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View Full Version : How does the heater/vent/defrost get actuated?

11-28-2000, 04:17 PM
Guys, my Blazer heater works great..........unfortunately it comes out both the vent and the floor (where it should). Is there some type of vacuum line that is run to one of the diaphragms that actuate this stuff? Seems it worked before we did the motor job, maybe we forgot to hook up vacuum? If so, where does it usually get its source? Full manifold vacuum? Moving the switch from vent to heat to defrost changes nothing on where the air comes out of.......its' in a '79 K5 with AC if that helps. Thanks in advance.......

[b]<font color=black>'79 - 406 TPI -<font color=orange> K5#5 - <font color=blue>See it at---&gt;

11-28-2000, 06:20 PM
Hey Frank,

Got the same problem ... been like that for about 4 years! Too lazy to check it out, but will tell you when it started. I changed a gas line one day and that either started it or problem occured as coincidence. I was sitting up under the hood for a long time, cussin and groinin and probably split a vacuum line. Most of the vacuum is centered in the black ball near the master cylinder, but I was working on the right side of the engine.

Plan to check it out when I replace the dash pad (someday), was actually thinking it's the hoses to the control in the dash.

Of course if it's the climate control switch, your favorite company sells them

I know that's confussing, but hope it helps.

11-28-2000, 07:43 PM
I've been meaning to start playing around in there myself -- I want to rig it so the heat comes from the defroster, vents and floor vents all at the same time. With the amount of heat this thing puts out, it drives me nuts that this isn't an option.

Maybe it's just my truck, but the vent setting works fine, the floor setting works fine, but the de froster setting sucks -- It blows more to the floor vents than it does to the windshield. There's a mystical 'Heat" setting, but it doesn't change anything.

Bigger fish to fry right now anyways -- Like trying to keep oil leakage below 2 quarts/week. ;)


Can Can
11-28-2000, 07:53 PM
The weak defrost and combined heat/vent problem must be a Blazer thing........Mine has the exact same problems. I hope someone can explain this and shed some light on a solution.......

11-28-2000, 08:43 PM
<font color=purple>The defrosters was a simple fix for me, I added a larger vaccum canister under the hood. If you run a larger cam, it will steal from the vaccum which is used for the defrosters. The system is vaccum fed, so increase the amount of stored vaccum so it is there when you need it for the defrosters. It worked great for me, my window clears in o time now. I hope this helps out .</font color=purple>

<font color=purple>L8r,

.Riz.<font color=purple>

6.2 man
11-28-2000, 09:47 PM
I jerry rigged mine . If you take out the glove box look at the heater .Youll see a long threaded rod pull that towards the kick panel .That will close the vents and run it to the floor .I wired it to the kick panel vent .I then used a hand vacuum tester and hooked it to the diaphram next to the accelerater .Apply vacuum to it and that will close off the floor vent and send it to the defrosters works great . e-mail me with questions this summer I'll fix it right

11-28-2000, 10:00 PM
The vacuum controls for the heat/ac need full vacuum to operate properly.

I bought a '77 K5 many years ago that always blew out of the floor vents, regardless of the switch setting. I pulled off the air plenum under the dash, expecting to have to perform all kinds of magic to get it working again. When I removed the plenum, there was an old, ragged looking BIC lighter. Looked like it had been there for years. I figured it fell down through a defrost outlet and got jammed in the rotating air door. Removed the lighter, put it back together and never had another problem with where the air came out.

<font color=black>HarryH3 - '75 K5</font color=black>
<a target="_blank" href=></a>

Can Can
11-28-2000, 10:07 PM
All this talk of vacuum is confusing me- I can actually hear something move at each setting(heat,vent, and defrost)but nothing seems to change much, except that on defrost I get a small amount of air coming from the defrost vents. I assume this dull thunk I hear when I change the setting is some sort of vacuum-powered valve- if so, everything should be hooked up right. If there isn't enough vacuum, can the valve/diverter not be closing fully or operating properly?

Riz- Where is the vacuum canister? Is it the 4" black ball on the firewall? What did you use to increase your vacuum storage?

6.2 Man- Does your plan to permanently fix your problem involve upgrading the vacuum supply?

11-29-2000, 04:44 AM
Speaking of defrosting...

