excessive heat coming from firewall: help please

[colonel mustard]

I have my A/C apart right now for a rebuild. This is looking at the heater box from the engine back towards the interior. The yellow box is the hot/cold blend door that is controlled by the lower slider on the HVAC controls. You are seeing it in the closed position (cold on controls). If your cable is out of adjustment, this door can be partially open.

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great pic thanks for posting that....is there a procedure to adjust the cable

 

[75gmck25]

Anything that fell down the passenger side defrost vent ended up in that plenum where the blend door is located. Its hard to tell what is jamming up the full travel of the door.

If you remove the glove box door, behind it you will see a vertical piece on the plenum, held on by about 3 screws. Once you remove that piece you will be able to see into the plenum and observe the blend door moving as you move the temp lever.

On top of the plenum there is a lever sticking up that is connected to the blend door inside, and a cable connected to the lever with a clamp and set screw. Moving the slider on the dash control moves the cable and also moves the blend door. IIRC, the adjustment is to loosen the set screw on the cable clamp, pull ithe dash control and door full cold to ensure it shuts off the door, and then tighten down the screw.

A couple other points:

Even in max cold position on the dash slider, there is full flow of hot water through the heater core. The dash air will get cooler, but the plenum itself will still be hot to the touch. For this reason, you never get really "cold" air through the vents unless the A/C works.

There are two vacuum actuators (round silver motors) that only affect the max A/C position. One is mounted up high where the air comes in through the intake at the bottom of the windshield, and the other is at the passenger side footwell. When you select max A/C (recirculate mode) it closes off the upper/outside door to flow only about 10% outside air, and opens up the footwell intake so that the A/C can suck in air from the cab. Even if one of these actuators is not working, A/C should still function - just no recirculate mode.

You also asked about the electrical part of the A/C. In most years, when you put the dash control on Max A/C or the temp slider in the max cold position (this is how it works on my '75), it will also turn the fan on high, regardless of the fan switch position. High speed fan uses a separate fuse under the hood and a fan relay under the hood. In some cases you may have only the lower speeds working (which go through the resistor pack), or you may have only the high speed working (direct power through the separate fuse). I think this still applies to your '85, but I've never checked the wiring diagram.

Bruce

 

[chengny]

great pic thanks for posting that....is there a procedure to adjust the cable

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Before going through that procedure, try this. Stick your head under the dash in the area of the blend door:

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While you are listening, have an assistant vigorously stroke the temperature lever from full hot to full cold. If the door is making contact at both ends, you should hear an audible thump when it hits. If you hear that, it would indicate the cable is properly adjusted and there is no need to readjust it.


This shows the door in the full cool position (i.e. no flow out of heater core):

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And in the full heat position (maximum flow through the core):

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[Rusty Nail]

so if I set it to MAX AC, the blend door will close?

so to summarize. yall are thinking the heat is coming from the heater core getting hot that hot air is being passed through the vents that aren't closing...???

Pretty much!
Leave the A/C controller on MAX AC when not in use.

My Blazer has jacked up HVAC vacuum lines - it's missing parts - and I get lucky with BI-LEV. (it's my only option that doesn't cause a vacuum hiss ATM )

 

[chengny]

This is correct:

your saying that that moving the temp lever to "cold" should shut the air diverter flap away from the heater core...regardless if the AC is functioning?

This is not:

so if I set it to MAX AC, the blend door will close?


The blend door is connected to the temperature lever by a cable. Moving the lever indexes the door and that is the only control.

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On the other hand, MAX AC is a setting on the mode select section of the control panel. The mode select lever is primarily used for controlling where the incoming is to be directed. It does this by changing the vacuum signal to the various actuators as required. The incoming air is then directed to the vents according to the chart below:

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Notice that there is no column for temperature control. The column that is headed "Heater A/C Door Open To" does not refer to temperature control, but rather whether the air will be directed to the dash vents or the floor vents.

