ac vacuum lines

[bwilhite1]

Well I guess that explains why a heater valve wasn't in the diagrams. Thanks again for the detail. I'll take another look at the blend door, but it's hooked up and seemed to be moving full travel when I had the evaporator out. I watched it close, but it sounds like it's not sealing correctly.

You can feel very cold air and warm air blowing out of the heater box at the diverter door (#2 in the diag). The door is slightly open, but I can't get it to move anymore and can't tell what full travel is for it.

Someone had mentioned the heater valve to me, and that their truck had one, so that's how I bit off on that. I thought that since the blend door was closing correctly, the hot air entering the heater box was due to proximity to the heater core and that it was not supposed to have water in it all of the time.

 

[chengny]

Thanks again for the detail. I'll take another look at the blend door, but it's hooked up and seemed to be moving full travel when I had the evaporator out. I watched it close, but it sounds like it's not sealing correctly.

Release the retaining clip and pull the temperature control cable off the crank arm - that moves the blend door.

First, a general automotive tip on this style of clip:

When installing, these clips are great. They are simply pushed straight down - onto the pin/rod/shaft - whatever they are being fastened to. Due to the angle of the locking tabs, they slip on to the shaft easily. Just hold it between your thumb/index finger, line it up with the pin. and give it a little push down. It will seat and lock in place.

On the other hand, when you go to remove one, that's a different story altogether. These clips can be a pain in the ass to pop off. And what's worse, about half the time - when they finally do release - they go flying off into the wild blue yonder and are never seen again.

Treat the clip as if it were a nut - and screw it off. To get it started, apply a little upward force (with a screwdriver tip e.g.) - and at the same time cause it to rotate (fingertips work well). You will see the clip start to climb right up the shaft. After the first revolution the screwdriver isn't even required, the tabs act kind of like a self-threading nut and drive the upward travel.

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With the cable disconnected, the blend door is now able to be stroked easily from stop to stop - locally at the crank arm. Get your head under the dash and twist the crank back and forth vigorously. Listen carefully (or maybe just listen normally - since you are so close to the action). As you stroke it fully left/right, you should be able to detect an audible "thump" each time the door seal hits the flange on either side. If you are able to hear that distinctive sound, you can feel comfortable that the damper is able to seal up tightly against both flanges. It is not unusual to find that the edges of the rubber gasket have been nibbled away by varmints. But no matter how raggedy the rubber seal is, it should not have very much effect on mixed air temperature.

The problem might be that the control cable is not adjusted correctly and is restricting the blend door from going to full cold. Check for proper cable stroke like this:

With the crank/cable still disconnected, rotate the blend door to the fully cold position (CW) and hold it there. Make sure the temp setting is all the way to cold at the control panel and the other end of the cable is secured to the bracket as it would be for normal operation:

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While still holding the damper tightly against the heater core flange, bring the loop at the end of the cable over close to the crank pin. Note the distance between the loop and the pin (if any). Be sure to do this without pulling on the cable in an attempt to close the gap. The cable is pushed out by the lever and the lever was moved to the coldest setting. So, under real conditions that is as far as the cable can be extended. If there is a gap - and the cable is reconnected to the pin - the blend door will be pulled away from the flange on the heater core side. And, depending on the degree of opening, that will allow some percentage of incoming air to be reheated.

That is just one possible cause for reheated air be able to mix with the cold air flow - with the temperature setting on cold. But it is a good place to start. If do you find that the cable is pulling the door away from the full cold position, adjustments can be made at both the cable end and the mounting bracket. Both use slotted bolt holes for mounting.

 

[75gmck25]

If you take a look at the service manuals posted on this site, you will find a section that describes the vacuum lines, colors, and how they are routed.

For my '75 the book also has a table that shows which lines should have vacuum and which lines should be closed for each position of the dash controls. I didn't look deep enough into the manual for your truck to find out if they have that table.

If you need to repair any vacuum lines, the parts stores sell GM hard vacuum line, and small sections of rubber vacuum hose can be used as a splice between sections of hard line.

Bruce

 

[bwilhite1]

I've been tinkering with this on and off for the last couple of months and getting ready to put it back together. I looked back through the vacuum diagrams on this thread to make sure everything is getting hooked up correctly and I have one last question. I have the RPO C60 vacuum setup, but my gray supply line goes through the firewall and is plugged into a vacuum port on an electric switch attached to the firewall near the evaporator coil. That switch has wires that run into the bundle that comes from the computer behind the glove box. I have no idea what that computer is for BTW.

