1987 300sdl climate control problems

[ARC]

Just returned from a 2000 mile trip. The car did great and the AC worked perfectly in the 100 degree heat. On the way home it began to rain and I adjusted the climate control to keep the windshield defrosted and the AC began to blow hot air no matter how I adjusted it.

If I shut the car off for a minute, the ac would work fine, but when the engine speed increased hot air would blow. I know that there is a vacum problem, and I have searched the forum for answers. I also discovered that the AC will work fine in the 'defrost' position. The temp wheel still works, the 'auto' button works fine, but cold air only when the 'defrost' button is pushed. All other buttons result in very hot air from the vents.

I would like your opinion as to the problem most likely being the push button unit; the ACC aspirator motor; the vacum pod near the steering column or something else. Actually, I have both heat and air, just not in the correct positions. I had also had problems with cooled air 'leaking' out the defrost vents causing the windshield to fog in the past, but I don't think that was a significant problem.

You guys are great !

[Jim H]

Originally Posted by ARC

Just returned from a 2000 mile trip. The car did great and the AC worked perfectly in the 100 degree heat. On the way home it began to rain and I adjusted the climate control to keep the windshield defrosted and the AC began to blow hot air no matter how I adjusted it...

This could be confusing. As designed, the DEFROST button engages high speed fan and full heat. Do you mean that after using DEFROST and then pressing one of the other A/C buttons, it no longer blows cold? After how long? For defrosting heat, the monovalve is opened to supply heat to the heater core and it takes a while for the evaporator to cool off the heat in air ducts.

Originally Posted by ARC

...If I shut the car off for a minute, the ac would work fine, but when the engine speed increased hot air would blow. I know that there is a vacuum problem, and I have searched the forum for answers. I also discovered that the AC will work fine in the 'defrost' position. The temp wheel still works, the 'auto' button works fine, but cold air only when the 'defrost' button is pushed. All other buttons result in very hot air from the vents...

Now, that sounds BACKWARDS...

Originally Posted by ARC

I would like your opinion as to the problem most likely being the push button unit; the ACC aspirator motor; the vacuum pod near the steering column or something else. Actually, I have both heat and air, just not in the correct positions.

ACC pushbutton unit, possible. Aspirator motor, not likely, it only affect how fast the ACC unit senses temperature changes, not the final temperature itself. Vacuum pods actuate flaps and dampers that direct the air, but don't heat or cool the air. 'Failed' mode is to leave the flaps in DEFROST position, so a vacuum leak could result in cool air from the defrost vents only. However, cool air does not always come from the center vents. See below.

Originally Posted by ARC

...I had also had problems with cooled air 'leaking' out the defrost vents causing the windshield to fog in the past, but I don't think that was a significant problem.

As designed, in full-cooling mode air comes out of the center vents with 'leak' air from the windshied vents. Once the temperature setpoint has been reached, the center vents are closed and all air is directed to the windshield vents. Cold air on the windshield will result in a cold windshield. Moisture in the air can then condense, causing local fogging just above the vents. That much would be normal.

Would you be able to comment on operation in light of what I tried to explain? Perhaps the sytem is operating more normally than it first appears.

Best Regards,
Jim

[ARC]

Push the AC button in either economy or max mode and hot air comes from the dash vents. Push the defrost button and cold air comes from the dash vents. (the temp 'dial' always in the max. cold position).

I know it sounds backwards - it is working backwards. I think that it must be something haywire in the pushbutton unit.

To compound the problem, yesterday I started the car and now everything is back to normal - defrost button gives hot air to windshield vents, AC buttons deliver cold air to dash vents. I didn't do anything at all to correct the situation, but for the time being it seems to be working fine. I am looking for a pushbutton unit just in case. Maybe the connections inside the unit are damaged.

Thanks for the ideas.

[Jim H]

Originally Posted by ARC

...To compound the problem, yesterday I started the car and now everything is back to normal - defrost button gives hot air to windshield vents, AC buttons deliver cold air to dash vents. I didn't do anything at all to correct the situation, but for the time being it seems to be working fine...

Ah, but you did. You let it 'think' for a while. I don't know if many know what goes on inside that unit, but there is something resembling a computer, with sensors, timers, relays, etc. It is a lot more than a vent position and fan speed selector switch box.

Yours is not the first 'miracle cure' I have read about in this forum. I hope it keeps working for you.

Best Regards,
Jim

[Brian Carlton]

I'm quite sure that it is the CCU (pushbutton unit).

They get flaky with age and exhibit some of the symptoms that you describe.

This one would give me a/c on all buttons, including economy. I finally changed out the CCU with another one from e-bay. It solved the problem.

Now I've got Klima issues. A/C works for three minutes and shuts down.