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Reverse Logic No Heat Problem

roboter

NAXJA Forum User
I've searched and can't find an answer. Any help greatly appreciated and sorry for the long post.

1992 XJ, 4.0L, auto.

I have no heated air coming out the vents at any blower speed with the heater control valve connected to the vacuum line. I start the Jeep with HVAC controls set to off, vacuum builds & pulls the diaphragm lever in, closing valve. Blower on, temp slide switch all the way to left, A/C works on max, norm, bi-level.

Blower on, temp slide switch still at left, vent, heat and defrost work as I hear the doors cycling and air blows out the proper vents. Engine at temp; hose is hot at input and return of heater control valve. I slide the temp to red, hear the door move but get no heated air.

I have replaced the heater control valve twice. I have check for vacuum leaks in the engine bay, checked vacuum check valve as good with flow at engine side, vac on control panel side using hand held pump.

Once I pull the vacuum line off the heater control valve, Vacuum releases diaphram lever out and it's toasty time all day long. Slide the temp switch to blue and cool air via blend door.

Valve bypass is not an option because of radiator flush setup so I'm thinking bad control panel?
 
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have you ever flushed the heater core? they tend to collect alot of gunk and will pretty much make the heat non existent. i just flushed mine. and it had this paste like substance and some gasket maker in it. heat works much better now.

also you can double check for heater valve operation in full cold it should be closed and anything else should be open.
 
have you ever flushed the heater core? they tend to collect alot of gunk and will pretty much make the heat non existent. i just flushed mine. and it had this paste like substance and some gasket maker in it. heat works much better now.

also you can double check for heater valve operation in full cold it should be closed and anything else should be open.
Did you even read what he posted?
 
Is the valve hooked up incorrectly? Not sure it would matter but maybe it's thinking that heater off really means on. Also, the flush setup you have shouldn't preclude you from deleting the valve.
 
I personally would delete the valve.

But, if vacuum is staying on at the valve, I would double check it at the HVAC control and see if it is still applying vacuum there. It sounds like something is wrong in the control head.
 
Is the valve hooked up incorrectly? Not sure it would matter but maybe it's thinking that heater off really means on. Also, the flush setup you have shouldn't preclude you from deleting the valve.


It does not matter as long as the valve opens you’re going to get heat if everything else is good. Even if the valve is bad, it defaults to open so you’re still going to get heat. It is recommended that you follow arrow on valve for supply in / this side up depending on your interpretation. Either way this allows coolant to flow in from the bottom tube and out the top of the heater core, which I'm guessing reduces air bubbles from turbulence.

I totally hear you on deleting the valve. But it was working and with my luck a bypass will clog the heater core.
:banghead:
 
I personally would delete the valve.

But, if vacuum is staying on at the valve, I would double check it at the HVAC control and see if it is still applying vacuum there. It sounds like something is wrong in the control head.

Agreed. I really hate messing with control unit because like everyone else, I'm plague with under spec wiring / connectors on the HVACs electrical side. I got toothpicks keeping the connector pins aligned as the last short melted and about set it on :flame:. :mad:
 
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