AC cab heater/AC control ?

[JIMs70GMC]

My 71 has a 1970 AC cab. No vacuum lines are connected to the engine. I put the cab on a non-ac chassis. I don't plan on getting the AC working this year, but would like to have heat. Right now no matter what I select I can not get air to blow on the floor. It seems like it always blows out the 3 dash vents and when I select defrost w/ the top lever I can feel a little air on the windshield. Do the vacuum lines need to be hooked up? Also, how hard is it to replace the heater core? Thanks,

[Robert1970C20rstbukt]

Check your air hoses under the dash and make sure they aren't rotted and are connected to the outlets. To replace the heater core you gotta pull the right inner fender to get to the bolts/nuts on the bottom of the heater box and pull the whole box out. Then there are four 1/4" or screws that hold the heater core to the box you gotta remove, then the core should come out. I just replaced mine about a month ago and it took me maybe an hour to do it all and put it back together.

[cdowns]

you have to have the ac vaccume lines hooked up for the heat to go thru the heater coil as far as the problem with the air routing there's a box on the floor that has a flapper valve(diverter) and it's a very common problem for the flapper to fail not allowing you to direct the air where you want it to go

[neonlarry]

^ I believe the heater core on a A/C cab is inside the cab. You will have to take the glove box out to get the box off to change the core. With the A/C ducting there is a diverter box that directs air flow it has a plastic flapper vale that is notorious for breaking at the pivot. The control looks like it is working from the outside but inside it does nothing. These can sometimes be fixed with fiberglass or metal straps or both.You might want to try that first I think new ones are around $70+.

[JIMs70GMC]

All the duct hoses are hooked up and intact. I'll check the flapper, neonlarry. Anybody confirm the core replacement?

[Harold Shepard]

Heater core is actually in the inner cab box, but you will have to get at the nuts that are on the firewall to take the inner box out. The heater hoses will have to come off but the replacement is straight forward after you pull the inner box. AC trucks are easier to fix than non ac trucks because of the inner box. Harold

[incoma]

One last note on the heater core, there are 2 wire retainers that hold the core in place. One on both ends of the core. These can be difficult sometimes to get lined up to rescrew to the housing.

[JIMs70GMC]

Got it off, broke the compressor switch in the process. What a PITA. Had to pull the glove box out, I'll need a new one now. Messed up two of the vent hoses, will replace those now. Found a vacuum diagram looks straight forward all the hoses are there. neonlarry was right the flapper don't flap in the diverter box. Now I have something to do this weekend!

[incoma]

Could you share the vacuum diagram? I'm putting my dash back in now, and I can't remember how to hook up all of the vacuum lines!

[JIMs70GMC]

I'll scan it in and post/email you this evening.

[dallas71]

Jims70GMC....I would also like to have a copy of that diagram.....

[neonlarry]

Here is a link to a post with vac diagram, just scroll down near the bottom:
http://67-72chevytrucks.com/vboard/s...threadid=58584

[JIMs70GMC]

Here's mine:

[cdowns]

if you'll read my post i told you about the flapper b-4 anyone else they just copied my answer

[neonlarry]

I'll be nice and just leave that one be.

[JIMs70GMC]

sorry cdowns, When I was replying I didn't scroll all the way back up. Thanks to everybody! I'm ready for the winter.

[incoma]

Thanks for the diagram Jims70GMC!!. That's helpful!!

[GMC Jim]

Jim... I replaced the compressor switch on my air conditioning system...... Installed a toggle switch under dash in line with the blower control switch.... I use this switch to control air-conditioned air vs outside air without have to move any levers...I feel this adds to the versitality of the system as far as air temp control is concernced....

[GMC Jim]

also I disconnected and plugged the vacuum hoses that actuate the outside air and the re-circulated air diaphrahms. Removed the kickpad flapper valve . I now used the lower kickpad recess for radio speakers and grilles. The system now uses outside air at all times; a plus in my opinion especially if any passenger is a smoker!!

[JIMs70GMC]

Do you all see a problem with removing the flapper in the diverter box? That is if I can't fix it. Priced them new at $90.

[chickenwing]

Yes. The defrosters in these trucks are bad enough as it is. If you just take out the flapper, the air flow will be split between the floor outlet and the defrosters. Don't think you will ever see in the rain again. I tried my hand at tig welding a new flapper with the old diffuser epoxied on. I failed miserably. Will try again with 2 thin gauge pieces of tin, lots of self drilling screws and even more epoxy.

[mrein3]

Originally Posted by chickenwing

Yes. The defrosters in these trucks are bad enough as it is. If you just take out the flapper, the air flow will be split between the floor outlet and the defrosters. Don't think you will ever see in the rain again. I tried my hand at tig welding a new flapper with the old diffuser epoxied on. I failed miserably. Will try again with 2 thin gauge pieces of tin, lots of self drilling screws and even more epoxy.

The pivot on the flapper is a metal rod covered with plastic. I cut away part of the plastic to expose this metal. Then I MiG welded a piece of sheet metal to the rod and pop riveted the plastic flapper door to the metal piece.

Make sure you tack it in to place so when you move the rod it opens and closes the flapper all the way. You could weld it with the rod in the wrong position relative to the switch it activates.

It looks kinda rough but once installed you can't see it and its WAY stronger than the crap the General put in there. I've had mine in there that way for over a year now.

[chickenwing]

I had it all rigged up correctly. My skills welding just STINK. The shop manager let me use the tig but all the qualified techs were out on the flightline. Should have delayed the project for another day.
Rich

[GMC Jim]

Has been over 10 years ago; but I repaired the flapper using 2 part expoxy resin and fibreglass cloth.....Can't recall the specific details but do recall using an epoxy mix with filler material ( aluminum filings ) mixed the filings with the expoxy { to the consistency of peanut butter} and applied arpound the metal rod.. then after the mix had partially cured,, ( about 15-20 minutes ) wrapped fibreglass cloth aroung the rod and on both side of the flapper valve..... and saturated with epoxy mix.... You dont' have to use aluminum filler; most any filler such as silica that can be obtained at marine supply outlets or most anywhere expoxy products are sold.... This repair has been quite satisfatory over the past 10 + years and I feel was stronger than when new ....

[GMC Jim]

More detail on the doors (flappers) used with factory Four Season air conditioning system. Downstream from the blower, the air encounters a temperature door. Movement of this door directs blower air thru the air conditioing eveaporator core and/or the heater core...Temperature is controlled by position of this door directing required volume of air thru the evaporator and heater core. Next door in the system is the outlets door. This door controls the temerature conditioned air to the floor outlet and/or the center dash and ball dash outlets. Next door is the defroster door. This controls the volume of air desired to the floor outlet and/or the defroster outlets.
Asd I mentioned in previous post; I diconnected the vacuum hoses controlling the outside air and recirculating air valves ( doors )and sealed the doors in postion for outside air only.
When you have the system dsisassembled; check the condition and proper travel of all three doors to ensure best efficient operation. Another item that is of impotrance in the efficiency of the system is the blower motor. Standard heater blower motor specs are :- Amps draw= 6.25 max; RPM 2550 min./ 2950 max. Blowers used with air conditioning systems :- 16.25 amps; 3700 min RPM. The blower designed for the air conditioning system produces much more air and should be used for more with heater only sytems for better performance. Thes blowers are interchangeable and will fit either sytem.
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