I've been working on my '98 Jimmy reclamation project. When I got it at the beginning of the month it didn't run because the intake manifold gaskets were bad and the oil was full of coolant. I suppose it could have been started but I didn't want to start it. I replaced the gaskets and did some other maintenance while I was at it, all thanks to the collective knowledge of the Blazer Forum.

Once I got it running I started working on the other issues: the power mirrors are intermittent, the ignition lock cylinder doesn't always return to the OFF position (which I've mentioned in another post), and the HVAC controls don't change the temperature of the incoming air from hot.

After doing some searching in the 2nd Gen section I found out about the blend door actuator and how to access it without removing the dash. I took out the motor and noticed there was no wire connected to it. After searching around in the dash for a while I found the wire that should have been plugged in. I plugged the wire into the motor, started the truck, and it worked! When I rotated the temperature selector the motor turned. I reinstalled the motor and put everything back together and when I tested it out the temperature changed properly. This leads me to believe that someone previously had removed the dash and not connected everything properly.

Now that the temperature changes properly I have to look at the next HVAC problem: no air goes to the floor vents. Thanks to all the great information I found here I know what to check out next. It seems that the diverter door in the heater box breaks. If all of the actuators have proper vacuum then I have to investigate the possibility of the broken door.

I'll let you know what I find.

 

right...

 

I'm back to work on the '98 Jimmy and I tore into the HVAC system this evening. I removed all the kick panels and discovered that one of the return springs was off one of the actuator levers. That helped the door move properly. The greater problem I discovered is that the HVAC control head leaks vacuum at the switch. I swapped the control head with another one I got with the truck from the previous owner. The controls worked properly for the first five minutes but then that switch started leaking as well.

Is there a way to improve the seal for the switch or should I just find a new control head?

 

Check for fluid in the vacuum lines. Lots of discussion on this topic (transfer case vacuhm switch is a good place to start). Both my trucks have had t-case fluid in vacuum lines and both times i had to replace the HVAC control unit (after cleaning out the lines and replacing the t-case vacuum switch).

 

Could be one of the actuators has a leak, or maybe a vacuum line wasn't put back on correctly when the dash was out the first time The table at the bottom tells which ports have vacuum in each mode switch position. Make sure the vacuum reservoir in the driver front fender is connected too.

 

I've already done the obvious things. I checked for fluid in the vacuum lines and found none. I also checked all of the actuators by hand and with a vacuum pump and they checked out OK. The previous owner replaced the vacuum lines from the manifold to the tee and the reservoir and all of those are OK. The second control unit I installed worked for about two minutes until it started leaking. During those two minutes it properly routed the vacuum to the right actuators. It's definitely the vacuum routing switch in the control head. I even took both switches off and rigged up a test hose and blew through it and the switch leaked. It's definitely the switch.

This morning I called Chevrolet of Puyallup to check on the availability of a replacement control unit. I've had a lot of problems with a lack of intelligence at that dealership in the past but recently they've been taken over by another dealership so I thought they would be smarter. No dice. He told me that the part is no longer available. I called Sunset Chevrolet in Sumner and in five minutes the parts guy had one on the way up from Portland. I'll pick it up around noon tomorrow.

 

I installed the new heater control unit today and it works great. The mode door actuator moves a little bit slowly so it's a little slow to transition to heat but I can live with that.

 

One other thing—many thanks to Captain Hook for posting that vacuum diagram. I have a shop manual for a '99 S/T Series and apparently the vacuum diagram for that year is slightly different from what's in my truck. I went crazy checking vacuum on the various lines to the actuators and not getting the results in my book because the vacuum diagram in the manual was slightly different. The one posted above was correct!

 

Sometimes there are changes from year to year with vacuum lines. The schematic posted above is for 98 manual HVAC. Automatic climate control is totally different from manual HVAC.

 

My book is the '99 S/T series book and it definitely shows different vacuum line routings. In fact, my diagram is for RPO C68 but my truck is RPO C60. Apparently there was a significant change between the two years.

Thanks again!