Freezing over in heater box ?


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Sep 18, 2018
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Hey guys,

i have my AC running on 134 for over a year now (everything is new, RV2 compressor, radiator (not original - much smaller, some universal chinnese crap), expansion, dryer etc), and it cools great.

When it's on for longer (say 1h +) it's starts to cool weaker, and air from the vents does not flow as much. The cure is to switch it off for 2-3 mins and back on. During this time i have water leaking onto my carpet from beneath the dash (heater box area). I believe it's freezing over.

The problem i am having is, that when i turn the car off, it leaks (probably melts all the ice from the heater box) A LOT of water from underneath the car (which is ok) onto the pavement. It's a lot of water, like over a gallon. The real problem is, a lot of the water leaks into the cockpit, usually onto the passenger floor (wet carpet), but if the car is tilted to the back, it really floods the inside of the car. I mean FLOODS - like over a gallon of clean water.

What could be the problem ? The water passage from the heater box is blocked, or too small for the water to escape?

Please let me know.

I am now sacrificing comfort, so as not to flood my carpet. The problem has much instensified in very hot (+90F ) and humid air.

What to do ?

cheers for any input :)
Sounds like the drain tube is blocked or to small. and also the box might need to be resealsed. Yes it sucks.
To fix the leaks inside the car get a gasket kit from DMT: Mopar C Body 69-73 AC Heater Box Rebuild Restoration Seal Gasket Kit

Is the thermal expansion valve sensing line set up properly?
There may need to be a cycling switch installed when running r134 with the RV2 compressor but I'm not sure. There are 2 places off the top of my head I would contact for tech help, Classic Auto Air, or Vintage Air.
The Valiants of that era had a cycling RV-2 compressor. The difference in the A-body and C-body systems was that the A-body systems did not have an EPR valve on the compressor inlet side, but used a cycling switch instead. EPR = evaporator pressure regulator, which kept the cooling at optimum levels without causing the evaporator core from freezing up.

As to the drain tube from the hvac case, DO make sure it is open and clear.

As to "hvac case leaks", these were FAR more common than anybody knows about, from my experiences. A main issue was in how the case halves were attached and the gasket which sealed them.

On one side of the case, there were slip-on items which had the nuts the attaching bolts screwed into. So far, so good.

On the other side, the side the bolts went through, it was smooth. The gasket was a somewhat stiff rubberized item. It had too much surface tension to effectively seal the case from moisture leakage due to the small gaps it could not get into due to the surface tension of the rubber. Too much torque on the hold-together bolts and the case flange might crack (being a fiberglass-type material).

I was at the local dealership one day when a customer (1969 Fury) had returned as she was still getting water in the rh footwell. I also knew that the local dealership techs were doing the repair as the factory specified, too. A new gasket was installed on her car.

I discussed the situation with the service manager, as a curious inquiry, as I knew we had had no such things on our '66 Newport. He noted that with the '69s C-body cars, they had deleted the drain pan the earlier cars had had in them. Obviously, the accountants decided the cost of the drain pan was "extra" when the bottom of the case should direct water to the drain tube?

Later, with my '70 Monaco Brougham, after I'd had it a while (it had 83K miles on it when I bought it), I pulled up the factory OEM hard vinyl floor mats. I had felt dampness on the upper edge of the exposed carpet. I discovered the complete rh footwell was more than damp, but not quite squishy! The bottom side of the mat was physically damp, too. I found a few drops on the bottom edge of the hvac case. That's when I first paid attention to how that area was sealed. When it was obvious that the gasket itself was not sealing well. NOR would it ever seal well, due to the gaps I mentioned above.

Additionally, any small dops which hit the upper carpet would get into the carpet and wick to the footwell area, under the mats, undetected. Possibly getting into the jute mat under the carpet for accumulation and or dispersion/evaporation due to the warmer underbody footwell (from engine heat and such).

Seeing the defects in the factory design and execution of the hvac case halves, how to fix it?

The obvious thing would be to replace the slide-on clip nuts with another style of retainer. A captured washer nut, possibly, which would allow for two flat surfaces the gasket could seal against? OEM production would not use them as one might get lost and not replaced, on the line, I suspect, choosing the "stay there" slip-on nut item instead.

Another option would be to forget the gasket and use a medium-width bead of high-heat silicone sealer (not available in those earlier years), being more heavy-bodied than normal silicone sealer. Without a robot installing the sealer (as happened in later years), variable results could happen.

So . . . here's my theory. Use the factory gasket kit mentioned above. Then, use the high-heat silicone sealer mentioned above and put a skin coat of it on all sides of the gasket, the flat sides and also the inside and outside edges. Let that cure for one day before installing. As to the slip-on clip/nuts, you can probably leave them installed, but wipe them off with a cloth and then put a dab of the high-heat silicone sealer on each side of them, making a smooth surface for the gasket to seal against. With a more gradual transition between their additional height and the case sealing surface they are installed on, rather than the very quick transition in elevations of the OEM situation.

Yes, a few more labor-operation steps to prep the case halves and gasket, but I highly suspect they will result in a better-than-OEM situation which should endure a very long time. With attention to details.

After seeing how the thick vinyl OEM floormats would block the evaporation of any condensate from the a/c case or from many temperature swings, I was much more diligent about checking for such accumulations! Knowing about this, I started to check the area every month of so, even removing the mats for things to fully dry out. Becoming less of a fan of those thick vinyl mats, too.

Hope this might help,
Icing up is not normal. Obviously the drain needs to be open, but if does'nt leak part of the time, I'd bet you are not able to catch the ice. This problem would happen with any type of mid temp application that ices up. The two main issues causing that are low air flow or low charge of the refrigerant. Make sure the heater core, evaporator and the blower wheel are clean, very clean, and the blower motor is up to snuff. If that all looks good, have the charge checked and weigh-in the proper amount if need be.... If there is no engineering for your particular unit, just add refrigerant slightly to reduce the superheat... this will effectively run the evaporator at a slightly higher temperature and stop the freeze ups.
Thank you for all the great input. Much appreciated. The issue is more complex than suspected. I will start with the drain tube. Then go for the refrigerant level, and finally onto the hvac box.

The entire box was rebuilt, and resealed, but not as mentioned above, and using just similar but not original materials.

I will probably get the gasket kit, and put it all together as CBODY67 mentioned above with an additional layer of heat silicone.

Probably better doing this than letting the carpet and floor rot out...
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If the coil is freezing over, I doubt if new gaskets are going to help any.

Yes, check the drain tube, but you need to address the evaporator coil freezing over.
I fought the same problem on my 1971 Town and Country. Had it apart three times, used silicone sealer to no avail. Bottom line it is a lousy design. This may sound hokey but I finally fixed it by using 3M black butyl strip caulk. I applied it all around the bottom seam and as far up as needed externally, without taking it apart. The biggest problem is the caulk wants to stick to your fingers as you apply it making it a tedious exercise. I did it at least 10 years ago and it hasn't leaked since. I also converted to 134a and removed the epr valve and it freezes up as well. I hope this helps. I was so glad this resolved the issue.