Is there a way to visually check to see if the blend door in my HVAC is intact, i.e., not broken? (for example - can a hole be cut somewhere so that the entire dash doesn't have to come off?)
Thanks for all your help Cruiser54, wherever you may be.
Robert

 

Just to make sure, the temp control is cable operated. Can you tell us why you think the door may be broken?

While looking at the blend door arm (crank), move the temp selector full range (cold-hot-cold) and make sure the arm moves. Then you can detach the temperature control cable from the blend door and turn the door manually to "feel" like it's swinging the door. It should swing fairly freely.

You could drill a small hole in the bottom about middle of the swing to verify the door sweeps OK. Just be careful you don't drill into other structure, the core or evap.

 

Yep, like Saudade said, the little lever under the passenger dash where the cable hooks up will tell you everything you need to know. It should move very easily.
That lever is fixed pretty well to the blend door so if it moves then your blend door does too. The only way it could disconnect is if it rusted in half and fell out.
My blend door was bound up pretty tight until I replaced my heater core and cleaned and lubed everything in the HVAC box.
Much of the resistance was in the cable itself so I removed it, ran cable lube through it while working it back and forth and it freed up nicely. Now I can move the temp lever easily with my pinky and it works great.
The little clip that connects the cable to the blend door lever is pretty weak by design, so theres a good chance it just slides back and forth on the lever if the lever is stuck.

I just posted instructions how to replace the heater core without taking the dash all the way apart in this thread: cherokeeforum.com/f2/heater-only-blows-warm-275873/

 

Everyone - thanks for the information.
When I had a mechanic replace the heater core (I was getting evaporation onto the inside of the windshield), I can't confirm that he checked all that you mentioned above. Some mechanics - perhaps those less conscientious - only fix what's obviously wrong and don't check other things. That being said, he may have only relied on the SOUND of the blend door moving when he moved the HVAC control unit lever.
So I took an hour drive last night and here's what I can tell you.
I drove on the freeway in 52 degree temps. I kept the heater closed and off for almost the entire time UNTIL I was 10 minutes out from my destination, which was now on city streets.
The temp gauge went up slightly in the slower traffic, to about 220 degrees. NO HOT AIR OUT OF THE HEATER VENT OR "HEAT" OR "VENT". Basically the temperature of the aire, with the blower on full and blasting air into the cab, was the same temp as the outside of the car air.
Just a reminder: the inlet and outlet of the heater core, when touched inside the engine compartment, were both HOT - with the outlet being slightly cooler, but still hot. Note: it was nitetime and too inconvenient for me to get out my infrared gun to check actual temps. Also, the radiator is only two years old and the thermostate was replaced twiec as a double check two weeks ago, as well as the heater control valve, now replaced 3 times. At some point maybe I will take some pics and upload.
BUT, here is one interesting thing: when I drove the car back home, the same one hour drive time, all the above conditions still existed with no heat coming out of the vent. I stopped to get a couple of slices - maybe ten minutes - with the car off. Got back in the car, with the blower and heater on, and for a brief moment, I had much hotter air and then within about 5 seconds, that air turns cold again. Also not that I checked the A/C it is fully working, icy cold air coming out.
So this is very curious - if the A/C is working, that would seem to indicate that the blend door (through the lever>vacuum pump on the heater control valve) is closing that blend door to keep the hot air out that would be coming from the heater control channel of that whole box down there.
So - kind of a mystery. I won't go back to the mechanic who put the heater core in for this issue. Personally, I could be wrong, but I don't think he wants to work on this truck anymore. I was getting some "vibes" from him when I picked up the truck AND my driver's side key lock was broken. Could be coincidence with the cold weather, but I don't want to go there and confront him on it.
Besides this being a mystery, all of you who read this keep in mind that I am not a mechanic and can do some things. I found another 2 mechanics who work on older vehicles and will take a look, but if I go there with all these different ideas, I have experienced some mechanics who don't want to hear it. Some mechanic frown on "tinkerers" and people like myself who want to make modifications to improve the vehicle. I am a Cruiser54 graduate and have done ALL but one (throttle body mod) of his list of modifications. So I have to do what I can first, and then go from there.
Here is my next attempt: I am going to pull the bezel off the dash and pull out the HVAC unit and see if anything is hanging up the lever. It will come out about 6 inches without un-connecting anything and at least I will be able to see/feel if any cables are hanging up preventing the lever from going even further to the right. Even though I can visually see that the heater control arm is moving when actuated by that lever, I don't want to physically move that arm for fear of breaking something inside the pump and then having to replace that again - Im' not that experienced with what looks or feels right with that valve.
There's the latest ! Thanks anyone for helping me along with this. It was only a week ago that I found out that there was thing thing called a blend door.