Did someone just stick that vacuum line on there when they removed the orignal motor and cut the vacuum lines? There's a connector on the end of the line like it's supposed to be attached to that switch, but I can't figure out why that computer would need to be attached to the AC controls, particularly with no supply. If it's necessary, I can add it back into the vacuum system, but didn't want to do just bypass it and forget about it without asking someon. Any advice is appreciated.

Here is a pic of the rear of the vacuum plug that connects to the control switch. Note the open tabs on each side. They slide over 2 plastic studs on the back of the switch and there are 2 of those spring steel stamped washer things that you push on and thy hold but are a bitch to remove. Most of the time the studs break on me but that will not hurt anything the harness does pretty good on its own holding onto the switch.

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Here is a step by step you can print out and see if this helps you on your quest. You can also download onto your smart phone so you have it on hand. I have been doing this lately on any projects I been having to tackle.

AC, Heat, Vacuum, Harness, Hoses, Servo, Diagnosis, Troubleshooting, Diagram, Switch

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[bwilhite1]

I've got the heater box removed and repaired and getting ready to hook everything back up. Do you have any input for my last post? Also, I assume I should tap constant vacuum from the manifold and not use the ported vacuum plug in on the front of the carburetor? Thanks!

Thanks again for the detail. I'll take another look at the blend door, but it's hooked up and seemed to be moving full travel when I had the evaporator out. I watched it close, but it sounds like it's not sealing correctly.

Release the retaining clip and pull the temperature control cable off the crank arm - that moves the blend door.

First, a general automotive tip on this style of clip:

When installing, these clips are great. They are simply pushed straight down - onto the pin/rod/shaft - whatever they are being fastened to. Due to the angle of the locking tabs, they slip on to the shaft easily. Just hold it between your thumb/index finger, line it up with the pin. and give it a little push down. It will seat and lock in place.

On the other hand, when you go to remove one, that's a different story altogether. These clips can be a pain in the ass to pop off. And what's worse, about half the time - when they finally do release - they go flying off into the wild blue yonder and are never seen again.

Treat the clip as if it were a nut - and screw it off. To get it started, apply a little upward force (with a screwdriver tip e.g.) - and at the same time cause it to rotate (fingertips work well). You will see the clip start to climb right up the shaft. After the first revolution the screwdriver isn't even required, the tabs act kind of like a self-threading nut and drive the upward travel.

You must be registered for see images attach

With the cable disconnected, the blend door is now able to be stroked easily from stop to stop - locally at the crank arm. Get your head under the dash and twist the crank back and forth vigorously. Listen carefully (or maybe just listen normally - since you are so close to the action). As you stroke it fully left/right, you should be able to detect an audible "thump" each time the door seal hits the flange on either side. If you are able to hear that distinctive sound, you can feel comfortable that the damper is able to seal up tightly against both flanges. It is not unusual to find that the edges of the rubber gasket have been nibbled away by varmints. But no matter how raggedy the rubber seal is, it should not have very much effect on mixed air temperature.

The problem might be that the control cable is not adjusted correctly and is restricting the blend door from going to full cold. Check for proper cable stroke like this:

With the crank/cable still disconnected, rotate the blend door to the fully cold position (CW) and hold it there. Make sure the temp setting is all the way to cold at the control panel and the other end of the cable is secured to the bracket as it would be for normal operation:

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While still holding the damper tightly against the heater core flange, bring the loop at the end of the cable over close to the crank pin. Note the distance between the loop and the pin (if any). Be sure to do this without pulling on the cable in an attempt to close the gap. The cable is pushed out by the lever and the lever was moved to the coldest setting. So, under real conditions that is as far as the cable can be extended. If there is a gap - and the cable is reconnected to the pin - the blend door will be pulled away from the flange on the heater core side. And, depending on the degree of opening, that will allow some percentage of incoming air to be reheated.

That is just one possible cause for reheated air be able to mix with the cold air flow - with the temperature setting on cold. But it is a good place to start. If do you find that the cable is pulling the door away from the full cold position, adjustments can be made at both the cable end and the mounting bracket. Both use slotted bolt holes for mounting.