How do you defrost your back window? Mine gets really foggy from the action in the back seat and need it defrosted sometimes!! My heater doesn't seem to get enough back there and the defroster don't either. Do I need to get one of them corded thingys and mount it in the back and plug it in the cig lighter when need it?

J**'s whats for dinner!

11-29-2000, 02:40 PM
My Jimmy suffers the same as you folks....really warm feet and, fogged up windshield. I think this is a great topic because if the vent controls are run by vacuum (we know they are). Where is the vacuum originating from?? The booster for the brakes?? I have hydroboost, no vacuum any thing for the brakes. Was there a vacuum for the diesel motor and was it inadvertantly disconnected when I put the gas motor in??? My question is this, does anyone really know how to go about a proper fix on this system??? If so please spell it out for those of us who are clueless, I'm tired of a foggy windshield....thanks....js

11-29-2000, 03:02 PM
I'm not sure how they get vacuum when you're running one of those oil burners.

I recall reading that the diesel powered Cadillacs had an electric vacuum pump under the hood for operating the vacuum accessories. Your truck may have had the same type of setup.

If my old brain can remember, I'll try to look up the diagrams in the factory service manuals at home tonight.

<font color=black>HarryH3 - '75 K5</font color=black>

11-29-2000, 05:40 PM
Harry, if you can help out with the lookup, you are king!

I tore half my dash apart today........first off, the vacuum on mine originates at the small black vacuum canister mounted on top of the blower motor on the firewall. I have vacuum there, confirmed it by pulling the hose off. From there it goes through the firewall to ?????? I think the back of the climate control there is where my question is - THERE ARE FIVE VACUUM HOSES GOING TO THE BACK OF THE UNIT........AND THERE ARE 7 "PORTS" ON THE there supposed to be two open? Seems everything worked before, and there is no "missing" hose laying around under the dash area, I even took out my d$%n CD player to make sure I could see well enough. I ran a direct vacuum line to either of those ports with NO luck. Can actuate the doors by hand with the threaded rod behind the glove box (idea that was offered last night), but can't get the unit to actuate them themselves. Man, is this frustrating..........sounds like a bunch of you have the same problem, so let's solve it! Keep those ideas coming!

Thanks for all the feedback, by the way. 'Appreciate it'....

[b]<font color=black>'79 - 406 TPI -<font color=orange> K5#5 - <font color=blue>See it at---&gt;

11-29-2000, 06:07 PM
BlazzinOR, I'll try to remember to look that up for you. As for the "empty" ports, they could be there to release vacuum when you switch between modes. There needs to be a vent somewhere or the vacuum will just be trapped in the lines and not release. So those are probably OK.

The few times I've had a GM heat/ac control apart, the vac hoses were terminated into a silicone rubber type "connector" that plugged onto the back of the control unit. It should be pretty obvious if one of the hoses got ripped out of that.

<font color=black>HarryH3 - '75 K5</font color=black>

11-29-2000, 06:11 PM
Good point. Another possible tie-in here..........relates to the cruise control (another guy mentioned his didn't work either).........mine doesn't work now either, and the vacuum hose that goes to the cruise control actuator goes/comes from under the dash it possible it there is a vacuum switch under the dash that kicks on when you actuate your cruise control, and that valve is tied in somehow with the under dash controls for the heater? (I realize this is a stetch, but why is the vac line for the cruise control going into the firewall??) Didn't have time to dig back into it and trace it, hope someone knows the answer............

[b]<font color=black>'79 - 406 TPI -<font color=orange> K5#5 - <font color=blue>See it at---&gt;

11-29-2000, 07:25 PM
one thing to check is the actual valve. Take the heater control out and split the valve from the rest of it (a few screws) take that apart and you will find a tan rubber disc with a few little nubs on it. If the nubs are broken off it will only kinda work when it feels like it. Dont know if GM will sell just the rubber. Took me 3 junkyards to find a good one. Everything works great now.

6.2 man
11-29-2000, 09:28 PM
I first hooked up a hand vacuum pump to the diaphrams . I applied vacuum to make sure the work . Then I see how long they hold vacuum . There are two valves one is on the end of the heater housing towards the gas pedal . the other is in the center between the heater halves ( can't see it). I took my glove box out on top there should be some lines I followed one that went to the center of the case that feeds the inner one .The inner one controls vents if it doesnt shut everything goes out the vents ( mine doesnt work ) .This one has to close off the vents before heat or defrost will work . The outer one by the gas pedal controls heat and defrost . with vacuum applied the defrosters open up . This summer I'm redoing my vacuum line and control . My diesel has vacuum pump and it works great .

