Defrost- no air flow

[Lazerwolf]

Not sure if this should be in this forum or heating/cooling. Winter is here in the rust belt, so time to tinker with the car while it's in the garage.

Late this fall, took the 1974 Charger for a drive over to shoreline about 20 miles away. Grand Haven for the Michiganders. Thunderstorm rolled in off lake and trouble began. No leaks but windshield fogged up bad. Turned on defrost but think I could have blown more hot air on window than it was putting out. Wiped a spot to see and headed home, leaving defrost on with fan on high. By time I got back home about 30 minutes later, had a couple of spots smaller than a basketball to see out of. It's probably important to mention the car originally had AC, but everything under hood was removed prior to my purchase and I've not gathered parts to put it back on yet.

Couple of things I noticed when I got home and had time to play. Fan speeds and temperature control (hot/cold) seemed to work fine. When I put the selection on VENT, I got a high flow of air out of dash vents. Putting it on HEAT, I get about a 10-20% air flow on the floor and 80-90% out of dash vents. Selecting the DEF option, I might as well shut it off since doesn't seem to be noticeable air flow anywhere even with fan on high.

My knowledge of the heater/defroster is limited to the selector switch on the dash. Any help or information would be appreciated.
Todd

 

[soundhd]

First I would say that a shop manual should show you how the heating/defroster system all goes together and works (assume it is a non A/C car). If the fan is indeed working correctly then there is some sort of blockage somewhere.....just have to get into/under the dash and check it all out.......if you do not own an shop manual for your year & model do you self a really big favor and purchase one.....will save you a lot of time and grief if you do your own repair work.....anyway you want to check and see that the cables are connected and moving the correct parts that direct the air flow to where ever you want it to go.

 

[Dennis H]

OK, not hijacking - but on my 69 Coronet the control box is cable action. Not sure if it is the same on your 74. I too have good flow everywhere except the defrost. When using defrost, it pushes good for a few seconds then something starts banging like a rubber mallet hitting the dash pad. Sounds like a door is jammed or something. Don't need defrost often here so been getting away with it until the wiring and dashwork is done. Good luck.

 

[Lazerwolf]

Unfortunately Dennis, it appears the 74's were vacuum operated not cable operated.

Further investigation this week and looking through the 73 Service Manual (don't have a 74), I think I might have a clue. There is a vacuum line that comes off the back of the temperature control module that runs through the same rubber grommet in the firewall as the temperature control cable. Not sure if this is only on AC cars like mine. The service manual says this goes "To Engine Vacuum". However my vacuum line runs through the firewall and is coiled up in the corner by the brake booster. I believe this should be connected to the fitting on the back of the intake manifold where the brake booster vacuum hose is attached. The aluminum fitting on mine only has one 3/8" port that has the brake booster vacuum connected, but I've seen originals on Ebay with 2 or 3 ports.

Could someone with a 73-74 AC car verify this? Any help is appreciated.