Odd heater control issue, checked several things, doesn't seem to be blend door

[unpredictable1]

I have a 99 Explorer with the 3 knobs for hvac controls. I have an odd issue, the truck frequently won't let me switch between hot to cold and vice versa. It happens over time and I can seemingly reset it for a bit but I have no idea what does it.

I removed the actuator for the blend door, it operates fine, even when the temp is in a state where you can't change it between hot/cold if you rotate the temp knob with the actuator out and in your hand, the actuator pin for the blend door rotates back and forth. And it's strong, no clicking, if you try to hold it with your fingers it still turns the pin with authority.

I can put a fingertip in end of the blend door shaft and rotate it open to close without any binding or noise, operates super smooth, you can hear and feel a reassuring "thunk" as it seats from hot to cold side. Reinstall the blend door actuator, and the heater controls will work perfectly fine for a time until it doesn't.

No idea what this could be, she has proper coolant level, the entire cooling system was redone within the past year (waterpump, tstat and housing, hoses, fan clutch) This problem existed long before any of this work was performed but were maintenance items I needed to do anyways. My next step is I'm going to drill a small hole in the blend door box behind the glovebox the next time it happens and put my borescope camera in there to see what the heck is going on.

Any ideas on where else to look?

 

[J_C]

I'd wonder if the blend door actuator has hardened grease and/or oxidized contacts inside. They seem to be held together with plastic friction tabs on the sides, possibly allowing it to open without breaking them if they are warmed up, then clean the tracks and contacts and apply a light coat of dielectric grease... is what I'd try, or just replace the actuator as it isn't expensive, around $25 + S/H on Rock Auto.

Similarly the dash knob might have hardened grease or oxidized contacts by now, but I've no idea if it can be opened and cleaned, maybe spraying some contact cleaner inside and working the knob back and forth several times would help.

Granted it's a 26 year old vehicle, could instead have a bad wire or connector corrosion by now.

 

[amj441]

I'd wonder if the blend door actuator has hardened grease and/or oxidized contacts inside. They seem to be held together with plastic friction tabs on the sides, possibly allowing it to open without breaking them if they are warmed up, then clean the tracks and contacts and apply a light coat of dielectric grease... is what I'd try, or just replace the actuator as it isn't expensive, around $25 + S/H on Rock Auto.

Similarly the dash knob might have hardened grease or oxidized contacts by now, but I've no idea if it can be opened and cleaned, maybe spraying some contact cleaner inside and working the knob back and forth several times would help.

Granted it's a 26 year old vehicle, could instead have a bad wire or connector corrosion by now.

I agree, when setting the temperature see if you can set it to one of the in-between temperatures. dont set it to full cold or full hot. set it to mostly warm and mostly hot. you may just have dirty contacts on the temperature control knob.