 

[chengny]

Yeah, somebody - at some point - was either making a joke or had no idea what he was doing. The gray tubing that starts at the HVAC control panel, transits the firewall and shares a grommet with the electrical portion of the engine side harness - should go nowhere other than:

1. An unported tap on the carburetor, or

2. Straight to a convenient tap on the intake manifold

The pressure/vacuum switch - that you found the gray tube connected to - is unrelated to the HVAC system. Whatever it is (probably the MAP sensor for the ECU), it certainly isn't capable of providing the HVAC system with the vacuum supply required to operate the various air flow control dampers. The switch only senses vacuum, it doesn't provide any.

 

[bwilhite1]

Thanks. I needed a sanity check before bypassing it.

 

[75gmck25]

The gray tubing should be connected to the vacuum reservoir (its called a vacuum tank in the last diagram you posted), and then the evaporator should have a vacuum line running over to the manifold. The reservoir ensures you still have vacuum for the HVAC controls regardless of engine vacuum.

Bruce

 

[chengny]

The primary function of the gray section of tubing is to connect the vacuum operated components of the HVAC system to the best available source of negative pressure. In the case of a gasoline fueled engine, that would normally be the intake manifold (or a port on the carburetor that is under the throttle plates).

The function of the vacuum reservoir is to maintain negative pressure in the control system when the normal source (manifold vacuum) is unavailable. The reservoir tank has only one tap - located at the bottom. There is a tee inserted into the vacuum supply tubing. It is located in the section that is on the engine compartment side of the firewall. The off-sticker from the tee is connected to the vacuum reservoir tank via another length of tubing.

In addition to the tee, there is also check valve in the supply tubing. The check valve is located in the section of tubing that connects the vacuum source to the tee. It is oriented so as to only allow air flow towards the carburetor.

When available at the manifold, the vacuum is primarily used to stroke (and also hold in the selected position) the various diaphragm operated dampers associated with the air handling portion of the HVAC system.

In addition to operating the HVAC controls, the negative pressure that is developed in the manifold is also applied to the vacuum tank - via the tee and interconnected tubing.

When vacuum is unavailable at the manifold (or whenever manifold pressure rises above the pressure in the system/reservoir tank), any air flow will tend to be towards the system/reserve tank. It is at that point that the check valve (and reserve tank) come into play.

Due to the air flowing in the wrong direction, the check valve will snap shut. This effectively isolates the control system from the higher pressure within the manifold. With the primary source of vacuum no longer available, negative pressure for system operation is temporarily supplied by the vacuum stored within the reservoir.

Since there is essentially no ingress of air from atmosphere - in a well maintained HVAC control system - other than when the dampers are being moved, there is little to no airflow into the vacuum tank from the control panel. And, if the check valve is tight, there will be no flow of air from the manifold into the system. Consequently, the reserve tank can maintain system vacuum for a considerable amount of time.

For the most part, loss of vacuum in the manifold is the result of a WOT condition caused by heavy acceleration. But the vehicle can't be accelerated forever, at some point the driver lets off the gas pedal and the throttle plates are trimmed in the closed direction. With the engine still pumping air - and the plates causing a restriction to airflow into the manifold - pressure quickly falls below what remains in the system. The check valve opens and allows manifold vacuum to begin evacuating any air from the control system and re-establish the reserve vacuum in the tank.

 

[blancawilliams]

Very useful post. It helps me to find out the problem.

 

[asltrfl]

Heater control system

WOW! Chengny, you are awesome. I've been tearing my hair out for days, studying and figuring out this system. I was darn close, but because of my cursing, my wife found this site/thread. Now it completely makes sense. I just got a new A/C Delco "control valve" ( the vacuum selector valve), a new "one way" valve, and a "Vacuum Reservoir", and I'm ready to go this weekend with all new lines tee's etc.

The last owner gutted the engine bay, it looks nice but was missing all the critical hvac components. By the way, the A/C Delco vacuum selector valve is GM # 16037769. I got it on ebay for $20 shipped, brand new in the Delco GM packaging. It is the exact one that came in my '85 k1500 and probably fits '82-87 and many other years too. Dorman makes an aftermarket vacuum reservoir (auto zone etc) and napa had the one-way valve.

THANKS again!!!

 

[ninety6]

This thread is awesome. I've through it a few times. I've got an 86 C10 Silverado with AC.

My truck will not blow full dash... Selector works, and the flap starts to move to dash, but it only moves about 1/4th of the way, so doesn't close off the floor vent and direct to dash.

