2nd Gen Non-Engine/Transmission In cab heat

[Billy Diesel]

I have a problem with the in cab heat staying on hot, regardless of the HVAC control positions. I brought it in once to the dealer with the condition occurring, and they said they couldn't help me at the time. I explained to them that it was an intermittent problem, and could they look at it now? Nope, schedule an appointment. Long story short, they couldn't duplicate the problem, and then asked me to bring it in when it was acting up... . (I think the irony was lost on them)



Any thoughts suggestions as to the culprit? If this is something that just needs some lubricating every once in a while, no sweat. If this is a part that needs replacing, I need to know soon as I am quickly running out of warranty. I am my own warranty station on stuff I break, but I didn't alter this part! Yet... .



Thanks,

 

[rwitt]

Is the thermostat staying open and not allowing the engine to come up to temp? At an idle in cold weather these engines do not produce much heat and if the t-stat doesn't cycle you will get luke warm air in the cab. Also with the defroster on the air conditioning compressor will cycle on and off as a normal operation.

Rich

 

[illflem]

No coolant flow valves since the early '90s, coolant is always flowing though the heater core. The temp control opens/closes with vacuum what's called a blend door that regulates the amount of air flowing though the heater core vs. the outside air. Sometimes this door will stick especially if leafs or other debris is hanging it up or you have a vacuum problem either in the switch or a vacuum line.

 

[nwinters]

What year did they change to a vacuum operated blend door? I am positive on my 98. 5 that its a cable driven blend door.

Nate

 

[wyosteve]

I know my '95 had a vacuum driven door because I lost a vacuum line and it wouldn't work until fixed.

 

[nwinters]

Maybe I'm thinking of the wrong door. The door that mixes hot and cold (temp control) in my truck is cable actuated. All the vent positions and the recirculation door are vacuum actuated.

Air flows in, either fresh or recirculated. It passes thru the evaparator and hits the temp door, it either goes thru the heater core or goes into the cab depending on where the diverter door is. On my truck this is cable actuated, I think that all the A/C control knobs are the same for these trucks, so I think they are all cable driven 98+.

#ad


You can somewhat see the diverter door in this picture, its just behind the evaparator. The heater core is behind the evap. with the diverter door somewhat parallel to it. The door in this position would not allow the air to flow thru the heater core, the door would swing perp. to the heater core to divert air thru it, heat up, and enter the cab.

 

[Billy Diesel]

Engine temp remains normal the enitire time. I have tried switching between defrosters, floor, vent, AC, makes no difference. Same with blower motor speed. Everything is functionally normally except for the HVAC temp control. There is no ryhme or reason as to why it starts. I have had it happen accelerating hard, cruising down the freeway, and just putsing around town. Condition lasts 30 minutes plus, and then will return to normal just as quickly.



Thanks for the ideas, I will keep you updated on what I find.

 

[illflem]

slo-ryde, the valves were eliminated in favor of blend doors because of valve sticking and a dead end created which caused sediment to accumulate in the heater core.

When you think of it it's pretty silly to be bringing 190° coolant into the cab when it's 95° outside, seems like you'd be fighting the A/C. I know a few people have installed a ball valve on a heater line to stop the flow in hot weather. If I lived in a hotter climate I'd consider the same.

 

[RBallard]

I'll add to this, since I am also suffering from the same fate. Mine from the start works to the extremes. With the dial either all the way cold or all the way hot. In between is either all cold or hot.



Just recently, it has gotten worse. When I let off the accelerator it blows hot air and then eventually goes back to cool air.



Anyone have that happen to them?



Bob

 

[illflem]

Bob, on yours it sounds like the dial itself is at fault allowing the blend door to change settings according to the amount of vacuum.

 

[Billy Diesel]

I finally got the truck in the other week. The technicain said my "actuator is wiggin' out. "



Anyway, had them fix it under warranty. Now they screwed it up so that I only have full hot, or full cold. No in-between. And this is not an intermittant problem anymore. Is this what you are describing Bob?



I get to bring my truck in one more time for this problem (it will be number 7), and my Lemon Law letter will be going with it. I think that is the only way I can motivate this dealer.

 

[RBallard]

That's exactly what I have. If it is set on cold, I can move the dial almost all the way to heat before it activates. Then once it is on heat I can move it almost (approx 90%) back to cold before it switches to cold. My in-between is where it last set.



It sounds like we have the same problem now.



illflem, thanks for the tip. I have to check out the price for a replacement. No matter how hard I beat it! - it still doesn't work.



wrhuston,



Let me know what the dealer does to correct yours.



Bob

 

[Billy Diesel]

Bob,



Mine is a little different. When the controller in the vertical position, this should be medium temp. When I move it one click to the left of vertical, that is full cold. One click to the right of vertical is full warm.



I will keep you posted on what I find though. It may prove to be usefull in your situation as well.