Old guy needs some help, as the title says only have vent air from lower only. Checked vacuum canister output port and have good vacuum there. Heater valve works fine,direction lever works on off and the default lower vents. Have taken out lower dash and Bottom AC ducts, I can only see 1 vacuum valve at this point,no loose lines. Where do I go from here

thanks

 

I'll ASSUME you're talking about a car with A/C, since the systems work differently. There are two vacuum diaphragms. The one you are looking at is the defroster door actuator. The one you need to check is the mode door actuator, which is on the firewall side of the HVAC box. The mode door actuator should see some vacuum in all lever positions EXCEPT Max A/C. Check this with a vacuum gauge. Also see if the actuator actually moves when you move the lever from HEAT to MAX. This is all shown in Chapter 1C of your Chassis Service Manual.

 

Thanks for the reply, yes it is an AC car, with the mode door actuator being on firewall side that means removing the complete heater box

 

You should be able to access the hose and at least feel if the actuator is moving without removing the box. To take that box off requires accessing the nuts under the passenger side front fender, which is not something you want to do if you don't have to. You really want to get a Chassis Service Manual if you plan to work in this car.

 

Thanks again, getting a csm is at the top of my list

 

There are currently over 15 original paper CSMs (not reprints) on ebay. The lowest-price one has a $15 starting bid.

 

Ordered,and thanks for your help

 

Got a chance to work on this today, connected the main vacuum feed line to the defroster and mode door actuators and they both function as they should. In my mind this only leaves the vacuum control valve or miss cconetions of lines to the valve.
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Connect a vacuum gauge to the hose that connects to the mode diaphragm and see what you get in the various lever positions. That selector valve does wear out, which can cause it to leak. This results in insufficient vacuum to operate the diaphragms.

 

Did not even move the gauge a little bit pulling 22 to 25 on main feed

 

Well, that narrows it down. Either the valve is bad or the line is plugged.

 

Got the right nut off for the control panel, dropped the steering column to get to the left nut. Will connect gauge to selector valve and post results

 

Well I feel stupid, worked 2 hrs on wrong nuts, could not believe they used a stamped nut to hold the unit in. Took the bore scope to see what the problem was and lo and behold a flange nut on both sides. Back on it Saturday

 

Qkay, I cry uncle on the 2 nuts holding the unitin, can see them with the bore scope and get a socket on them but none wants to fit. They feel like 7/16 can you confirm that. Sure wish the csm had gotten here by now.

thanks.


To qoute Jagger

We used to be young, good looking and stupid

now we're just stupid

 

Finally got the unit out, vacuum main feed before the T is 20/23 at the selector valve is 10/12. Do not get any reading at selector valve except for the defrost port. At this point I am tempted to get a rebuilt unit. Is the part guy good and reliable.

thanks again

 

New unit showed up today, hooked it up and air now comes out where it should

 

Well. done.

 

Joe, thanks much for your help it's been over 45 years since I have to troubleshoot a GM car. The CSM just showed up today