excessive heat coming from firewall: help please

[colonel mustard]

so heres the deal.

1981 k5 350 swap. factory AC and Heat equipped truck although it is not functioning. the heat doesn't turn on...nothing. its all disconnected electrically for now.

here my issue.

on the passenger side, excessive amount of heat is coming from behind the glove box area and venting out to the floor board. should I be looking at heat coming from the firewall??? maybe heat coming from some vent somewhere??? where should I start looking?

thanks

 

[4WDKC]

first the hvac system is never off, the duct doors will always be open I dont know if max ac setting caused them to recirculate the interior air and closed off the duct for exterior air blowing across the heater core. second check your heater control hoses for a valve that would block flow preventing hot coolant from reaching the heater core. Third check the firewall and ac box for leaks and seal any you find.

 

[dvdswan]

I'm with @4WDKC on this. I was thinking as well if the temperature control cable had come off last time you used heat. Far fetched but it can happen.

 

[Craig 85]

I'm in the process of rebuilding my A/C. I checked all of the vacuum pods to make sure they worked. There is a pod under the hood in the cowl area on the passenger side. This picture is off eBay and has a plastic unit, mine is metal. This pod and flapper close when the A/C is turned on recirculate (Max A/C). Otherwise it is in the open position (applying vacuum will close it) and will provide fresh air to the cab at all times.

Also make sure the hot/cold cable is adjusted correctly. The interior door may be open and letting air get by to the heater core.

You must be registered for see images attach

You must be registered for see images attach

 

[colonel mustard]

first the hvac system is never off, the duct doors will always be open I dont know if max ac setting caused them to recirculate the interior air and closed off the duct for exterior air blowing across the heater core. second check your heater control hoses for a valve that would block flow preventing hot coolant from reaching the heater core. Third check the firewall and ac box for leaks and seal any you find.

if the heater core is blocked from getting hot coolant...wouldn't the coolant in the heater core be cold (ambient temp)? thus meaning there wouldn't be hot air?? or am I thinking backwards on that.

so if I set it to MAX AC, the blend door will close? sorry so many questions...I haven't messed with ac heater controls....ever..

I'm in the process of rebuilding my A/C. I checked all of the vacuum pods to make sure they worked. There is a pod under the hood in the cowl area on the passenger side. This picture is off eBay and has a plastic unit, mine is metal. This pod and flapper close when the A/C is turned on recirculate (Max A/C). Otherwise it is in the open position (applying vacuum will close it) and will provide fresh air to the cab at all times.

Also make sure the hot/cold cable is adjusted correctly. The interior door may be open and letting air get by to the heater core.

You must be registered for see images attach

You must be registered for see images attach

does the electrical part of the AC need to be in working order? im in the process of re-wiring the truck so as of now it isn't functioning.


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...???

 

[colonel mustard]

first the hvac system is never off, the duct doors will always be open I dont know if max ac setting caused them to recirculate the interior air and closed off the duct for exterior air blowing across the heater core. second check your heater control hoses for a valve that would block flow preventing hot coolant from reaching the heater core. Third check the firewall and ac box for leaks and seal any you find.

where would this valve be located and what does it look like. I reread your post and it clicked in my small brain what you meant. makes sense

 

[4WDKC]

if the heater core is blocked from getting hot coolant...wouldn't the coolant in the heater core be cold (ambient temp)? thus meaning there wouldn't be hot air?? or am I thinking backwards on that.

so if I set it to MAX AC, the blend door will close? sorry so many questions...I haven't messed with ac heater controls....ever..



does the electrical part of the AC need to be in working order? im in the process of re-wiring the truck so as of now it isn't functioning.


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...???

Correct, if closed its cool air but if stuck open or removed like so many people do you get hot air all the time. I know newer cars function this way with the max ac not sure if these trucks do or not. It is possible and something than needs to be ruled out, best part is it doesnt cost money to check this out.

where would this valve be located and what does it look like. I reread your post and it clicked in my small brain what you meant. makes sense

yep just takes a minute to set in.

 

[82sbshortbed]

Maybe disconnect your heater hoses from the water pump and intake and put a plug back where the hoses were. Then you're sure not to have hot coolant flowing to your core. My heater doesn't work and the hoses are disconnected.

 

[SquareRoot]

A good time to upgrade. Remove everything & install Vintage Air. No cables, vacuum lines/pots, excessive wiring...

Nice cool, r-134. Everything is completely separated by the firewall. Boom done.

 

[colonel mustard]

A good time to upgrade. Remove everything & install Vintage Air. No cables, vacuum lines/pots, excessive wiring...

Nice cool, r-134. Everything is completely separated by the firewall. Boom done.