 

Got enough coolant?

 

Got enough coolant?
Yep - I measure it everytime before I start it up with a popcycle stick that is marked with a zip tie: from upper water level to the top of the pressure tank inlet (Mac's Radiator aluminum tank).

 

Insufficient, Erratic,
or No Heat
Low Coolant Level
Incorrect thermostat.
Restricted coolant flow through
heater core.
Heater hoses plugged.
Misadjusted control cable.
Sticking heater control valve.
Vacuum hose leaking.
Vacuum hose blocked.
Vacuum motors inoperative.
Blocked air inlet.
Inoperative heater blower motor.
Oil residue on heater core

 

My "wag" is that when your mechanic replaced the heater core, he did not insure the blend door was seated correctly. Look at 89Laredo's excellent diagram and you can see the function of the door. The door has stubs on each end that act like hinges in little cups made into the housing. Easy to miss align when replacing the heater core. Good Luck!

 

Is that dotted line to the left of "Cool" where the lever cable is from the control unit (AC to heat)?

 

The double dotted lines are the gates in their other positions. Yes, that is the one the cable attached to.
If you look under the passenger dash, can you move lever ? My bet is that the cable is slipping on the blend door lever. If the lever is moving then you're not getting coolant through.

Do you have the vacuum controlled valve on the heater hoses?

 

Yes - and I checked that. the Lever on the pump moves about 90degree or more when the AC/Hot lever is moved to the right (and to the left)..
Haven't looked under the dash. I am going to remove the unit from the dash (bezel comes off + 4 srews on the unit) to see if there is more "play" available and if that makes a difference.
I have to wait till I can move my truck as I live in the City and don't like to work on it when its parked down the street from my apartment.
Thanks, RR

 

If your core is filthy inside the lines, it will not heat your cabin.

Here


You are welcome

 

Quote: If your core is filthy inside the lines, it will not heat your cabin.
Reply: See Post #4 - the heater core is brand new.

 

OK - I haven't been able to visually check the blend doors yet as my newest heater contol valve is not working properly (this is the third one; I finally found a MOPAR one that I will install soon).

So - I have installed a third heater control valve ("HCV"; Chinese made) and the arm on the valve moves somewhat when I move the lever on the control unit.
Note: Last year I had a vaccum pump installed (Leeds) on my brake booster because my overall vacumm is low at app. 17 psi (260k miles and I need a valve job on #1, which is 30% lower than the other valves, as measured in a leak-down test). The brakes are incredible now, better than I can remember before the install.

Is it possible that the vacuum to that heater control valve is so low that the heater control valve butterfly door won't open or close all the way? Which could also explain why no heat (not cold, but mildly warm). Stated another way - maybe the 3 heater control valves I have installed previously were actually OK, but just not getting enough vacuum?

I just received the Mopar valve and tested it by unhooking the vacuum line (car running) from the inline heater control valve and hooking it to the new, uninstalled valve. The new valve open and closes quickly - but the wild card here is that since it isn't installed, there is no coolant flow to hinder the new valve's butterfly door from opening/closing.

At this point, I guess there are 2 things to do (still hoping someone replies whether a separate vaccum pump can be installed to actuate that heater control valve vacuum pump). 1) try the new Mopar HCV and see if it works and 2) after I take out the existing heater control valve, chech it with the vacuum hose that goes to it when it is installed and see if it opens/closes as easily as the new Mopar one I tested before I installed it.

Oh man . . .or get a valve job . . .
Thoughts?

 

Have you checked the temp of the core's inlet (bottom one) and outlet hose (top one)? The expansion bottle and core are plumbed in parallel. The heater valve only allows or restricts flow to the core. The valve needs vacuum applied to block the flow so the valve should be open (allow) when there's no vacuum so a vacuum pump won't help unless the vent doors are not working. In that case, airflow would default to Defrost.. I've had my heater valve disconnected from vacuum for years and no issue getting heat when needed (vent outlet reaches 110-120F). So disconnect and plug the vac line and see if that gets you more heat.

You can bypass the heater valve but it's a bit more involved than with the later open system. Look at the "Y" pipe on the outlet side. You would need to make something similar.