Mysterious HVAC Issue

[turbo79]

'92 I have a mysterious issue with the a/c-heating system.

The heating system has been struggling to get warm, I thought it might be something clogged in the heater hose/pipe system coming from the engine, but the other day - a sunny day so no heat needed - I had pushed the vent button and just out of curiousity I turned the temp up and felt the temp climb as I did this. Turned it back down and the temp went down. Then I pressed Def, and turned the temp up and and it stayed cool!

I do know that the A/C operates a little (off and on in short bursts to keep condensation down) in all positions except Vent. So what part of the system should I check to see what is happening? It seems like the a/c comes on in Def in particular and and then does not cycle off again, keeping the output too cool. I can turn it up to 90 deg. and the Def output still stays cool, in Vent it will turn up.

Details:

New heater core, New blendoor actuator (within the last 5,000 miles)
A/C works well when it has to cool the car
all hoses checked for flow
Tested EATC unit, no codes

:braincramp:

 

[Mr Anonymous]

Blend door actuator.

 

[turbo79]

Blend door actuator.

That doesn't make sense if it goes cool to warm and back to cool when I move the temperature while in Vent, and not in any other position.

Which circuit or functionality tells the blendoor actuator which position to be in when Vent is selected, and when the other functions are selected?

Which circuit or functionality tells the A/C to switch momentarily off and on (to cut down on moisture) when the other functions are selected?

Do you know the answers to those questions?

What I'm saying is that the blendoor activator is a passive motor that responds to an electrical signal. Where does that signal come from? What tells the system to not run the A/C when Vent is selected, and to cycle it on and off when in the other positions?

Tom F.

 

[rubydist]

the eatc head unit is what tells the a/c to turn on or not. the head unit also commands the blend door position.

it seems to me from your description that the blend door is moving (although they can get intermittent on their way to being dead) so it might be the eatc head unit itself that is sick.

I would watch the a/c compressor clutch with the eatc set to defrost at various temperatures, to see if I thought the a/c was cycling properly for the setting. I would also try to confirm that the blend door is moving as it should in all of the air direction settings.

 

[turbo79]

the eatc head unit is what tells the a/c to turn on or not. the head unit also commands the blend door position.

it seems to me from your description that the blend door is moving (although they can get intermittent on their way to being dead) so it might be the eatc head unit itself that is sick.

This blend door moves OK, but if the if the EATC head was 'sick' like you say, then maybe it wouldn't shut off the a/c short blast that happens periodically when you're in any position besides Vent (for decondensation purposes). Most all post '86 (incl. EEC-IV I believe) A/C systems have this feature, I believe.

I would watch the a/c compressor clutch with the eatc set to defrost at various temperatures, to see if I thought the a/c was cycling properly for the setting. I would also try to confirm that the blend door is moving as it should in all of the air direction settings.

If you mean Def, Floor, Panel & Floor etc. then that all works just fine. According to the HVAC system diagram I have, the air direction is controlled by a different door than the one that lets in air from the heater core or the evaporator unit (blend door).

Guess you know that.

What I'm looking at is what a short or open circuit in either of two important sensors would do:

The sunload sensor on the dash
The in-vehicle temperature sensor (up above the glove box when you pull the door down by releasing it)