Help w/ heating & A/C problem

[snampro]

Car: 1994 Viper with factory A/C

Problem: ventilation system blows air out the defroster port on the top of the dash regardless of setting.

Description: I rarely use the A/C or vent in my car (probably not since May 06), but I took my wife to dinner last night and she turned on the A/C and we thought nothing happened. When I got home I did a little troubleshooting and here’s what I found:

With the fan switch on high, temp on cold, and selector switch to A/C I hear the compressor engage, fan kick on, and cold air blows out the defroster ports.

Move the selector switch to vent and the air continues to blow out the defroster and the compressor disengages as it should.

Move the selector switch through off and to heater, still air blowing out the defroster.

Move to mix of heater/defrost, still air out the defroster and compressor engages

Move to defrost, air out the defrost ports with compressor still engaged.

Summary: Fan works, compressor works, switches work, but the internal ducting is not responding…any tips before I start taking the dash apart?

Thanks in advance,

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Ben, aka “snampro”
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[Tom, F&L GoR]

You are not getting vacuum to the two dashpots that control the air doors. You can get to the dashpots from under the dash on the passenger side. There is a plastic cover next to the fusebox - no need to take the dash off. Both dashpots should have a vacuum hose attached.

Since nothing is happening, the vacuum source may be disconnected. That is the thin, hard plastic line on the passenger side valve cover. Find this one and you can suck on it to "actuate" the doors by hand. Or by mouth, as the case would be.

If that still doesn't work the doors, then the "manifold" that attaches to the back of the panel with the heat/AC knobs may have come off. Now you do need to remove the dash. If you need any service manual pages, let me know.

[snampro]

thanks, I have the manual and am headed to the garage now...

update: Back from the garage and its none of the easy stuff:

Removed the small vacuum line from the passenger's side of the INTAKE MANIFOLD. Sucking on the line nets nothing, no doors move, and the pressure doesn't hold, i.e. I'm just sucking air through the line.

Removed the knee bolster and plastic cover to access the heater/defroster and vent actuators. All lines are connected. I can manually actuate the doors with no problem, and there appears to be no vacuum when I remove the lines with the car running and moving the selector knob. I also tested the actuator by using a hand suction tool and was able to make both move and hold a pressure. I then plugged the tool into the intake manifold line and it would not hold the pressure...

So, it must be the "manifold" under the dash. The manual doesn't really give any good pictures or descriptions of the manifold...any suggestions?

Thanks for your help so far, at least I have a long list of things that are not broken. I think I'll start removing the dash.

[snampro]

I've got the dash (gray part) off the car and there appears to be another under-dash that needs to be removed. I also appear that I will have to remove all the gages, radio, and hvac controls...any advice?

[Tom, F&L GoR]

Just a minute...

[Tom, F&L GoR]

I ran out to the garage and pulled off the gray part - it should look like this?



Well, not really, I had replaced a gauge before and took pictures. Looks like the knob assembly should come out by removing the four screws (two on either side.)

The manifold is the image on page 8E-8, Figure 17 (938E-17), heater control. It's the stuff on the bottom left.

[Tom, F&L GoR]

...rocker cover, intake manifold... those big red things.

[Randy]

I hope you verified the vaccuum line that attaches to the intake manifold is attached before taking apart your dash. As in, open your hood, and make sure the vaccuum line is plugged in on the outside top edge of the passenger side intake, about four inches forward of the firewall - i.e., no tools involved. This is the most common cause of this problem in my experience.

[snampro]

yes, I did make sure that line was connected. I said above I removed it from the manifold and attached a vacuum puller to it and was unable to pull a vacuum.

I removed the knob assembly and accessed the manifold. I removed the black line from the check valve and used my vacuum tool to pull a vacuum. I then turned the ignition on and was able to actuate all functions properly, so the problem is not between the manifold and the actuators.

I also removed and checked the check valve and it functions properly. now, when I plug the line running through the firewall and then pull a vacuum on the line that plugs into the intake I continue to get nothing. So...this means to me that the line running from the intake manifold to the check valve and on to the control manifold is leaking somewhere between the intake manifold and the firewall...

I also have another problem: when I removed the line from the intake manifold the fitting came with it. I appears that this fitting does not screw in, but was pressed in and possibly sealed with a glue or something...any ideas on how to reattached it? I'm thinking high-temp silicone and a slight tap...

Thanks again,


Ben

[snampro]

ALRIGHT!!! I found the problem, the vacuum line from the manifold must have leaned against a coolant pipe, the engine, or the trans because it was burnt through in a spot. I'm going to replace it with some tubing and run it behind the right wiper motor and into the cabin using the rubber grommet just inside the door. The stock routing is an engineering blunder...

now, any suggestions on reinstalling the fitting to the manifold? I may carefully tap the hole an put on a new fitting. I had to do this with the fuel pressure relief valve and had no problems.

thanks for all the help tom. since I have the dash off I'm going to paint it!!!

pic of the burnt tube below:

[Tom, F&L GoR]

My memory may be going, but doesn't the plastic tubing just push into the tiny rubber elbow on the manifold? If so, you only need to replace the tubing and splice (maybe with larger rubber tubing that the original fits inside?) to connect to the OEM tubing. Vacuum will generally keep them together pretty well.

Or, just cut out the section with holes and splice it and reuse everything else.

It's always the little things, isn't it!

[snampro]

the fitting I'm talking about is not the rubber elbow, I'm talking about the metal "nipple" that the rubber elbow plugs into, it pulled right out. I put a tiny bit of jbweld on it and tapped it back into place. Its pretty snug just like that, and I'm sure when the jb dries it will be fine.

I replace the entire line because the hole was fairly close to the center of the length and would have been difficult to repair. I ran the new vacuum line behind the right wiper motor, into the cabin through a rubber grommet near the passenger's right knee, and under the glove box to the knob assembly. I have yet to start the car because I'm waiting to make sure the port on the intake holds, but I did use the vacuum tool to verify proper installation and everthing is working well.

Thanks for the help,


Ben