HVAC issues

I have a '98 Jimmy 4dr 4x4 that I have owned for just over 4 years. Ever since I got it there has been constant issues with the Heat or A/C or combination of the 2. It has the manual HVAC controls. Sorry this is so long but I might as well put it all down the first time. Here it goes.

I got it in June so the heat was not even a thought till one day I could not defrost the windshield on a foggy morning. All I was doing was freezing the water to it instead. Come to find out I has a 90% plugged core that the dealer happily (after much discussion with the used car department and the service manager) backflushed the heater core and also fixed the wiper motor controls. Well I had heat now. More than I knew what to do with. No amount of running the A/C could get cold air out of any vent I selected. The vents all changed with the selector but all I got from them was heat.

Good news is that that only lasted about a week and since I was so busy at the time I put it off for 6 months. I had to back flush the core again. I also changed out the HVAC controls mounted on the dash. For a short time I could slowly move between somewhat cold to somewhat warm. Then the core plugged up again and all I got was cold.

My neighbor, a Cadillac mechanic and former Chevy mechanic, helped me disassemble my outer heater box, inner fender and remove my dash. About 4 hours total including a test drive and repair of a former alarm/remote start install by some hacker with wire taps. I had heat again and it worked well for about 2 months. I had to do a flush and fill of the dexcool and another gentle backflush of the core and I had heat again and for the next year. However the selectability eventually stopped and I was just about stuck to all full cold or full hot.

I've been putting up with that too long and want to get it fixed. I have also switched to the green stuff 100% in a 50/50 mix so I hope the plugged cores are behind me.

I purchased a blend door motor from GM. I just have not installed it yet because the only way I can see getting to it is with the dash out again or with the passenger air bag removed. Neither task I am comfortable doing alone. Are there any other controls in the cooling system that would point to why I can't blend the temp how I want it?

Make sure you have good engine vacuum to the AC/Heater controls.
Lot of problems withthe vacuum tanks and plastic and rubber vacuum lines.
Get a vacuum gauge and test vacuum all of the way from engine to heater control.
Also make sure the heater controls are switching good.
Good luck

The vacuum supply to the controls is good and I get changes to where the all hot or all cold air is going to when I spin that dial.

It seems to be with the electrical thermostat dial not really controlling any blending. The 3 pin connector on the rheostat does seem to be good. I forgot what the ohm reading were but they were identically opposite of each other when using my fluke to read the different combination of pin pairs. When the outside and middle were compared to the other outside and middle the readings were low-high and high-low with the spin of the dial.

I'm thinking it is the blend door motor and I have a replacement it is just a job I am a bit leary of since the right side airbag needs to be removed to get any sort of room to get the 2 screws removed.

Go up in the how to section and you will find some info on bleed door motors and how to do them.
http://www.automotiveforums.com/vbulletin/forumdisplay.php?f=2069

Let us know how it goes.

I spent an hour looking at the how-to pages last night and came up dry. Maybe I was looking at the wrong page. Either way thanks for referring me to the right spot MT.

It sounds like it's probably your blend door motor. Alldata says your'e supposed to roll the dash back to access it, but if you think you can get to it by removing the air bag, do it. I know it can make you nervous if you've never removed an air bag before, but just disconnect the battery to disarm the system and you'll be all right.

I spent an hour looking at the how-to pages last night and came up dry. Maybe I was looking at the wrong page. Either way thanks for referring me to the right spot MT.

You are welcome.
There are down in the S-10 forum.
Good Luck and let us know how it goes.

Ya rolling the dash was not fun the first time we did that to remove it when the core was replaced. That is a 2 person job and there are entirely too many things to remove. by the time I would be ready to roll the dash I would be done doing the job thru the glove box hole.

I did however follow the directions MT posted a reference to. It took me an hour and I just completed. I can tell you the screws are too long by about half to make it easy on the mechanic's hands.

I used the 7/32 socket and a 1/4-3/8 adapter so I had something to grip and spin the socket. Once the screw was loose the 1/4 drive ratchet would not ratchet any more. Why are there so many sharp surfaces under there? I imagine it would be easier to remove the front seat and if I had smaller hands.

Anyway the new motor is a 4 pin unit and I was a bit concerned but after looking at the connector that feeds it there are only 3 wires used and all the pins were there on the new one that are in the plug.

I got it all put back together and started the truck back up and gave it a whirl. Seems now I have a variable climate control again. I took the old one apart and the motor and gear set works correctly when I supply 12v to the motor directly. May have fried the electronics inside of it.

Now I just need to tackle that leaking intake manifold gasket (felt the leak at the back of the block at the head/manifold/block joint). That one will take me a bit longer than an hour.

I guess if I were going to make things perfect I would also replace the broken seat adjuster on the passenger front seat, the gate struts and mount a real class 3 or 4 hitch instead of using the 3500/350 pound bumper that is too high for my trailer in some situations. I'd also like to figure out how to wire up removable amplifier power/signal cabling for a small removable subwoofer I have had sitting in my garage for 5 years waiting to go into one of my vehicles.