The vacuum is working - at least partially -- the flap opens (partially) and closes when it should based on mode selected. I can push the door fully closed by hand (and when i push and release i hear air moving). nothing seems to be binding, and the plastic clip on the door is fine. All other actuators and such work as they are supposed to...

So my question is -- do i have a vacuum leak on the AC vacuum line or selector valve, or do these actuators go bad? I don't hear or feel any vacuum leaks. All my vacuum line is still very plyable and doesn't seem to have any cracks (of course i can't see it all).

I guess i'm asking for help diagnosing based on the fact that the vacuum partially operates the mode door.

 

[75gmck25]

It would be worth your time to buy a vacuum pump tool similar to this one. They probably have them at most auto parts stores, or you could buy one of these online. https://www.amazon.com/Mityvac-MV80...8&qid=1498855263&sr=1-25&keywords=vacuum+pump

First use the gauge to check your engine vacuum, which should be about 18-20 inches at idle if you have a stock engine. It might be as low as 12 inches if you have a long duration cam. FYI - Also do an online search for engine troubleshooting with a vacuum gauge and you will find all kinds of good info on finding problems with your engine based on vacuum readings.

Then hook the pump up to the port on the vacuum motor that seems flaky, and see how much vacuum it takes for the door to go full travel. Most of the time it only takes about 10" to open it completely. If it fails that test you just need to buy a new vacuum motor.

You can also use the vacuum pump to pull vacuum on the line running to the vacuum reservoir, or the line that runs from the engine to the reservoir (which would simulate the engine running). By gradually working through the system you will find where you have a leak or do not have enough vacuum to operate the HVAC.

You can also use the pump to pull vacuum on your distributor vacuum advance, check the operation of a cruise control vacuum servo, etc.

Bruce

 

[ninety6]

Thanks for the tips, good info. I'll check out that vac. I did use a vacuum pump (like the kind for pulling vacuum on A/C systems) and hooked up my supply line (going into the cab) to it-- that pulls good vacuum, but, i didn't have a proper adaptor, so while my connection worked, it wasn't fully sealed. not really the proper tool -- but i was able to use it at least start troubleshooting. With that, i was able to confirm that all other functions and actuators were working properly, so my issue is specific to AC/mode actuator only (or something in that line of course).

So if the actuator/vacuum motor takes too much vacuum to open, replace that -- if it leaks, find the leak and fix it.

Stock 305 stock EVERYTHING (except probably tires, battery, brake pads i assume, and the 2 brake hoses i replaced this year). ole' girl only has 63k on her.

Does sounds like i need the proper tool to further diagnose without just poking or replacing stuff.

I could also use that pump to verify all the other vacuum lies in the engine bay.

I don't have a lot of experience with vacuum lines... but i know a lot more after troubleshooting this problem!

 

[ninety6]

Fixed it! yay!

So, i got the little mightvac -- cool tool. I was able to verify the supply line was good (as i thought) and that engine vac is good (about 17). So i tore the dash back apart and took the selector valve off the control assembly through the radio hole (just pushed that out of the way).

got the little round clips off to take off the vacuum line assembly -- without breaking anything -- using a fork.
I then tested each vacuum line for all the actuators. ALL held vacuum fine with no leaks.

So i'm thinking bad actuator.... the ac-mode actuator. Of course you can't get to that without taking the whole freaking thing out of the dash. After messing with it for a while - i realized that it only took 3 pumps on the tan hose to get to 10" - and the other ones took like 10 pumps of the might vac - thought that was strange.
So i started tracing the line. Ended up finding that the tan vacuum line was MELTED shut behind the floor vent, UNDER the tape that holds the lines all together!! (not really sure how it was melted, it's not that close to the floor light, not near the radio.... ???????)

So, no leak -- but no vacuum was getting through. I cut out the bad part, patched it using some small fuel line, and everything works great now... yay! flap closes all the way and blows good air through the dash. This thread was instrumental in finding this issue... even though it's atypical, i was able to use the info and diagrams and pictures to narrow down my issue. Thanks all!

Originally i thought the AC Mode Actuator was partially working -- but it wasn't. It was just that the heat one did (moving to floor). So when i turned from AC to heat, the door shut. once that line vents -- it just opens an inch or so. My actuator wasn't doing that, it was just back in it's neutral position.

So, 5 minute fix that i did under the dash that wouldn't have required taking anything apart-- just took me about 8 hours over a couple saturdays and dismantling the dash twice to find the problem.

So, now to find where my A/C system is leaking, and probably replace the noisy compressor..

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