My '89 S-15 has problems with the heater/ AC. No matter what I do ll I get is defroster. The temp works fine, but nothing works with regard to switching from defrost to heat or AC. Have changed the vacuum control unit and checked all diaphragm... Nothing seems to be wrong, yet I have no system working. What can be the problem? How does the vacuum get to the control unit on the dash? I am at a loss...HELP!

 

Did you check the blend door?

 

I made sure the cable is working properly and it is. I ran a vacuum line from under the hood and made sure all bellows are working and all the doors open and close, but nothing except the temp (which is cable operated ) works. It will not switch from defrost to heat, to max air, to vent, or to anything vacuum run.

 

Hello

I just finished repairing my 94 Jimmy by swapping out the climate control unit.

My original issue was a bit different from yours as for I couldnt get heat to save my life....even after a new heater core. I have a thead in 1st Gen section, however no one has replied or commented just yet, but may hold some use for future forum users.

Anywho, when changing out my climate control I was having a hell of a time making all the connections to the unit ( 2 harnessess, that stupid cable clip which is still jacked on mine and then the vacuum connection deal).

If you do a search on eBay, you will more than likely find a pic of the climate control unit withe the vacuum lines attatched, since most salvage yards just snip the lines when pulling the unit.

Long story short, just infront of those vacuum ports within the climate control is a wheel that plays pretty much the middle man between vent selections as for it is rotated back and forth with a plastic gear as the **** selector is lifted or lowered per the user.

It would be my guess that the little wheel in there has come apart and its stuck in last known postion, or the mess of vacuum lines has pulled free from climate control unt.

Just my. 02 that I can throw your way to maybe help.

Goodluck!

 

Thanks for responding. My issue was no vacuum to that vacuum control thingy in the control panel. I had nothing, so naturally no doors open or close leaving the default open giving only defrost. I needed a vac-wiring diagram to tell me which line fed vacuum to the unit so I could trace the broken hose or replace it. I tried everyone from Autozone to the GMC dealership. Everyone has the electrical wiring, but no vac diagram anywhere. I ended up getting a hand pump vac-tester and running through the lines making sure which line opened which door. Once I did that by process of elimination I figured out which one was the feed line but never 100% found where it goes through the fire wall or where it hooks into the engine system. I got it working by drilling a 1/4 inch hole and running a new line through and connected it to the vacuum line between intake and the big black ball that holds vacuum when the engine is off. I don't know where the factory live is broken or which one it is, so I still have an engine vacuum leak, but it isn't a big issue so I've been driving it as is, until the weather gets nicer and I will try again to find and cap it. At least I have AC and heat now. The engine has just shy of 3000 miles on the new rebuilt 4.6L now, and I replaced all the AC components. Replaced the rear differential, and got all the 4 wheel drive working properly, so its almost a new 1989 S-15 Jimmy...except the digital dash cluster comes and goes, and the body is still a bit rough, but who wants too much new on the outside to knock off on some tree offroading anyway? LOL ..... Thanks again for your response.

 

10-4 partner...I must have misunderstood your post...sorry I couldnt have helped more. However, on the flip side...congrats on keeping one of these beast alive and on the road...build sounds awesome!