I have a 99 Ram Diesel and it has had this heater problem since I purchased it but now it is worse. I start the vehicle and turn the heater on and it blows cold (as it should) I then drive some distance and the temp gauge on the dash indicates the engine is warming up but the heater is still blowing total cold air. I then hear a click (Sounds like a damper is opening) and immediately hot air blows. Sometimes I can get the damper to open if I turn the heater temp control back and forth rapidly. When the truck was on warranty I had them look at this problem a number of times and they always told me this was normal. I do not live in a very cold climate (Sacramento) so it doesn't take a long time for the coolant to heat up but whatever tells that damper door to open is not doing it soon enough. It now takes about 15 minutes of driving on the freeway at 70 mph for the damper door to open and the temp gauge has indicated normal running temp for 10 minutes before it opens. When I go to a cold climate (Tahoe) the problem is much worse.

 

Kinda sounds like a blend door problem. Do you have decent vacuum from the pump? I have not had to get into mine so I don't know how hard it is to check the door. Somebody will chime in with the answer.
Tom

 

I would check your vacuum hoses under the dash and under the hood.

Just my 2 cents worth...

 

I would bet that the adapter for your blend door is broken. Actuator is under the dash passenger side toward the middle. About the size of a deck of cards with a three (?) wire connector to it. Roll the carpet back, cut, hack, peel, destroy (whatever it takes to get it out of the way) the sound deadener. Remove the front screw, and then the rear just loosen until you can get the actuator down far enough the access the adapter. Chances are it is cracked, or broken in two.

 

That's soooooo 12 VALVE!

 

Oh well...

 

I did as J BODY suggested and removed the front screw of the actuater and loosened the the rear screw and I was able to remove the adaptor. It is about one inch long, one half inch in diameter and has a male and a female end. It doesn't appear to be broken. Does anyone else have a suggestion?

 

I really don't think it is a mechanical problem, I think it is an electrical problem. Whatever provides the signal for the door to open when you start up your vehicle and initially it blows cold air like a/c then when the engine warms up and you hear a door open and then immediately the air coming out of the heater vents blows hot. Whereever the signal is generated from to tell that door to open is waiting way too long to send the signal. I now drive for miles with the engine warm in Sacramento mild climate before that signal is sent to change the air coming out of the heater vents from freezing cold immediately to very hot. The vehicle has had this problem since it was new but is much worse now.

 

I would vote for the actuator, but get a manual and do some trouble shooting first.

I had to replace the actuator on my car, and it was 100 x harder to get to (had to remove COMPLETE dash AND windshield). it was acting the same way, and then it just stuck. Stuck on lukewarm air and it was 30 deg outside.

 

There is no electrical control for the blend door. I am still leaning towards a vacuum leak. My Haynes manual shows there is a vacuum plug on the control and there are seven different ports on it. The plug is located on the control assembly and there could also be a leak in the control assembly itself. It also says to pull the vacuum harness away from the control and put 8" vacuum on each of the seven ports. If one does not hold it is leaking. You can also check the blend cable while you are there.
Tom

 

On a 1999 the BLEND AIR DOOR is electronically moved by an ACTUATOR at the bottom of the HVAC. The only vac on the 1999 is the MODE control. That's the TOP **** that provides max a/c, a/c vent, bi-level, fresh air vent, floor, floor/defrost, defrost. The COLD/HOT **** (bottom) has a three wire connector on the back of it which feeds inputs to the actuator. The HVAC control head is made by Lucas (the prince of darkness) which also made those fine multi function switches we so love. I can only remember changing one control head ever for any prob and it was most likely a fan switch issue. I have changed a couple of actuators, but they rarely are an issue either. Most common prob I see is the broken, cracked adapter that has already been verified as good.

 

I may have fixed the problem by just taking the adapter out and putting it back in. The heater starts to blow other than cold air about 3 min after I saart it now and I don't hear that door opening. However it hasn't been cold in the mornings. The jury will still be out until we have cold mornings and it still works.

 

The problem appears to be fixed. It was cold out this am and it worked fine. I took the truck to the Dodge dealer a number of times shortly after I purchased it and all they ever said is that it appeared to be working just fine. Finally the problem got so bad I did something. Possibly the adapter was installed wrong and by me taking it out and putting it back in, it is fixed.

 

Just to throw this in here, if for nothing else but for some more ideas to later searchers. Check that your **** is not slipping on the control shaft. My daughter had one that didnt get cold at all. The **** was slipping, but you couldnt really tell in that the **** didnt feel loose or unstable in any way. Go figure.

 

I had a 94 in the shop that had the stripped ****, now as it stands I always check the **** also. Sometimes it is the obvious that we overlook isn't it?? Goodluck,,Rick