96 heater core hot all the time

Team Mudshark

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Sep 4, 2019
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96 LT 4 x 4 292K miles. All I find are old threads about this so here's a new one. I got my AC fixed after 13 years of none. 1 day after, I have cold water pouring out of the heater core box. I pulled it and the heater core is hot all the time. I replaced the heater valve and the controlling solenoid valve. No change. I replaced the HVAC control panel not too long ago with a used part. Can someone tell me what the default setting is for the heater valve? I assume that when the AC is running, the heater core should not be hot. Is this true or is it always hot but not in the airflow. I've been thinking that the hot core and the cold air are mixing and condensing into rain. It's like turning on a cold water hose as soon as the AC is turned on. Now, my recirc actuator is disconnected at the moment as it clicks constantly when it reaches the end of it's travel. I have the valve in recirc position. I also now have no defrost heat. I can get plenty of cold defrost air. Of course right now the cover is off the heater core so it's hanging unboxed. Maybe that's the reason for that? Anyway I also have never had decent airflow to the left side of the dash. I have cleaned out leaves and such from in front of the evap core but not much change,. First up, I need to know what controls the heater valve and how it is supposed to work. My weird defrost airflow, I understand may be of my own making with the box removed. The other 2 door actuators move and appear to be working. I can't fix it until I know how it's supposed to work of course. Mind you somewhere on the property I have the factory service manuals but I'm old and lazy and if I can get someone to tell me what I need to know it will save me a lot of time, effort, and beer. Thanks.


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Jan 1, 2008
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The water valve is closed when vacuum is applied by the solenoid. Feel the bottom of the valve - the spring loaded plunger goes up into the valve when it’ is closed. There is a small (maybe 1/8�) vacuum line that connects the water valve to the solenoid. A second hard-tube vacuum line connects the solenoid to the PCV valve fitting.

The solenoid is energized (and vacuum applied to close the valve) when the “max†cold detent is selected on the temperature dial.

The intent is to maximize a/c performance by having a (somewhat) cooler heater core since it is immediately downstream of the evaporator. The temperature damper/door adjusts airflow to pass through the heater core based on dial setting.

The water valve goes back to open when the truck is shut off.

The water valve solenoid has two wires to it: one power and one ground. With the dial set to max cold, you would have 12V on the solenoid connector.

As for the defrost, it’ is controlled by the actuator to the right of the gas pedal. The actuator will have two linkage arms, check that they’ are intact. Poor airflow from the left side is pretty typical for the three of these that I’ have had. There I’s just too much drop across the length of duct, airflow is going to be less since it’ is the most distant from the blower.

Without the cover on the heater core, I’ would think that you would no’t have much hot air at all in the ductwork.

If you’’’ are getting a lot of water escaping from the case (and assuming that it is no’t antifreeze), check to see if the evaporator drain is clogged. I suppose it could be backing up into the adjacent heater core area of the case. With a functioning a/c, you should see it dripping from the black rubber elbow on the passenger firewall, kind of near the heater hose lines.


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