Heat A/C duct work help

[Wes Harden]

Not sure a vacuum gauge is necessary for this job, but as huge tool whore I am not going to discourage anyone from getting tools.
Unless you have a huge rumpty rump cam your engine will make enough vacuum.
Need to inspect all the vacuum lines. There should be a plastic ball with 2 vacuum line connections, near the windshield wiper motor on your truck.
1 hose from manifold vacuum, below throttle blades, to ball. From ball to in Dash controls.
Sometimes you will find a check valve in the source hose to ball. It isn't necessary if it is gone. Check valve is useful if you are wide open climbing long hills.

 

[85-K5Blazer]

EDIT **** Sorry ... I did not see Wes's post before I posted mine ... Yea what he said ... LOL.


Just for grins, have you checked the vacuum lines going from the engine to the vacuum tank and also coming out of the tank ... making sure none of the lines are cracked or broken ?

Mine is a diesel so I don't have a vacuum tank ... I have a vacuum pump specifically for A/C and the EGR Crap ...


From Wes's Picture above.

 

[wildweaselmi]

Thanks everyone.. I'm trying to trace the vacuum lines now.

When I was looking before it appears the vacuum line comes in at the base of the carb and goes directly into the FW

I need to see if I have source vacuum before I start tearing my dash back apart to try and trace these vacuum lines.

 

[Wes Harden]

If that port doesn't have vacuum at idle, look for a pipe plug in intake manifold, near where your hand is. A threaded nipple in an intake runner will do the job.

 

[wildweaselmi]

look for a pipe plug in intake manifold

I was looking around on the intake manifold and I don't see anything but again not sure what a pipe plug would look like. I am assuming like the rubber cap people put on the carb when not using the port but thats my assumption.

A threaded nipple in an intake runner

Not sure I see a spot on the intake to insert a threaded nipple. Going to try and see what ports are on the intake manifold that came with my 383 stroker crate motor. Maybe it will help me understand a bit better.

I have confirmed I don't have a vacuum reservoir like the ball everyone is mentioning. Not sure if I once had it and it got removed at some point (probably the point where my selector stopped working). I still have the big vacuum tank behind the drivers side headlight but only have the gas tank fumes connected.. the other two are plugged with rubber helmets.

 

[Blue85]

You can check for vacuum with your finger. Unplugging a line with vacuum should also raise the engine idle some amount. If you have a mighty-vac type tool you can test the HVAC system with that, too.

I didn't see any details on the truck. Does it have a carb or a distributor? Just wondering what else may not be working right due to deletion by convenience.

The big black can up front is the charcoal canister. Sounds like it's not hooked up, either.

 

[wildweaselmi]

I think I know what the issue is, without testing which I need to do to be sure. I believe the vacuum port the heater controls are plugged into is for Timing Spark and not constant which seems like it would cause a bunch of issues regarding trying to change settings. Looks like I should be using the Full Manifold Vacuum (the small one since the big one is for brakes). When connected to the back if I start on defrost and start the truck up I get air blowing at the defroster (GOOD) and then I change to vent and air moves to the front vents (GOOD) and then I switch back to DEFROST (nothing). Not sure if I get more vacuum at the beginning and then just not enough to switch it back. I really don't want to turn my truck off and start it back on just to switch it back to defrost when I need it. lol

Does it have a carb or a distributor?
holly fuel injection (terminator) and the holly hyperspark distributor as well since they work so well together.

The big black can up front is the charcoal canister. Sounds like it's not hooked up, either.
Yea the only thing plugged into the canister is the fuel tank vent. Other two are not connected.

I am starting to believe I need to add the vacuum canister and considering the check valve so the engine doesn't rob any provided vacuum

 

[Blue85]

Sounds like you're on the right track. If you can get the ball and check valve, it can only help. Someone here may have an extra. Maximum vacuum should be at low idle speed (well, other than decel), but even at regular cruising conditions you should have enough vacuum to pull those actuators.

 

[Wes Harden]

Definitely hook to the manifold vacuum port the small one. I am guessing you did try that port and the air flow switched to the vents after that?
Normal default position should be defrost. That means if there was no vacuum anywhere you get air flow through defroster.