The mode select lever also serves to start/stop the A/C compressor. Obviously if it is set to MAX AC or NORM AC the compressor is enabled and will cycle on/off as required. In addition to the A/C settings, the compressor is enabled when in the BI-LEVEL or DEFROST positions.

 

[HotRodPC]

wow great info. so let me attempt to dumb this down ...

your saying that that moving the temp lever to "cold" should shut the air diverter flap away from the heater core...regardless if the AC is functioning?

Yes it should because it's going to move the blend door to the cold side in the air handler and block off the heat. At least that's the way it should work.

I think it's also been mentioned, if you don't want the heat in the cab, you can also bypass the heater core and just look your heater hoses on the engine/radiator so no hot coolant is running to the cab.

 

[75gmck25]

Once again - its usually worthwhile (and simple) to remove the panel covering the blend door area and take out any junk that may have fallen in.

You may be really lucky and the plenum is empty, but I found a variety of pencil stubs, pieces of paper, match books, bullets, and nuts and bolts in mine. It was all the stuff my father-in-law used to leave up on the dash until it fell down the defroster vent. .

Bruce

 

[colonel mustard]

Yes it should because it's going to move the blend door to the cold side in the air handler and block off the heat. At least that's the way it should work.

I think it's also been mentioned, if you don't want the heat in the cab, you can also bypass the heater core and just look your heater hoses on the engine/radiator so no hot coolant is running to the cab.

I wish I could not have heat. But I need heat to be able to enjoy the truck in the fall and spring where I live.


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[HotRodPC]

I wish I could not have heat. But I need heat to be able to enjoy the truck in the fall and spring where I live.


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75gmc brought up a good point about the blend door being hung up from junk falling down in there. If you don't get it worked out though, you could always bypass the heater core for summer come about October, hook it back up. It's just a mater of moving hoses. Total of 2 hose clamps and that's it.

 

[QBuff02]

It's the souls of all the dinosaurs you're burning. Small price to pay for a good time. but if you think you're uncomfortable, think about the poor T-Rex that your intake just swallowed. In all seriousness, if you can't quite figure it out or make it bearable with all of the above mentioned items I would throw in a set of valves to the heater core hoses and be done with it. And then you take the hot coolant completely out of the equation. Almost all heavy equipment has a manual set of valves inline on both the supply and return lines. If you wanted to get real trick, and I've done it, The newer equipment has electric valves controlled by a rheostat and we've retrofitted a couple of them into some older equipment. It's a thing where some guys hate having to open up the engine bay doors and hunt down the manual valves because they can totally be in pain in the ass/hard to reach places. Built by Engineers, right? Lol You could go to a heavy equipment dealer, Agricultural dealer, or maybe even a good hardware store and source them. They make all kinds from pipe thread to o-ring in all different shapes and sizes. You could even buy a couple of T's and make your loop just off the firewall. Here's a pic of screw in ones I keep on hand at my shop.

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[colonel mustard]

It's the souls of all the dinosaurs you're burning. Small price to pay for a good time. but if you think you're uncomfortable, think about the poor T-Rex that your intake just swallowed. In all seriousness, if you can't quite figure it out or make it bearable with all of the above mentioned items I would throw in a set of valves to the heater core hoses and be done with it. And then you take the hot coolant completely out of the equation. Almost all heavy equipment has a manual set of valves inline on both the supply and return lines. If you wanted to get real trick, and I've done it, The newer equipment has electric valves controlled by a rheostat and we've retrofitted a couple of them into some older equipment. It's a thing where some guys hate having to open up the engine bay doors and hunt down the manual valves because they can totally be in pain in the ass/hard to reach places. Built by Engineers, right? Lol You could go to a heavy equipment dealer, Agricultural dealer, or maybe even a good hardware store and source them. They make all kinds from pipe thread to o-ring in all different shapes and sizes. You could even buy a couple of T's and make your loop just off the firewall. Here's a pic of screw in ones I keep on hand at my shop.

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I really like this idea....even when I get everything fixed I think its a great idea. im trying to imagine the plumbing in my head. seems like you would need 2 T sections ??

