Heater blows, but doesn’t redirect air to defroster.

[SpriceyStuff]

Hey guys. I got my blower motor working in my 74 Dart and now I have heat again! Unfortunately, however, when I switch it to defroster mode it doesn’t redirect the air that direction. It keeps blowing air through the under-dash vents. How does this stuff work, and how do I fix it? I can see from underneath that the ducts from the heater box to the DEF vents art intact. At least the one on the passenger side is. If it means anything to you, it’s a factory A/C car

 

[SpriceyStuff]

I happen to have another heater box out of another 74 A/C Dart laying around. I see a cable that operates what appears to be a door that controls whether air blows by the heater core or not. There’s a vacuum canister on the other side with 2 hoses coming off of it. Sucking and blowing on them moves a rod a little, but I am unable to determine what that actually does

 

[67Dart273]

Do you have a service manual. If not send me a PM "conversation" Free by the way

 

[Oldmanmopar]

Check you vacuum source. Or you may have a bad control switch.

 

[Dave999]

chances are the vacuum switch is busted. 99.9% eletcrically working but 99% are vacuum circuit broken. never buy a second hand one, ever, they are all broken.... i could have been a rich man had i been told this....!

Why??
Due to the delay in actuating anything by vacuum, (inconsistently availble vacuum) people have a habit of pushing too hard on the buttons and they crack the vacuum section internal to the back half of the switch unit.
then even if the black pipe is conencted to the vacuum port on manifold or carb
pressing the buttons on the control just makes the air leak into your inlet smaller or bigger...
and vacuum does not build high enough to trigger any of the vacuum actuators.

OR someone removed the inner fender mounted water tap and vaccum actuator in the heater matrix pipe-run, leaving the red and white pipes for the actuator tied at one side, this results in exactly the same leak of air into the system.
and nothing works.
this fender mounted tap is expensive to replace and unless someone dismantled and greased it with plumbers tap grease it will have siezed either open or closed.
i have not tired cheap ebay replacement.

any way.. here is my winter "bodge"

pull the slider across to hot on the control. provided its cable runs from the control across to the top of the AC/heater unit, and its outer cable is clamped effectivly using one of those chrylser specifc cable clamps to the L shaped bracket on the top of the unit. it should open the door to the heater matrix for fan air. trouble is the vacuum activator for outside air is probably still open and you can't easily reach it becasue its on the back of the unit hard aginst the bulkhead.
once you have done this you will get warm air, but it will then only come out of the bottom of the unit. I think you got this far....

Now
get down on the passenger side floor and find the rod from the vacuum actuator that you can see near the kick panel, the rod that runs across the bottom of the unit
it naturally relaxes into its "push air out at the floor" position
pull it all the way against the tension you feel, It will come out 1/2 an inch or so from the actuator, the rod may have a necked in section (bonus) and you will, hear some flaps, flap to a new position.
put a cable tie around the rod it to stop it going back. which it will do with a certain vigour.
put another cable tie around it to stop it pushing the first cable tie along the rod due to vibration
blob of rubber glue if you want. something you can strip off easily in the summer months

you now have both hot air from matrix and cold air from the plenum combining to give warm air out of the unit and it shoould hit the screen when the fan is on....
provided the plastic trunking from the base of the air outlet runs up under the dash and has both cardboard tubes connected to the dash vents.
this style of cardboard tube is, still widely used for forced air heating in american campers caravans and RVs
so you can buy as much as you like in imperial sizes

this plastic bit came in two varieties, and is for want of a better explanation, a Z shaped square section tube held to the front of a square outlet on the aircon or heater unit by two self tapping screws. one style for normal heaters (fat square plastic) and one for aircon (starts as fat square plastic but has a narrower section in the long upright of the Z to allow it to fit behind the under dash aircon diffuser without being crushed by it.
both end at 2 circular spigots coming out front and side for mounting the cardboard flex tube that runs up to the dash vents.

I can't explain the vacuum circuit here but a service manual of roundly the right year off of mymopar.com will have an exceedinlgly detailed aircon section

you don't need any of the evaporator or pump under the hood to have a working set of vaccum flaps directing cool (outside temp) and hot air.
don't be pulling on red and white pipes that go behind the unit
putting them back on is a removal and re fit
You can get the unit out of a car but its more like remving the car from the unit.
most would suggest taking out the screen and the dash frame
you don't need to, BUT the unit has to come out in parts, 2 or 3. and you are working blind

dave

 

[SpriceyStuff]

Do you have a service manual. If not send me a PM "conversation" Free by the way

I have this thing

chances are the vacuum switch is busted. 99.9% eletcrically working but 99% are vacuum circuit broken. never buy a second hand one, ever, they are all broken.... i could have been a rich man had i been told this....!

Why??
Due to the delay in actuating anything by vacuum, (inconsistently availble vacuum) people have a habit of pushing too hard on the buttons and they crack the vacuum section internal to the back half of the switch unit.
then even if the black pipe is conencted to the vacuum port on manifold or carb
pressing the buttons on the control just makes the air leak into your inlet smaller or bigger...
and vacuum does not build high enough to trigger any of the vacuum actuators.

