AC blows out floor and defrost vents?

[davidijaxson]

Originally Posted by CamP

I've read that if both the cruise control doesn't work and the dash vents don't blow, the cause is either the vacuum check valve under the plenum is bad or the vacuum line to it is disconnected. Both use a common vacuum line that originates on the right side of the plenum.

I have a buddy that?s a pretty decent mechanic. We have checked everything external to the plenum. Or at least I believe we have. I am guessing next step Is to locate a new check valve and then remove the plenum. Or is there some type of test you can perform on the check valve to determine If it is bad?

[402Z]

I?m having the issue where I have cold ac but it only blows out of the foot and defroster vents. I?m guessing it is possible the HVAC programmer then?

[DRM500RUBYZR-1]

Originally Posted by DRM500RUBYZR-1

Insufficient vacuum is often found to be the culprit for the inability to direct air out of the selected vents.
The programmer is often suspect, however there is about a 1 in 3 chance that the supply vacuum is the actual cause. Squashed, kinked, cut, or varmint chewed hose in that order have all been found.
If the supply is good, then the hose connections on the programmer are the next stop.
These components are all now about 30 years old.
Always diagnose first before replacing.
Rarely do we replace the C-4 control head, (unlike the c-5) with most problems of failure to direct traced back to vacuum lines or programmer connections, then the programmer itself.
Take your time and check carefully and you will soon have full control of your HVAC.

Marty

OK, if you simply want to fix it by replacing parts that may be fine, go ahead and remove the programmer and send it to Batee.

If he finds it to be fine then you know that you have a vacuum problem, or a diverter door hanging up either because it is stuck or receiving insufficient or no vacuum to move it. If the programmer is fine, then based on the vent mode selected, the programmer via vacuum, operates the diverter to send air to the selected output.

The proper way to figure this all out is to follow the troubleshooting guide in the FSM as it logically tests the functionality in the correct sequence.

The control head tells the programmer what to do, but it requires vacuum to execute the instruction.


Marty

[carman49]

I had the problem of the AC only blowing out the defroster and floor when I bought my car a few years ago. The blower speeds and temperature settings all worked on the control head, just not the switching modes.
I first wanted to make sure I was getting vacuum to the programmer.
Getting there is fun.....
Remove drivers seat and under dash panel. Lay pon the seat tracks with your feet in the rear storage area and your head under the dash. Remove one bolt holding the programmer in place. It has slide in clips at the top to hold it. I removed the vacuum connector and checked for vacuum with the engine running at the source hose. I had good vacuum. I then applied vacuum to each hose with a Mityvac and checked that the vacuum motors held vacuum. They each held and when I removed the vacuum I heard each one clunk to the closed position. I called Batee and he had me run some resistance checks of the inside and outside sensors. They were all in specs according to him so he had me send him my control head and programmer.
The previous owner had a shop replace the programmer with a used one in the past.
He called me and told me that the programmer was not the correct one for my 90, it was for a later ZR1 and that the programmer for a 90 ZR1 is a one year only part for only a 90 ZR1. He said that the compressor should not have came on, but it did. He replaced the vacuum solenoid assembly and sent the programmer and control head back to me after checking it all out. I installed the parts back and everything works perfectly.
It is so good to have cold air out the dash vents here in Texas summers.

[GOLDCYLON]

Originally Posted by carman49

I had the problem of the AC only blowing out the defroster and floor when I bought my car a few years ago. The blower speeds and temperature settings all worked on the control head, just not the switching modes.
I first wanted to make sure I was getting vacuum to the programmer.
Getting there is fun.....
Remove drivers seat and under dash panel. Lay pon the seat tracks with your feet in the rear storage area and your head under the dash. Remove one bolt holding the programmer in place. It has slide in clips at the top to hold it. I removed the vacuum connector and checked for vacuum with the engine running at the source hose. I had good vacuum. I then applied vacuum to each hose with a Mityvac and checked that the vacuum motors held vacuum. They each held and when I removed the vacuum I heard each one clunk to the closed position. I called Batee and he had me run some resistance checks of the inside and outside sensors. They were all in specs according to him so he had me send him my control head and programmer.
The previous owner had a shop replace the programmer with a used one in the past.
He called me and told me that the programmer was not the correct one for my 90, it was for a later ZR1 and that the programmer for a 90 ZR1 is a one year only part for only a 90 ZR1. He said that the compressor should not have came on, but it did. He replaced the vacuum solenoid assembly and sent the programmer and control head back to me after checking it all out. I installed the parts back and everything works perfectly.
It is so good to have cold air out the dash vents here in Texas summers.

Thanks for the close out. It helps out the next guy or gal

[CamP]

I never followed up on my vent issue. It turned out to be the vacuum line to the ac controller was pinched between the plenum and injector housing.

[Young1]

That is correct. Cruise and AC share a common vacuum source on RH plenum. Secondaries vacuum source is the pump and LH plenum. Stuff under the hot plenum shrinks and ages and gets loose. Not as likely under the dash. I would also consult the FSM and follow the diagnostic tree. Vacuum stuff is pretty easy. Electrical not so much. Your patience and diligence keeps the legend alive.