 

[colonel mustard]

This is correct:

your saying that that moving the temp lever to "cold" should shut the air diverter flap away from the heater core...regardless if the AC is functioning?

This is not:

so if I set it to MAX AC, the blend door will close?


The blend door is connected to the temperature lever by a cable. Moving the lever indexes the door and that is the only control.

You must be registered for see images attach

On the other hand, MAX AC is a setting on the mode select section of the control panel. The mode select lever is primarily used for controlling where the incoming is to be directed. It does this by changing the vacuum signal to the various actuators as required. The incoming air is then directed to the vents according to the chart below:

You must be registered for see images attach

Notice that there is no column for temperature control. The column that is headed "Heater A/C Door Open To" does not refer to temperature control, but rather whether the air will be directed to the dash vents or the floor vents.

The mode select lever also serves to start/stop the A/C compressor. Obviously if it is set to MAX AC or NORM AC the compressor is enabled and will cycle on/off as required. In addition to the A/C settings, the compressor is enabled when in the BI-LEVEL or DEFROST positions.

gotchya...that DIAGRAM IS NICE !!!

 

[QBuff02]

I really like this idea....even when I get everything fixed I think its a great idea. im trying to imagine the plumbing in my head. seems like you would need 2 T sections ??

Well even I learned a little something today.. I got to thinking maybe there was an easier way than plumbing it up with some extra T's (especially since there's not a lot of real estate right at the firewall/airbox area. And since you have a/c with the vacuum lines already available... this might be something for you to expand upon. I'm trying to remember if the systems on our vehicles are vacuum on or vacuum off..? the correct valve would need sourced depending on which way it works. either way, there's some pretty slick little valves available to do just this I found online. maybe worth going to your local auto parts store with this and seeing if they can match you up something to just cut the hoses, install this in line with the hoses, sneak a vacuum line over to it and presto.. no more hot coolant to the heater core! Lol Truthfully, if it was me on my engine where the brass elbow for the heater core line screws into the front right corner of intake, i'd just screw out the fitting, screw in a shut off in it's place and be done. As long as your coolant has another way to bypass there would be no issues, and no flow=no heat. But this setup might just fit the bill and be clean in both looks and operation in the process. Food for thought.


https://shop.advanceautoparts.com/p...es&gclid=CJSy_eDwluQCFWWOxQIdCd4Csg&gclsrc=ds

 

[Rusty Nail]

This is correct:

your saying that that moving the temp lever to "cold" should shut the air diverter flap away from the heater core...regardless if the AC is functioning?

This is not:

so if I set it to MAX AC, the blend door will close?


The blend door is connected to the temperature lever by a cable. Moving the lever indexes the door and that is the only control.

You must be registered for see images attach

On the other hand, MAX AC is a setting on the mode select section of the control panel. The mode select lever is primarily used for controlling where the incoming is to be directed. It does this by changing the vacuum signal to the various actuators as required. The incoming air is then directed to the vents according to the chart below:

You must be registered for see images attach

Notice that there is no column for temperature control. The column that is headed "Heater A/C Door Open To" does not refer to temperature control, but rather whether the air will be directed to the dash vents or the floor vents.

The mode select lever also serves to start/stop the A/C compressor. Obviously if it is set to MAX AC or NORM AC the compressor is enabled and will cycle on/off as required. In addition to the A/C settings, the compressor is enabled when in the BI-LEVEL or DEFROST positions.

Thank you!

For that totally radical vacuum diagram..

 

[colonel mustard]

So looks like multiple problems.
1. The actuator arm on the blend door looks broken

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2. Is the top of the blend door actuator arm supposed to be attached to something ?

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3. If I move temp control slider to cold , the heater core becomes fully exposed. If I slide it HOT the door shuts and you can no longer see the heater core. Is this correct ? Seems opposite of what I have read in this thread.

Also there are multiple broken tabs on the entire box. Ugh. Man this is turning out to be a chore. But will be worth it when I’m done.

If any of y’all have that broken white piece in the picture holding the actuator arm to the blend door please let me know. I need it.


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