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Old 02-13-2021, 03:29 PM
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DarkMonohue DarkMonohue is offline
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Fix your heater control. Vacuum ports explained!

Seems like heater controls come up every once in a while, so maybe this will help some people get them working right.

For reference, this is on my '85 J20. I believe this unit was used from '71-'85. The earlier control unit appears to be built about the same but is not identical. It is turned vertically rather than horizontally, and I can't be certain that the vacuum ports are mapped the same way, so you'll want to verify that yourself. In both cases, there are three vacuum lines attached to a round cast metal vacuum control valve. The back half of this valve is rotated to one of three different positions according to which button is pushed and the front half remains stationary.

The control unit is attached to the instrument cluster. You will need to remove the instrument cluster to get the control unit out of the vehicle.
Here's a look at the heater control unit from the left end prior to fully removing the instrument cluster. This is how the vacuum lines were connected when the unit came out of the truck. The vacuum lines are arranged incorrectly in this photo, but you can see that there are three vacuum lines, as well as a foam-like filter element in the center.




Here is the unit on the bench, ready for disassembly.




To disassemble, pull the temperature control knob off the lever, remove the two screws holding the front panel and buttons to the main housing, then pull that front panel away. Next, use a pick or awl to remove the black strap-style retainer holding the filter into the center of the disc valve and remove the filter, then remove the screw holding the front of the disc valve to the control assembly. The disc valve and the rotating spindle that drives it can then be pulled out.

Here the spindle and disc valve are removed and disassembled. Note that the flat spring shown on the spindle is shown installed incorrectly. It should be installed so that the center presses against the back half of the vacuum disc valve.




The disc valve often becomes sticky if it has not been used for some time, so it may need to be gently tapped with a wooden tool like a hammer handle to break them free. I also chose to lap the mating surfaces flat using 1500 grit wet-or-dry sandpaper with some soapy water on a pane of glass. They are smoother and flatter than they look in this photo.




The vacuum control disc valve has three hose nipples on it. On mine they were numbered 3, 4, and 1 in order from vehicle front to rear (left to right when looking at the disc valve). There is also a vent in the center covered by a little round foam-like filter element so that parts of the system not in use can release their vacuum and return to their normal position. By applying vacuum to each port with the controls in each position you can determine which ports communicate with which with each button pushed.
Here's a table comparing control position to port communication. In this table, "filter" means the purge filter in the center of the disc valve:




Now we can determine which hose goes where. With this table, the correct hookup is manifold vacuum to 3, heater inlet door to 4, and defrost door to 1. Your ports may be numbered differently, but the port position on the valve is likely the same function on all of them.

On my truck, the hose with the white stripe goes to manifold vacuum; the one with the red stripe opens the air inlet door to let air into the heating system; and the one with the green stripe redirects air from floor heat to the defroster vents. So on my truck the correct hookup is white stripe to 3, red stripe to 4, and green stripe to 1.

The steel parts of this control unit are, at least on my '85, rough stampings with a lot of sharp edges. They might benefit from a quick deburring and smoothing of any surface that rubs against anything else. They are also unfinished steel, so they are prone to rusting up over time. It would not be a bad idea to thoroughly clean the raw steel parts up and then protect them with something like a coat of gun blueing, or even some DIY electroplating, if you're set up for it. Also make sure that the felt contact pads you see in the photos are present and in good shape to help keep things quiet.

Like many other things, these units respond well to a cleaning and light lubrication, and getting the assembly out of the truck is the hardest part of the whole operation. Get to it!
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'85 J20 Old Man Truck, bought @ 65K miles - not great, but better than walking.
Member, FSJ Prissy Restoration Association
High quality junk here: intro thread and slow build thread

Did you know? Willys is just Willis spelled differently, but pronounced the same. Neither Willy nor his apostrophe are involved.

Last edited by DarkMonohue : 02-13-2021 at 05:38 PM.
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  #2  
Old 02-13-2021, 04:36 PM
oregonphil oregonphil is offline
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That is a great how to description. I have not done this rework myself. Did you need to replace the old filter (with what) or just clean it and put it back in?
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Old 02-13-2021, 05:12 PM
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DarkMonohue DarkMonohue is offline
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Quote:
Originally Posted by oregonphil
That is a great how to description. I have not done this rework myself. Did you need to replace the old filter (with what) or just clean it and put it back in?
Thanks, Phil, I hope it helps people out.

The filter was just fine. It really only sees airflow when the controls are switched from one position to the other, and then only for the fraction of a second it takes for the vacuum in the circuit you're switching away from to vent to atmosphere. So it isn't breathing huge volumes of air, and where it's positioned up under the dash, it's not seeing much dust. I just added a couple of drops of 3-in-1 oil to it to help keep the disc valve lubricated.

I should have mentioned a few other things about cleaning, refinishing, and lubricating the mechanical parts. I'll edit them in now.
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'85 J20 Old Man Truck, bought @ 65K miles - not great, but better than walking.
Member, FSJ Prissy Restoration Association
High quality junk here: intro thread and slow build thread

Did you know? Willys is just Willis spelled differently, but pronounced the same. Neither Willy nor his apostrophe are involved.
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Old 02-17-2021, 08:01 PM
jeepman42 jeepman42 is offline
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Great pics & info . as with many others, we have not figured out the pic part. Cheers to you!
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  #5  
Old 02-17-2021, 08:19 PM
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hey,does anyone here know how to.......
 
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hey brother, i have one of those kicking around somewhere - want it? i will never use it PM me

steve
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Old 02-19-2021, 12:37 AM
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mwood65 mwood65 is offline
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Holy vacume hoses Batman!

