'71 Heat/defrost problems

[Murphy IRE]

It's a K20 with a 400sb. The heat is always on and very little makes it to the windows on defrost. I've ordered a new heat valve and will install tomorrow. The PO installed an OEM A/C, I don't know if it has ever worked. I'm trying to determine why the fan doesn't work on high (separate thread -I have located the heat resistor and A/C relay). I'll check them tomorrow.
But I found two more problems The "S" Duct is all torn up. The PO used silicone and tape (NOT very well) and there were huge gaps at the connection. I see I can order one from LMC for $13+. If I can't repair, that's an option. THE REALLY BIG problem is the Duct Air Selector box. The flap works fine, but all the rubber is gone leaving a 1/2" gap all around on the inside (LMC=120+). I think I can "Gerry Rig" some rubber and adhesive and put the $120 toward taking my wife out to dinner.
Has anyone had to made a simular "Rig"? If so, what matterial did you use to make the new seal?
Any other thoughts on the heat and other suggestions, A BIG THANKS in advance.

[JimKshortstep4x4]

When you buy your hot water control valve make sure of the one needed. Some are closed with vacuum and some are opened with vacuum. Here are pics of the the two schematics for the vacuum routing so you can compare them to your system.

Jim

[Murphy IRE]

HUH???
In the middle of the night, it dawned on me that I need to check the vacuum on the lines and/or replace the lines. How do I determine which valve I'm suppose to have? Thanks for the diagram. I've never seen the passenger side vent open! I'm OK with putting a manual lever there and just going with recirculated air for the A/C (That will be the last thing).

[Murphy IRE]

Started checking the vacuum. The small valve on top of the control assembly doesn't hold a vacuum. I made sure I moved the levers around and when I put it on inside, the vacuum leak was just barely slower. Should it hold a vacuum??
The actuator valve does draw down, but it loses all but 1lb. IS this normal???

[mikemileski]

Sean, when I got my factory air '72 GMC about six months ago, the blower would only work on low and medium, but not on high. I had a guy I trust tell me that it was probably the blower relay that was bad. I bought a new one for about $15 and now it blows on hight too. I am also in the process of getting my vacuum stuff working. I have a three port system like you do. I was told in another thread that one port could be blocked off and the system plumbed like a two port system. I really don't know what the advantage would be in doing that. I am also wondering if all three ports are common, meaning it doesn't matter how you hook up the three little vacuum hoses. If they're not all common, then it would mean that each hose has its own specific port that it gets hooked to. Does anybody know the answer to that question?

Mike Mileski
Tucson, AZ

[leddzepp]

Originally Posted by mikemileski

I was told in another thread that one port could be blocked off and the system plumbed like a two port system. I really don't know what the advantage would be in doing that.
Mike Mileski
Tucson, AZ

The advantage is simplification and less vacuum hoses going everywhere. Looks like the OP has already done that.

[dwaite72lnghrn]

I believe it is AC only. Non AC units used a cable to close the heat coil door.

[Murphy IRE]

I pretty sure it's AC only. I have AC so my system may be different. But this is where I would start.
So, I would put a vacuum pump on to your heater valve and see if it holds a vacuum. If it doesn't, that could be one of your problems. Also, I would replace your vacuum hoses. Between the two, that should do it. Someone sugested to just install a manual valve (it's open or it's closed). That could be another option. On no AC, you may have cables only.
Good luck, let us know the outcome.

[gonline71]

Finally got it fixed this weekend. And, non a/c vehicles only have the cables. I removed all of the ducting and cleaned inspected each piece. The seals were not perfect, but they should still work fine. The problem ended up being the main heater door by the heater core. It wasn't closing all the way even when the swich was pushed all the way left. It just needed some adjustment.

Thanks guys

[Murphy IRE]

I finally got all the heat to shut off during the summer! However, I had it up in the mountains and needed to get the heat onto the windshield. It blows really strong out of the vents, but it still wont direct much of anything to the windshield. Should I try adusting the cable on the box (pg 126 in the LMC catalog - of course it's not labeled as a part for sale). There is an old foam flap that's supposed to divert the air the two defrost hoses (it's really worn). So any help is appreciated
1) Name of the part
2) Any suggestions???

Thanks,
Sean

[70'402]

LMC has the foam cover for the defrost flap in a kit, part #36-0563. You might be able to get it separately from one of the board vendors also. If you can't get air to the heater or defrost then you probably have a broken flap
in the air selector box that bolts to the heater core housing. The flap is plastic and crumbles where the shaft goes through. I fixed mine with some epoxy and fiberglass cloth.

[red71cheyenne]

Originally Posted by Murphy IRE

I finally got all the heat to shut off during the summer! However, I had it up in the mountains and needed to get the heat onto the windshield. It blows really strong out of the vents, but it still wont direct much of anything to the windshield. Should I try adusting the cable on the box (pg 126 in the LMC catalog - of course it's not labeled as a part for sale). There is an old foam flap that's supposed to divert the air the two defrost hoses (it's really worn). So any help is appreciated
1) Name of the part
2) Any suggestions???

Thanks,
Sean

I've always called it the flapper, but thats the part that shuts the heater off and puts the air flow up to the defroster. Had to do some work on mine when I rebuilt the heater box. If you can get to the flapper, clean it up(rust) and replace the foam on it. Adjust the cable attached to it to fully close off the heat airflow and you should be good on defrost. Mine works like a champ now.