Mysterious A/C vacuum tube

mtriplett · Jun 26, 2010

So I bought a 94 RT/10 a year ago, and the previous owner told me the A/C wasn't working, and I only needed to replace a vacuum tube. I rarely drive it because the fan doesn't turn on, and I don't want it to overheat.

I've tracked the fan problem past all the connecting wires and through the relays (all check out) and to the A/C Compressor Clutch Relay. It may be that the Clutch Relay is not allowing the entire system to work correctly. I have the sneaking suspicion that the vacuum tube may have something to do with it.

Problem is, it's not apparent where the vacuum tube connects to. It comes off the top-left side of the head (when you're facing the engine from the front of the car). Even the Dodge dealership couldn't figure out where it went. None of their schematics even show it as being part of the engine.

Below are a few snapshots to show you what I'm talking about. I'm hoping some of you have the same tube and can let me know where it goes (better yet, post a pic).

Thanks for your time

Murray

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crviper · Jun 26, 2010

Mine is hooked up, hold on will see where it goes!

crviper · Jun 26, 2010

Ok I just took my car apart for ya. And hope that it helps! On the third picture try to download it and zoom where I shine the light. The vacuum hose goes into the fire wall. And if you can that it's and outlet on the firewall!

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crviper · Jun 26, 2010

At least it's a starting point!
Ps. Oh be sure to remove the diffuser or the cowl cover. If you move that cooling line like I did in the pic you should see on yours either broken vacuum house going intonthe firewall!

AZTVR · Jun 26, 2010

That tube eventually goes to a very small check valve sitting behind the radio, and goes to the vacuum operated flapper door in the heater/AC evaporator assembly. It needs vacuum to hold the door in the A/C position so that the air goes through the evaporator core; otherwise the air goes through the heater core. The typical complaint is that the cooling shuts off at wide open throttle (very little vacuum) because that valve is not working and has no vacuum reserve. The fix for that is installing a different valve. It looks like yours is not even hooked up. You have to remove the radio to get to the valve. (This is all based on an '02. Someone can say whether I'm full of it in relation to a '94. The drawing in the parts book looks the same.)
http://forums.viperclub.org/rt-10-gts-discussions/578869-ac-shuts-off-when-gas-pedal.html

Does the A/C clutch engage? If it doesn't operate; then I would suspect the low pressure switch is preventing it from operating due to lack of freon.

mtriplett · Jun 27, 2010

That's fantastic information, guys. Thanks CR for going the extra mile to give me an excellent visual. And AZ, that will help in diagnosing the problems. I'll keep you posted on my progress.

Murray

crviper · Jun 27, 2010

I wish this issue was mentioned when I had my dash and radio apart. Removing the cowl was easy. But I am sorry I won't be taking my dash off again! Lmao! But I do hope that AZVTR and I helped you out in any form! Mine is a 94 as well so I can know at least that our setup is identical. And my factory air works great! It does cut off when you go are at WOT. But resumes when you are lower rpms. At idle cold air it still decent, but if your at cursing speeds it gets really cold! And the brand I use to fill my system is Artic Freeze ultra synthetic. Can get that pretty much any auto parts store! Oh don't mind the racing steering wheel. I installed one on my racing ridding lawn mower! Lmao!

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crviper · Jun 27, 2010

Please do my friend! and if there's any information you need from the shop manual I be happy to send ya pics or illustrations! Oh by the way my anniversary of me having my viper a year is this coming week. I bought my 94 on June 30 2009.

jdeft1 · Jun 27, 2010

Ummmm, It's all good fun.. but that is the vacuum line pickup for the basic function of the incredably advanced, pnumo-actuated hvac system on gen 1 cars...

As for the car over-heats: (specify please..) Not a vacuum issue IMO, Check the temp sensor....

Jack B · Jun 28, 2010

AZTVR said:
That tube eventually goes to a very small check valve sitting behind the radio, and goes to the vacuum operated flapper door in the heater/AC evaporator assembly. It needs vacuum to hold the door in the A/C position so that the air goes through the evaporator core; otherwise the air goes through the heater core. The typical complaint is that the cooling shuts off at wide open throttle (very little vacuum) because that valve is not working and has no vacuum reserve. The fix for that is installing a different valve. It looks like yours is not even hooked up. You have to remove the radio to get to the valve. (This is all based on an '02. Someone can say whether I'm full of it in relation to a '94. The drawing in the parts book looks the same.)
http://forums.viperclub.org/rt-10-gts-discussions/578869-ac-shuts-off-when-gas-pedal.html

Does the A/C clutch engage? If it doesn't operate; then I would suspect the low pressure switch is preventing it from operating due to lack of freon.