The timing on this was uncanny since I have to pull mine out tomorrow and figure out what's wrong.


The fan runs (and has been replaced not too long back)


No heat comes out anywhere. No air at all for that matter when the fan is blowing. It has new defroster hoses also so their not shot.


I either got no vacuum from the engine or the control switch is maybe not working and it's basically staying in the off position?
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  #7  
Old 02-19-2021, 12:54 AM
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DarkMonohue DarkMonohue is offline
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Yeah, sounds like either no vacuum to the controls, or controls jammed up, or vacuum lines missing or leaking or out if place, or the vaccum motor at the air inlet door has failed, or...

Tripwire, I'm not sure who you meant that offer for, but I'll take that heater control unit if nobody else needs it.
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'85 J20 Old Man Truck, bought @ 65K miles - not great, but better than walking.
Member, FSJ Prissy Restoration Association
High quality junk here: intro thread and slow build thread

Did you know? Willys is just Willis spelled differently, but pronounced the same. Neither Willy nor his apostrophe are involved.
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Old 02-20-2021, 04:53 PM
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mwood65 mwood65 is offline
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So to be sure I am reading it right...


Manifold vacuum to the one port and the other 2 just go to the appropriate vacuum dash pots on the heater & defrost under the dash?


Mine got delayed but home to pull it apart in the next day or so...


It's nice and sunny now so I may wait till were in the middle of a blizzard to work on it.....That's how it usually plays out it seems.


Would you happen to know the size of the vacuum lines off hand?
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Old 02-20-2021, 06:23 PM
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DarkMonohue DarkMonohue is offline
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Quote:
Originally Posted by mwood65
So to be sure I am reading it right...


Manifold vacuum to the one port and the other 2 just go to the appropriate vacuum dash pots on the heater & defrost under the dash?


Mine got delayed but home to pull it apart in the next day or so...


It's nice and sunny now so I may wait till were in the middle of a blizzard to work on it.....That's how it usually plays out it seems.


Would you happen to know the size of the vacuum lines off hand?
Yeah, that's really all there is to it. I have a way of making things sound more complicated than they really are. Here's the quick and dirty way:

1. Disconnect all hoses from the control unit. Start the engine and turn the heater fan on.
2. Move controls to the OFF position. Try the hose that supplies manifold vacuum on each of the three ports. Only one of them will hold vacuum without an audible vacuum leak. Leave the hose on this port.
3. Move controls to the HEAT position; one more port will now have vacuum. Attach the hose from the heater inlet door to this port. You will hear the heater inlet door open and will now have airflow at the lower heater outlets.
4. Move controls to the DEF position; the final port will now have vacuum. Attach the hose from the defrost blend door to this port. You will hear the blend door move and will now have airflow at the defroster.
5. Try the controls in each position. Verify that the system works correctly in each position and that you have no audible vacuum leaks.

That's really all there is to it.

I'm not sure what size the vacuum hose is, but can't imagine it is anything unusual.
__________________
'85 J20 Old Man Truck, bought @ 65K miles - not great, but better than walking.
Member, FSJ Prissy Restoration Association
High quality junk here: intro thread and slow build thread

Did you know? Willys is just Willis spelled differently, but pronounced the same. Neither Willy nor his apostrophe are involved.
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  #10  
Old 02-20-2021, 09:11 PM
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mwood65 mwood65 is offline
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Quote:
Originally Posted by DarkMonohue
Yeah, that's really all there is to it. I have a way of making things sound more complicated than they really are.




No, you had a great write up.


I know people who have met me think "Now there's a damn handsome and obviously intelligent guy"


They would be wrong...lol (on both counts)


Thanks though for simplifying it for me, I will be tearing it apart tomorrow.


I wish I could find hoses for the defrosters. I put new hoses on there awhile back from Ace hardware but they don't quite fit right.


Anyways, Thanks again, this will help me very much tomorrow!!
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  #11  
Old 02-20-2021, 09:52 PM
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DarkMonohue DarkMonohue is offline
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Defroster hoses shouldn't be too tough to find. Maybe this will help:
http://www.ifsja.org/forums/vb/showt...t%2A+hose% 2A

Let us know how you make out with the work tomorrow!
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'85 J20 Old Man Truck, bought @ 65K miles - not great, but better than walking.
Member, FSJ Prissy Restoration Association
High quality junk here: intro thread and slow build thread

Did you know? Willys is just Willis spelled differently, but pronounced the same. Neither Willy nor his apostrophe are involved.
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  #12  
Old 02-21-2021, 02:20 PM
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mwood65 mwood65 is offline
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I replaced all the vacuum lines and got them all hooked up properly,


The problem now seems to be the vacuum pull off thing for the heat/defrost isn't working. It stays on defrost and wont switch to heat.


The thing just stays collapsed like when vacuum pulls it open but when you shut off the vacuum it stays in the same position. It doesn't leak vacuum though



Is there a spring inside that should push it the other way when you remove vacuum or is there a spring inside the air distribution box??


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Old 02-21-2021, 10:33 PM
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78 WIDETRAC 78 WIDETRAC is offline
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Quote:
Originally Posted by mwood65
No, you had a great write up.


I know people who have met me think "Now there's a damn handsome and obviously intelligent guy"


They would be wrong...lol (on both counts)


Thanks though for simplifying it for me, I will be tearing it apart tomorrow.


I wish I could find hoses for the defrosters. I put new hoses on there awhile back from Ace hardware but they don't quite fit right.


Anyways, Thanks again, this will help me very much tomorrow!!






I bought some duct hoses and have Plenty extra to pass on. PM me
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