...

In what way? Temperature of the heated air coming out in the back, or the airflow amount moved by the blower fan?

 

[oh05ext]

the temp. theres tons of airflow. its weird. if the front is on defrost the rear heat doesnt get hot but if the front is on the dash vent the rear heat is fine.ive probably got soem effed up actuators but in this cold im not checking crap.even my beer freezes in the garage overnight.

 

[The_Roadie]

....even my beer freezes in the garage overnight.

Need higher alcohol content beer, it seems.

View attachment 32418

 

[oh05ext]

anything over 8.5% alc goes in the fridge with the other good stuff.cheap beer gets garage time.

 

[hockeyman]

if the front is on defrost the rear heat doesnt get hot but if the front is on the dash vent the rear heat is fine.ive probably got soem effed up actuators but in this cold im not checking crap.

Yep, mine does the exact same thing. Either we have the same "ghost-in-the-machine", or it could simply be a bad design from the factory. Either way, I'm not too worried about it. The front heat works stellar and all of the A/C vents work great in the summer. Life's good!

 

[The_Roadie]

I'm thinking that when you select defrost mode up front, the designers thought you were serious about losing visibility out the windshield, and the system shuts off the rear heater core coolant flow using the firewall valve. So all possible heated coolant is used up front. I'll check the shop manual - I don't have an EXT/XL so I might have skipped over mention of this before.

 

[oh05ext]

Hey roadie do you know a way to trick the firewall valve into the open position at all times so I dont have to buy the $170 pump kit

 

[The_Roadie]

Connect a source of switched 12V (like from the RUN bus) to the gray/black wire at the solenoid valve. But I don't think that will work. The pump cures a design mistake that the coolant flow is so low when the valve is open, that the long tubing run to get to the rear heater core allows the heat to be wasted on the way.

 

[oh05ext]

hmmmm let me check that out this weekend. unless, i can find a pump to rig up in the meantime. i was thinking of using a supercharger coolant pump or something along those lines.

 

[RyanEAS]

. The rear heating performance was so poor on early models like yours that GM added a liquid booster pump to send more hot coolant to the rear piping, located on the passenger side firewall. You may or may not want to spend the money retrofitting this pump kit.

I "May" be one of those fine folks willing to spend the money retrofitting with this pump kit you're talking about. Where would I get more information on that? I'm assuming with this pump kit for the rear heat, that it circulates more coolant to the rear heater core faster and thus allows heat to come from the rear vents sooner?