Defrosters, Less Vent Holes Better?

[Fleetwood Meadow]

In my ‘51 Dodge the defroster works absolutely terribly. When I took the vent cover off that runs from one side of the windshield to the other I found 11 slots, each about 2” long, spaced along the windshield. The defroster channel is made of metal. The inlet from the blower motor is on the passenger’s side I am assuming that the purpose of the design is to get large amounts of air flow spread evenly across the front windows. That is not the case. The passenger’s side windshield starts to defrost after about 30-45 minutes. The driver’s side never gets clear. So the question arose, if I found a way to close off half of each slot, making them all 1” long, would that build pressure inside the defroster channel and push the air out of the smaller holes more powerfully actually defrosting the window? I noticed that when I put tape over every other slot the air movement was felt across the windshield to the other side of the car. 

[plymouthcranbrook]

I have a 52 Plymouth Cranbrook and  I find the biggest issue with my defrost is the fact that it uses air in the car to blow on the windshield. The defrost doesn't come directly from the heater like a more modern car. No way to eliminate humidity that way. I have better luck using the heater and cracking a vent window. 

[Rusty_OToole]

Chrysler products of that time used a complicated heater system with a blower intake behind the grille, a heater on the inner fender and cardboard duct work into the body. My first move would be to be sure the blower was working correctly and all the air was not leaking away. By the way I have more than doubled the output of an old heater fan just by cleaning the dust and dirt out of the fins. Take the fan off, soak it in soapy water, clean off every spec of dirt and give it a thin coat of gloss spray paint. It is tedious to get every bit of dirt off but necessary to keep it balanced. Oil the bearings with synthetic motor oil and be amazed at how much air it moves. Make sure all the wiring connections are good including ground.

You could also clean out the duct work and blow  thru the heater core with a compressed air hose from the back, it is probably full of dirt, leaves etc too. For good measure you could fill the heater core with CLR suitably diluted, and flush with the garden hose.

The idea is to make sure everything is clean and working as the factory intended. If you do this you should have no more trouble with your heater or defroster. I have done this kind of overhaul on other old cars with good results, it takes time but the cost in money is almost nothing.

[Rusty_OToole]

To put it another way your problem is not defective design, it is lack of air. Possible causes, blower fan clogged with dirt or running too slow, ducts full of dirt leaves mouse nests, rusted out or leaking hoses, ducts etc. Be sure the fan is working correctly and everything is clean and tight and it should work fine. They did not design the system to not work, it must have worked when it was new. Make it as the factory intended.

[Fleetwood Meadow]

Here is the issue with this car. I should have mentioned it in the original post. This car does not have the Mopar All Weather system in it. The heater core is on the firewall with a blower motor on the inside of the firewall. It has no name on it and it uses a cable to move the lever in the housing to push it from floor to defroster. The only thing on the inside of the engine bay is really just a hose going from the grill to the firewall. The hose from the blower motor to the defroster track is 2.5” in diameter. I’m not sure when this heater was put in or if there was never a heater from the factory but it does not appear to be something that would have come out of the factory. 

[Rusty_OToole]

In that case the heater is probably held in place by 2 bolts, most of them were. Easiest way to fix it might be to take it off the car and work on it on the bench. Blow out the core and service the blower fan as described and go from there. If it still won't do anything you could try taping over some of the holes with duct tape and see what happens.