ToxicFox

Okay, so I drive an '89 with the straight 6. The vacuum line that runs to the heater control valve always has suction, which makes it so that I do not have any heat at all when I switch the vents over to heater. What is it exactly that controls the vacuum for the heater control valve, and how do I fix it? And no one say "bypass the heater control valve." The valve itself works fine. It's the vacuum that controls it is the problem.

cruiser54

Dash heater controls.

Bypass the heater control valve with a piece of tubing. Plug the hose with something. Make sure your heater blend door is adjusted so it closes fully and you'll be fine.

ZINGGG

Works fine for now, maybe. It'll leak soon enough. Bypass it and take cruiser's recommendation

cruiser54

I know I went against the OP's wishes, but I lightly bumped one of those valves with my elbow while doing a repair under the hood, and it split apart and sprayed me with hot coolant. They serve no purpose at all, except to fail at the most inopportune moment and cost you money, and likley stranding you.

Wouldn't it be fun to have one let go on the trail, miles from nowhere, and losemost your coolant?

ToxicFox

To close out this post, and for anyone else that may be watching it. I finally got around to fixing this and the issue was rather simple. Inside the climate controls on the dash there is a vacuum plate that slides to cover or expose different holes to change the vacuum for the different vents. On the far left side of this plate(the side closes to the fan speed dial), is a small peg that moves up and down as you change the temp slide. The mounting bracket for controls housing had broken which had allowed that peg to come free of its guidance track. Everything works great now.

DFlintstone

Very cool of you to get back after all this time! Thanks for the info.

HellCat

Hello All,

Sorry for reviving an old post, but after a few days of searching this is the closest I could find to the problem I have.

I have replaced my heater control valve along side the valve cover in the engine bay, but i now have the same problem as OP, where the vacuum to the heater control valve won't change.

I have checked the cable retention bracket, but that is fine. As I move the temperature slider, the cable actuates and moves the small peg around - as it should.

So what does the small peg do? What does it actuate??


The vehicle is a 97 xj 4.0 auto.
If anybody has an information, I would be very grateful!

Thanks

93_xjcherokee

You shouldn't have a heater control valve on a 97. At least I think they bypassed it in like 95

HellCat

Strange.
It was definitely there - it exploded a few weeks ago leaving me stranded.
So I have replaced it... Though I never checked if the original one worked via the vacuum actuator.

CCKen

You don't have a '97 is the problem.


In '97 the climate control panel changed from slider temp controls to rotary type **** controls.


Verify the year of your Jeep.

HellCat

Hi,

Thanks for getting back to me.
Let's say it's a '96 then. It was registered in the UK in 98, but its the pre-facelift body. Is there a way to tell from the VIN plate?


The climate control is the slider type, and the temperature control slider actuates the pivoting peg in the photo (borrowed from earlier post).

So what's on the other side of this peg?

ehall

That peg moves the blend door inside the dash ductwork, directing incoming air through the AC evaporator and/or the heater core as needed.

The thread is discussing the heater control valve in the engine bay. The HVAC controls have a little vacuum manifold with lines that manipulate different diaphragms (pic), one of which passes through the firewall and connects to a valve in the engine bay that controls coolant flow to the heater core. When temperature slider is on Heat, the valve opens allowing coolant to pass to the heater core in the dashboard. When temperature slider is on Cold or the HVAC is turned Off, the valve is closed and prevents coolant from passing into the heater core.

cruiser54

Like this:

HellCat

Thanks all for the replies.

I made an assumption that the heater control valve was the sole control for the temperature. That was clearly wrong!

So the heater control valve is binary, and the temperature is controlled by the blend door. The fan motor must be running for the heater control valve to be actuated to the hot position (valve open).

Thanks again!

ToxicFox

Hellcat,

The heater control valve(HCV) is controlled by a small peg on the left side of the vacuum control housing on the top of the climate control unit. When you move the temperature slider, it raises an lowers the peg.

When mine wasn't working the vacuum housing, which is mounted to the top of the climate control housing by two screws, one of the brackets had broken and it moving the entire housing instead of just the peg, locking it in the cold position.

The bottom of the peg has a groove in it which allows it to move up and down on the track that is connected to the temp slider. If you pull the control housing out and remove the top cover of the vacuum housing, then you can see if the peg is moving properly. If it is not moving properly, then you have your problem.

If the peg is moving properly, then I suggest removing it at inspecting the sealer that covers the holes on the plate the vacuum lines go to.

If that is on good condition, then clean it.

If none of this is your problem, I suggest going to a junk yard and pulling one out of an older model. all the slider type units are the same.