I thought about that several times....was looking at some nice heaters on summit. but I wanna try this for pennies before I flip more coin than I really need to

 

[roundhouse]

if the heater core is blocked from getting hot coolant...wouldn't the coolant in the heater core be cold (ambient temp)? thus meaning there wouldn't be hot air?? or am I thinking backwards on that.

so if I set it to MAX AC, the blend door will close? sorry so many questions...I haven't messed with ac heater controls....ever..



does the electrical part of the AC need to be in working order? im in the process of re-wiring the truck so as of now it isn't functioning.


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...???

Easy way to tell is loop the heater hoses together , (you’ll need an adapter since they are different sizes ) to bypass the core so no hot water will be getting in the core

And go for a test drive

Have someone else drive so you can feel around under the dash

 

[roundhouse]

I thought about that several times....was looking at some nice heaters on summit. but I wanna try this for pennies before I flip more coin than I really need to

We got our Vintage Air setup from Rutters Rod Shop in North Carolina
Was cheaper than summit

 

[chengny]

so heres the deal.

1981 k5 350 swap. factory AC and Heat equipped truck although it is not functioning. the heat doesn't turn on...nothing. its all disconnected electrically for now. So you cannot operate the system blower at all - correct?

here my issue.

on the passenger side, excessive amount of heat is coming from behind the glove box area and venting out to the floor board. Does the hot air only seem to be from under the dash - behind the glove box? Is there flow out of any of the normal vents (i.e. defrost, dash vents, floor vents above the transmission tunnel). Does the volume of hot air coming into the cab seem to be related to road speed?


should I be looking at heat coming from the firewall??? I would imagine that you would be able to see a hole in the FW. maybe heat coming from some vent somewhere??? There is no cross connection to any system that would supply the air handling plenum with hot air. where should I start looking? I would work backwards. Maybe look at the flex hose that connects the main plenum riser to the RH dash vent.

You must be registered for see images attach

You must be registered for see images attach

They're pretty flimsy and after 37 years it may have come off.


thanks

These C60 AC systems are referred to as the reheat type. The air that is to be supplied to the cabin (whether from the recirc duct or from the outside) is drawn into the fan suction. Total output is then discharged through the evaporator coil.

See the image below. If the mode select lever is in one of the AC positions, the compressor will run as required. If any other mode is selected, the compressor will not operate.

You must be registered for see images attach

The evaporator outlet temperature is somewhere around 30 F with the compressor running. If air at that temperature issued from the dash vents, it would be way to cold for comfort.

So, to bring it up to a reasonable temp, the air is directed to the heater core. By the use of a diverter door it is caused to either bypass or pass through the heater core in proportions required to maintain the desired outlet temperature.

The flow of coolant that is supplied to the core is constant and unable to be controlled. I'm sure you can see where this is going. Because you have disabled your AC system, all you can do to cool the supply air down is to move the temperature lever at the control panel to COLD.

Doing that should cause all air flow to the ducts to bypass the heater core - if the temperature cable and linkage are working properly.

Justin and Mike suggest above that to confirm that the air is being heated by the core, restrict the flow to the core. You might want to take a pair of Vise-Grips and gently squeeze down on either the supply or return. The flow doesn't have to be completely shut off. Just enough to make a determination if its the heater core.

 

[colonel mustard]

These C60 AC systems are referred to as the reheat type. The air that is to be supplied to the cabin (whether from the recirc duct or from the outside) is drawn into the fan suction. Total output is then discharged through the evaporator coil.

See the image below. If the mode select lever is in one of the AC positions, the compressor will run as required. If any other mode is selected, the compressor will not operate.

You must be registered for see images attach

The evaporator outlet temperature is somewhere around 30 F with the compressor running. If air at that temperature issued from the dash vents, it would be way to cold for comfort.

So, to bring it up to a reasonable temp, the air is directed to the heater core. By the use of a diverter door it is caused to either bypass or pass through the heater core in proportions required to maintain the desired outlet temperature.

The flow of coolant that is supplied to the core is constant and unable to be controlled. I'm sure you can see where this is going. Because you have disabled your AC system, all you can do to cool the supply air down is to move the temperature lever at the control panel to COLD.

Doing that should cause all air flow to the ducts to bypass the heater core - if the temperature cable and linkage are working properly.

Justin and Mike suggest above that to confirm that the air is being heated by the core, restrict the flow to the core. You might want to take a pair of Vise-Grips and gently squeeze down on either the supply or return. The flow doesn't have to be completely shut off. Just enough to make a determination if its the heater core.

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?

 

[Craig 85]

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?

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.

You must be registered for see images attach