If it doesn't switch back that means an air door motor/vacuum pot still has vacuum when it shouldn't. Double check the vacuum line connections to the pots.
If all the line are correct I would suspect the rotary vacuum switch leaking internally. They are not really serviceable, but you have 2 now ( I think) gently take the old on a part, clean all the hard grease off the rubber, apply a thin coat of new Sil Glyde https://www.amazon.com/Napa-7651346-Silicone-Lubricating-Compound/dp/B0054DWS1W reassemble

 

[wildweaselmi]

Definitely hook to the manifold vacuum port the small one. I am guessing you did try that port and the air flow switched to the vents after that?

correct, i actually found something is plugged into the small
manifold vacuum port. When tracing the hose it goes up to a T which one side had a small plastic line that was broken off (assuming that was the original vacuum source that went into the firewall) and the other side of the t goes towards the drivers side.
I took the broken plastic piece off that T and put the vacuum source line to it and that’s when it started to work when i start the truck up and it works for moving the selecter once and after that it stops working.
My assumption is I don’t have enough constant vacuum which is I ordered the $10 vacuum reservoir and check valve and see if that works. should be here sunday I think.

rotary vacuum switch leaking internally.

Does anyone know where I can buy a replacement? i took the old one apart to clean it and it broke (the plastic legs to hold it onto the rubber broke off).

 

[85-K5Blazer]

What broke exactly .. Send picture ... Does the other HVAC controller dash unit have one working ?

 

[Wes Harden]

The vacuum rotary switch. I was afraid that might happen.
I will look around.

 

[wildweaselmi]

What broke exactly .. Send picture ... Does the other HVAC controller dash unit have one working ?

i’m using the original and the one that broke is the one that was on the back of the used HVAC control i bought of ebay

I will look around.

i really appreciate it. i’m not having any luck finding one myself

 

[Wes Harden]

well my google fu was not up to the task. GM part number 16037769. Best suggestion I have atm is to call this http://thepartguy.com/ac-controls-chevytruck.htm guy and ask if available. Maybe post an add in our https://ck5.com/forums/ck5/general-parts-for-sale.16/ section want to buy.

 

[85-K5Blazer]

Status ? Were you able to combine the two HVAC Control units into a single working unit ? If I get out to the salvage yard this weekend I will also look for a controller if you still need the Vacuum Selection Switch.

By the way the controllers by LMC DO NOT have the vacuum selection switch on them. They come with a new Fan Switch, but not the Electrical selection switch or the vacuum rotary selection unit.

 

[Wes Harden]

Good to know.

 

[wildweaselmi]

By the way the controllers by LMC DO NOT have the vacuum selection switch on them. They come with a new Fan Switch, but not the Electrical selection switch or the vacuum rotary selection unit.

good to know for sure… that backup plan is out the vented window.

I’ve searched pretty hard looking for a solution for all of us but everyone says the same thing.. “those have discontinued for years”. My thought is the same as yours… start pulling these rare parts from scrapped squarebody donation vehicles.
I did get something put together that i believe will work but unable to test at the moment

on a holding pattern while i figure out why she won’t start now. Ain’t one issue it’s another.
I confirmed spark is good
I confirmed fuel is flowing (replaced both fuel filters to be sure no blockage)

I have a feeling it’s the holly efi so going to update the firmware and see what happens but i have to load windows onto something since holly is old school and tuning only uses windows… bummer

 

[85-K5Blazer]

Okay @wildweaselmi and @Wes Harden here are pictures of what a new LMC Controller looks like ... I bought one and it took forever to get in ... as you can see you have to use parts off your old one ... if they work .. UGH ... I am currently using my original controller with new Fan and Electrical Selection Switch.

See Next Post ...

 

[85-K5Blazer]

Ok @wildweaselmi ... I did go to the salvage yard today and located a 1984 C20 and a 1985 C20. Both had A/C, unfortunately the 85 Controller was gone .. I was going to get both of them.

I did pull the 1984 Controller ... it's a gamble, I paid $15.99 with tax and to cover my cost plus shipping it's yours. I really wish I could have gotten the other one as the truck looked in decent shape.

Let me know what you think ...

Thanks !!!

 

[wildweaselmi]

Thank You @85-K5Blazer but I successfully have rebuilt the unfindable part. NOW the issue is I believe my fuel pump is dead and of course its an intank version AND I upgraded my tank to 40gal which is around 3/4 full so I’m looking at trying to syphon enough fuel out so I can manage to drop the tank. Here’s the stupid part.. the issue very well could be the fuel pump relay but I’m not finding it anywhere. Really wish I had an inline fuel pump mounted on the rail somewhere just because easier to resolve if break down in the middle of a trail. Someone mentioned that you can’t hook an inline fuel pump when you have an intake fuel pump.. I guess it won’t work which is too bad since that would be a nice PLAN A (intank fuel pump) PLAN B (inline fuel pump).
As soon as I can test the rebuilt piece for the HVAC, I’ll let you all know.