A little confused, the PCM shuts down the ac at wot.

AZTVR · Jun 29, 2010

AZTVR said:
That tube eventually goes to a very small check valve sitting behind the radio, and goes to the vacuum operated flapper door in the heater/AC evaporator assembly. It needs vacuum to hold the door in the A/C position so that the air goes through the evaporator core; otherwise the air goes through the heater core. The typical complaint is that the cooling shuts off at wide open throttle (very little vacuum) because that valve is not working and has no vacuum reserve. The fix for that is installing a different valve. It looks like yours is not even hooked up. You have to remove the radio to get to the valve. (This is all based on an '02. Someone can say whether I'm full of it in relation to a '94. The drawing in the parts book looks the same.)
http://forums.viperclub.org/rt-10-gts-discussions/578869-ac-shuts-off-when-gas-pedal.html

Does the A/C clutch engage? If it doesn't operate; then I would suspect the low pressure switch is preventing it from operating due to lack of freon.

Jack B said:
A little confused, the PCM shuts down the ac at wot.

I will certainly defer to your knowledge on that one. I don't dispute it. I've read posts that agree and others that disagree; however, the AC clutch relay is connected to the PCM so I certainly believe that feature would be part of the design..

In any case, I can report that at heavy acceleration (possibly not totally wide open throttle ?) the cooling air in my car would immediately turn warm as if it were not going through the evaporator. Replacing the vacuum switch with one of the ones recommended in the various related threads solved that issue. Perhaps I was not getting to the point where the PCM disabled the compressor clutch; and I was only experiencing the vacuum operated door issue.

mtriplett · Aug 10, 2012

Update - I was able to snake the vacuum tube through while I had my dash apart and plug it into the back of the AC unit. The vacuum enables the control of the heater unit's dial that switches everything from floor to face to defrost and AC. Topped off the AC and now it's blowing cold. Wow

Also - fans were coming on, but only at 207 degrees. But then the engine cools quickly back down to 190 or so. When they say Vipers run hot, they weren't kidding. Also, radiator cap needed replacing.

Thx again.

Murray

Randy · Aug 10, 2012

Neither Gen 1 nor Gen 2 disable the A/C clutch under any throttle load - the reason it goes hot is due to the loss of vacuum (another side effect of WOT) which opens the flap in the ventilation system and bypasses the evaporator. You should know this simply because turning off the clutch does not make the AC evaporator instantly hot - the only way the airflow COULD turn instantly hot is by not blowing air by the evaporator.

In my case, I resolved this by putting a vacuum check valve on the vacuum line that is the subject of this thread, and, for a few months, I had A/C under higher throttle.

DrumrBoy · Aug 11, 2012

Good info. I clicked on it b/c of the term "vacuum tube"....I was thinking Marshall Amp vacuum tube and I was dying to know why Gen 1s felt a need to use them on the A/C system :lmao:

Camfab · Aug 11, 2012

That's funny I had the same thought except I was thinking old TV set. OK probably 80% of the forum has no idea what I'm talking about.

JoelW · Aug 11, 2012

Camfab said:
That's funny I had the same thought except I was thinking old TV set. OK probably 80% of the forum has no idea what I'm talking about.

Same thing here. Vacuum lines? I knew Dodge was accused of using "old" technology but vacuum tubes? Whew, time for a cold beer!

AZTVR · Aug 11, 2012

DrumrBoy said:
Good info. I clicked on it b/c of the term "vacuum tube"....I was thinking Marshall Amp vacuum tube and I was dying to know why Gen 1s felt a need to use them on the A/C system

Camfab said:
That's funny I had the same thought except I was thinking old TV set. OK probably 80% of the forum has no idea what I'm talking about.

Geezer alert !!! :alert:

(uh, yeah, that was the first thing that popped into my mind also.)

DrumrBoy · Aug 11, 2012

Guilty as charged! :headbang:

Viper Grenade · Aug 14, 2012

That vac line goes through the fire wall and is used as the vac source for the Vac operated relays in the HVAC system. Such as selecting the floor vents, dash or defrost.

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