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No, not with this design ( if you are talking about a liquid valve ), Chevy of this era has the hot hose direct to the heater core from the intake manifold, from there it goes straight back to the top of the radiator.

The only control for heat / cold is a few blend doors, the heater core is always charged.
when the heater core blend door is closed there is no air flow through the plenum.
these doors get sticky all the time, and the electronic module that controls them cannot overcome the force needed to fully open or close them.
( or the gear set in the modules crack, also common, but this problem has been addressed by replacement. )
I guess then you can open the doors somehow... and then it will be about 6 months until you need to be worried about closing them.
 

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Discussion Starter #22
Dug into it a bit over the weekend... I removed the black plastic on the cowl (just below the windshield) and removed the cabin air filter. From there, I could see the blower fan and the recirc door. I was able to verify that the recirc door is working properly. Unfortunately, closing it while I'm driving doesn't do much to improve the temp.

these doors get sticky all the time, and the electronic module that controls them cannot overcome the force needed to fully open or close them.
( or the gear set in the modules crack, also common, but this problem has been addressed by replacement. )
That really seems to be the most likely explanation, though I should point out that neither the old or new actuators clicked or seemed to be broken. In fact, the old actuator turns w/ such force that I couldn't stop it w/ my fingers. When I changed the actuator, I also tried to move the blend door w/ my fingers, but I couldn't. Of course, I couldn't get a very good grip on the cam that sticks out. I tried grabbing it w/ needle nose pliers and still couldn't move it, but I didn't want to apply too much force and damage it. But given how strong the actuator is, it didn't surprise me that the door was difficult to move. I guess I don't have a good feel of how much force should be necessary to move it.

Anyway, the question that I really haven't been able to get an answer to is what is the best way to access the blend door? I have an exploded diagram of the HVAC system, but I can't tell how much of the dash needs to come apart to access it. I hate to just start taking it apart becuz I'll probably take out a lot of stuff that doesn't need to come out, but I think I'm at the point now where I don't have any other choice.
 

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For my truck the entire dash and one of the ducts had to come out. Its a 05 Silverado. So if your exploded view looks similar to the exploded view of a 05 Silverado...there you go. I've noticed that most of the GM hvac systems are similar. My truck has dual climate control with rear seat vents so they are likely close.
 

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A few thoughts, flush and fill coolant system. Air in coolant system. Crud inside heater core. Have you tried a higher temp thermostat? In winter I've often seen people cover half of the front of the radiator (I see newer Ford trucks with a louver system built in front of the rad itself). I heard from a mechanic once that if the AC refrigerant sets loose from the system it will cause dust to stick inside in all the heater vents causing restrictions. Get yourself a 2001 Civic, they had heat like nobodies business.
 

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Discussion Starter #25
Something very interesting happened on my way home yesterday. I had the air coming out by my feet, and it was about 90° (remember it should be in the 140-160° range). Then I switched it to vent, and got noticeably warmer. I had the IR thermometer w/ me, and the temp steadily climbed to about 130° before I got caught up w/ traffic and had to put the thermometer down. Shortly thereafter, I switched it back to come out by my feet, and the temp dropped back down to about 90° again. I tried switching it back and forth to vent several times, but I could never get it back up to full temp. The temp control was all the way hot the whole time.

This indicates that there's nothing wrong w/ the cooling system, the heater core, or the actuator. So this lends even more evidence to the blend door...

According to the diagram I have, the blend door and the mode door are connected but should be able to move independently of each other. Maybe there's some obstruction that the mode door temporarily moved out of the way, or the bushing between them is worn. The blend/mode door part is #13 on the diagram (blend door on left, mode on right).

 

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Discussion Starter #26
Well I have an update... over the holiday break, it got so bad that I basically had no heat at all.

But I found out about an actuator reset procedure: with ignition key in the Run position, pull the HVAC fuse inside the car, wait 60 seconds, and reinstall the fuse. Wait another minute or so with the key on for the actuator to finish realigning. Unfortunately, this had no effect after 4 attempts.

But I thought about it some more and realized that I can hear the actuator as it goes thru its realignment. It whirs and then grinds to halt when the blend door becomes fully closed, then whirs again -- presumably to move to whichever spot it thinks it should be in based on the temp setting on the HVAC controls. So I decided to remove the fuse again right as the actuator grinds to a halt. That should keep it closed, or, if it's jammed, as fully closed as it can get.

The problem is that w/ the fuse out, the blower fan and mode selection don't work. So I drove home w/ the fuse out, left the car running, and reinstalled the fuse. When the fan kicked on, I had full 140°F+ heat!! Well, at least for a few seconds until the blend door actuator finished realigning:(

Still, this told me a lot. There's no problem w/ the cooling system, the heater core, the actuator, or the blend door being jammed. I can get the blend door into the correct position and get full heat, but I can't keep it in the correct position. And since the reset procedure doesn't work, the only other explanation is that the BCM either won't accept or doesn't retain the correct positioning information it should be getting from the reset procedure. So I think the solution is to get the BCM reflashed.

In the meantime, a pleasant side-effect of my reset experiment is that I have at least some heat now. It seems to change every time I drive the car -- sometimes it's 70°F, sometimes it's 110°F, and sometimes it's 130°F. It's certainly much better than it was!
 
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