Hi guys, I'm sure someone has already dealt with this one, but I haven't...Yet
My 99 ram's a/c blows nice and cold, until you put your foot into it, then it switches to defrost, which sucks..I thought about locking the ducts vaccum switches, but there is probably a better fix available. any suggestions ?? Thanks, Mike

Look at the vac line at the firewall as it goes into the cab (plastic line pass side) I would bet it has a nice worn spot in it causing a leak.
Have had to fix this twice on my 3500.

mike

This was discussed last week also, IIRC. Jeep GC doing the same thing - consensus was a failed vacuum check valve. If the vac line looks okay per the previous poster's response, this may be another possible source.

My 96' 2500 Ram does the same thing. It has done it for years. I have just dealt with it. I guess, I'll look at the vac line going into the cab as suggested. I saw a TSB before that had a diagram of which vac line needed an additional vac valve installed o cure the problem. I tried that as the diagram showed but then it stayed on defrost 100% of the time no matter where the switch was. So, I removed it.

The factory says:

Vehicle Application:
1996 Dodge Ram 1500 5.2L, GAS, Vin Y, Eng Cfg V8
1999 Dodge Ram 3500 Van 5.9L, GAS, Vin Z
1998 Dodge Ram 1500 5.2L, Vin Y, Eng Cfg V8
2000 Dodge Ram 1500 Pickup 5.9L, Vin Z, Eng Cfg V8
1998 Dodge Ram 1500 5.9L, Vin Z, Eng Cfg V8
2001 Dodge Ram 1500 Pickup 5.2L, Vin Y, Eng Cfg V8
1999 Dodge Ram 1500 Van 5.2L, Vin Y, Eng Cfg V8
2001 Dodge Ram 1500 Pickup 5.9L, Vin Z, Eng Cfg V8

Customer Concern:
Driving with the A/C on under light or heavy acceleration, the airflow will stop blowing out of the vent ducts and switch to blowing out of the defrost vents.

Tests/Procedures:
1. Check the vacuum check valve at the right side of the intake manifold and check the second check valve in the line to the left of the receiver drier.

2. If they are good, check for a leaking line or defective and leaking vacuum motor in the Heater Ventilation Air Conditioning (HVAC) system.

Potential Causes:
Plastic Line
Vacuum Check Valve
Vacuum Motor

Tech Tips:
When vacuum is lost to the system as in acceleration, the HVAC system will switch to the defrost position as a safety default.

Diagnostic Codes:
None

I have been trying to fix my 96 Ram 1500 for two years, I did everything listed, changed out all check valves and hoses, tested all the vac motors... no luck. I finally decided to pull out the A/C control switch, the vac lines run to it and it directs the vac to the appropriate vac motors. When I took it apart, you could see that this flat molded rubber gasket has raised ribs on it that seal the various passages so that the vac goes to the correct ports when it moves laterally. The gasket looks like it has shrunk about 10%. It also is lubricated with a grease like lubriplate and was dried out. I cleaned the surfaces and relubed it and reassembled it as a Hail Mary play. When I put in service, I ended up having to push up on the knob and stick a toothpick under it to wedge it up, but it works like a champ, no more dropping out on acceleration and my recirc setting works again which lets my A/C blow 11* colder. I called the dealer as this is a dealer only item and found that you cannot buy just the switch, you have to buy the whole panel assembly with temp setting and blower speed etc. This is about $160, so I think I will wait till winter gets here. It has worked perfectly for a couple of weeks now. Hope this helps and if you try it and it works for you, let us know!

BTTT.

There is a vacuum leak, and it may or may not be in the heater control system

Well, I still haven't had time to actually work on the truck, but I needed to use it this weekend, and was not willing to sweat my ass off again, so here's the way to get the A/C to blow out of the vents in the dash....Look under the dash, above the gas pedal, and you'll see the vaccum canister that controls the ducts. Hook up your Mighty Vac vaccum pump, and give it a few squeezes...and whattaya know, the vents blow cold air out again. If you need Defrost, release the vaccum pump, and out the defroster ducts it goes.
I know this is half assed, but for a quick temproairy fix, this is the way to go.... Mike

Yeah, sounds like the switch for sure. I just had the pleasure of shelling out $180 for the switch and panel, the good part is that everything works perfectly, the bad part is that a 75 cent molded rubber gate inside the switch was the only problem. Good luck!

Do you think that rubber gate could be repaired ??? How about some of the liquid rubber that you dip your tool handle into....just a thought. I still haven't had a chance to get into mine yet, too many other things on the to do list...Mike