OR someone removed the inner fender mounted water tap and vaccum actuator in the heater matrix pipe-run, leaving the red and white pipes for the actuator tied at one side, this results in exactly the same leak of air into the system.
and nothing works.
this fender mounted tap is expensive to replace and unless someone dismantled and greased it with plumbers tap grease it will have siezed either open or closed.
i have not tired cheap ebay replacement.

any way.. here is my winter "bodge"

pull the slider across to hot on the control. provided its cable runs from the control across to the top of the AC/heater unit, and its outer cable is clamped effectivly using one of those chrylser specifc cable clamps to the L shaped bracket on the top of the unit. it should open the door to the heater matrix for fan air. trouble is the vacuum activator for outside air is probably still open and you can't easily reach it becasue its on the back of the unit hard aginst the bulkhead.
once you have done this you will get warm air, but it will then only come out of the bottom of the unit. I think you got this far....

Now
get down on the passenger side floor and find the rod from the vacuum actuator that you can see near the kick panel, the rod that runs across the bottom of the unit
it naturally relaxes into its "push air out at the floor" position
pull it all the way against the tension you feel, It will come out 1/2 an inch or so from the actuator, the rod may have a necked in section (bonus) and you will, hear some flaps, flap to a new position.
put a cable tie around the rod it to stop it going back. which it will do with a certain vigour.
put another cable tie around it to stop it pushing the first cable tie along the rod due to vibration
blob of rubber glue if you want. something you can strip off easily in the summer months

you now have both hot air from matrix and cold air from the plenum combining to give warm air out of the unit and it shoould hit the screen when the fan is on....
provided the plastic trunking from the base of the air outlet runs up under the dash and has both cardboard tubes connected to the dash vents.
this style of cardboard tube is, still widely used for forced air heating in american campers caravans and RVs
so you can buy as much as you like in imperial sizes

this plastic bit came in two varieties, and is for want of a better explanation, a Z shaped square section tube held to the front of a square outlet on the aircon or heater unit by two self tapping screws. one style for normal heaters (fat square plastic) and one for aircon (starts as fat square plastic but has a narrower section in the long upright of the Z to allow it to fit behind the under dash aircon diffuser without being crushed by it.
both end at 2 circular spigots coming out front and side for mounting the cardboard flex tube that runs up to the dash vents.

I can't explain the vacuum circuit here but a service manual of roundly the right year off of mymopar.com will have an exceedinlgly detailed aircon section

you don't need any of the evaporator or pump under the hood to have a working set of vaccum flaps directing cool (outside temp) and hot air.
don't be pulling on red and white pipes that go behind the unit
putting them back on is a removal and re fit
You can get the unit out of a car but its more like remving the car from the unit.
most would suggest taking out the screen and the dash frame
you don't need to, BUT the unit has to come out in parts, 2 or 3. and you are working blind

dave

holy cow, what a reply. Thank you so much! I’ll take a look at this when I get off work today. I played with my extra unit sucking and blowing on the red and white hoses and I’ve determined that they actuate a door that appears to be a switch between outside air and recirculating air. I still have the “vacuum amplifier” unit on my inner fender, but I’m not sure if it’s properly hooked up or not. The car I believe was well used over the years, minus sitting for 17 years between 2005 and 2022. Maybe it all still works. Who knows? I’ll take a look though. Thank you again.

I know the cloth tube on the driver side disintegrated long ago, when I was redoing wiring under the dash due to a fire that burned a lot of it out.but there is a plastic duct tube that appears to have been a replacement part someone else put in before, and it goes up toward the dash.only trouble there is that it’s on the passenger side. Not sure how good that will be at defrosting the whole windshield, even if that is a defroster. Can you tell me anything about rear window defrosters? Is that something my car would have in 1974, or is that a later model option? Or just an option in general? I see it in wiring diagrams but I don’t know if my car would have even had that

 

[Dave999]

heated rear screen or a fan unit and duct would be obvious
screen would have visble heater wires in it
or fan and duct would be visible in trunk or on parcel shelf.

front. with only 1 cardboard tinfoil and fibre tube all you will do i heat the wireing

you need both to get any appreciable flow out of both vents

take off the dash pad (stud mount in both end corners and 1 in the middle usually) and take out the ashtray.... you will have enough access unless you have arms like the hulk....
dash pad has 3 studs and a load of clips once studs are undone it comes off towards you.

when you put dash pad back on just do up the middle stud...leave the ends undone to avoid it cracking in hot wheather

clamping taught old vinyl back on in winter and having the dash box flex and expand in the heat of summer will just crack the pad in 6 months time....

id go as far as to say anywhere where ambient temp can vary from freezing to too damn hot
with the seasons that all would benefit from undoing the nuts at the ends of their pads and throwing them over the hedge....learned this lesson in Australia...too damn hot..... 25 years ago and have a perfect dash pad ever since...

Dave