11-29-2000, 09:55 PM
OK, I found the info for diesels and how they get vacuum. There is a mechanical pump in one of two possible places. Some 6.2's have the pump attached to a drive that runs off of the camshaft, similar to a distributor on a gas engine.

Others drive the pump via a drivebelt and pulley on the front of the engine, similar to an emissions air pump.

The pump itself looks a bit like a mechanical fuel pump for a small block chevy. It is a diaphram type pump that works very similar to a fuel pump. It's just sucking air instead of sucking fuel.

For the cruise control question. If there's a vacuum line running inside from the cruise control, then I think it's probably an aftermarket cruise control. Some of them had a hose that ran from the vacuum motor for the throttle to a switch that attached to the brake pedal. When the brake gets pushed, the switch vents the vacuum to atmosphere, thus making sure that the cruise control releases. I haven't seen a GM factory cruise control that had this vacuum release.

If that hose is loose, it will act as a vacuum leak, affecting idle quality as well as the operation of any vacuum operated devices (and the cruise control certainly won't work!)

<font color=black>HarryH3 - '75 K5</font color=black>

11-29-2000, 10:09 PM
BlazzinOR, according to the factory service manual for a 1987 K5, there are indeed 5 hoses that go to the back of the control (I doubt that it changed much over the years).

I'll scan in the vacuum diagram and try to post it here for you to check out. If that doesn't work, I'll email it to you.

I also looked closely at the description for the factory cruise control and it indeed does mention a mechanical vacuum switch attached to the brake pedal, like the one I've seen on aftermarket cruise controls.

<font color=black>HarryH3 - '75 K5</font color=black>

11-29-2000, 10:53 PM
BlazzinOR, here's the scan of the vacuum diagram for you. I was going to attach it, but it looks like file attachments have been turned off.


<font color=black>HarryH3 - '75 K5</font color=black>

11-29-2000, 10:54 PM
Harry, thanks a million. I'll check out the cruise control hose tomorrow, it is factory cruise by the way.

If you get the diagram for the vac lines, post it or email it, owe you big time.

[b]<font color=black>'79 - 406 TPI -<font color=orange> K5#5 - <font color=blue>See it at---&gt;

11-29-2000, 11:04 PM
Harry, didn't show up, try emailing me. Thanks again.

BTW, see you have a TPI also.......they rock!

[b]<font color=black>'79 - 406 TPI -<font color=orange> K5#5 - <font color=blue>See it at---&gt;

11-30-2000, 09:22 AM
OK, I sent a copy to your email address.

And yes, TPI is pretty cool, but I don't think I'd ever install it again. Too much hassle to work on. The later LT1 and LS1 manifolds look like there's much less stuff in the way of everything else.

<font color=black>HarryH3 - '75 K5</font color=black>

11-30-2000, 12:06 PM
Just want to thank harry and everyone else for the info. Now I have a weekend project. Figure out how to make the vacuum work without the diesel parts.....js

11-30-2000, 01:38 PM
Just goes to show you how valuable this bulletin board is to all of us.

Great bunch of guys/gals, thanks for being a part of this community. Special thanks to Harry for the .jpg file on the vac diagram. Very clear and easy to understand. Now to dig into it............

[b]<font color=black>'79 - 406 TPI -<font color=orange> K5#5 - <font color=blue>See it at---&gt;

12-10-2002, 09:37 PM
My K-5 has the vacuum to shut off the C.C..... It was in an '87 k-5 and it goes to the brake switch. Also on my '78

12-10-2002, 09:40 PM
HarryH3, Can you send me a copy of the Vacuum Diagram?

01-02-2003, 01:02 PM
Could you send this diagram to me? What year is it out of? Thanks

01-02-2003, 01:24 PM
It's at If the direct link doesn't work, copy the link, then click on the link in my sig to get to my home page, then paste the link above into your browser address bar. /forums/images/graemlins/thumb.gif

BTW, the diagram is from an '87 factory service manual, but I don't think that they changed much between '73 and '91. The newer body style trucks (Starting in '88 with pickups and '92 with Blazers and Suburbans) use a completely different system.

01-02-2003, 01:47 PM