95 LT1/ZF w/Automatic AC system

The AC system was just not doing well at all. The temps coming from the dash were not that cold, and when set on 'auto' the fan would usually settle in around the 2-4 setting, even if the interior was warm.
I had weak airflow and it was not that cold on auto. I could set the system to manual for better flow, but that did not help a lot with the temperature from the ducts. If I set the temp to 60 it did better, but it was always noisy with the recirc on and high fan.

I checked several threads here at the forum, and the ones I found seemed to indicate a system, fan or freon problem.

I was about to set out with pressure gages and do the freon system analysis, but I first did a quick check of the temps at the dash outlet. Setting the AC system to 60 got me a mid-40's air outlet temp on a 85 degree day. So I had freon!

The main culprit I discovered was leakage from the RH cold air duct outlets to the inside temperature sensor. (The sensor is in the middle of the two RH air ducts.) The system thought the interior was always cold! And then the auto system would throttle back the blower and even adjust the temperature mix door to make the air warmer.

The sensor is directly behind the RH outlet assembly. There is a mating tube on the middle part of that assembly to seal against cold air leakage to the sensor. But the OE gasket is not really big or thick enough to form a good seal.

The flimsy OE gasket was replaced with about a 1/2 inch thick, 1-3/4 inch circle of foam material with a 1/2 inch hole punched in the middle to allow air in to the temp sensor. The circle of foam was tucked in behind two plastic layers in front of the sensor and the 1/2 inch hole aligned with the sensor. (Be careful not to damage the sensor)

The mating tube on the vent assembly fit snugly against that gasket when re-installed on the dash, preventing leakage from the cold air outlet on either side.

The difference is amazing! Cold air is back, and the thermostat settings actually now have meaning. Air flow is dramatically increased, since the sytem now is commanding more voltage to the blower motor.

So if your system seems weak, check this out along with the normal freon/system/fan motor checks. It only requires a quick removal of the RH vent assembly and putting in a more robust gasket.

 

The in vehicle temperature sensor is a thermistor. If it was not fully exposed to the colder air in the air duct, the resistance value sent to the AC Programmer would indicate a higher in vehicle temperature not a lower one.

This is the opposite of what you're saying.

 

Thanks, Hooked....

To clarify, the inside temp sensor is indeed a thermistor. It is attached at the vent outlet area, but at the end of a tube that draws air directly from the passenger cabin. (There is a small air pump on the heater core casing assembly to do this, and includes a tube running up to the vent area that connects to the temperature sensor.) The AC system design intends the thermistor to be exposed to 'passenger cabin' air.

Immediately adjacent to the inside temp thermistor are the two cold air ducts. If air from those two ducts is allowed to mix with the cabin air being drawn in to the tube, then the system thinks the interior is colder than the actual temperature in the passenger cabin.

I found if that mixing is excessive due to a bad/poorly fitting gasket, then the system gets the incorrect readings for cabin temperature. In other words, the thermistor is supposed to be exposed to 'cabin' air, but instead it is exposed to chilled air from the AC system.

 

If you had pictures of this fix I would try it. It's too hard to "imagine" what things look like that you were working on. Pics.. Pics.. Pics.

 

The first photo is the most important. It is with the RH duct assembly removed. There are 3 'layers' of plastic - 1- the level of the sensor, 2 - level of a plastic piece with about a 1-1/2 inch hole, and 3- a level of plastic with a larger hole.

This shows the old gasket - the new one goes in the same place tucked in between layers '1' and '2'. You can see on the old gasket the imprint of the vent assembly - it is offset from the gasket and leaked cold air to the sensor. Also, the old gasket did not have much 'body' and was easily deformed.

The other pic shows the view after the duct assembly is re-installed - you can see the yellowish new gasket. If I were to do it again, I would find a sheet of black foam from which to make the gasket.

 

Nice find. Thanks for the write up & pix, they are now part of my e-file
shop manual. :~)

Regards,
CK

 

Very nice pics. Good job. I am now gonna mess with mine. Well, at least look at it. I don't seem to have the same problem as you. Good write up...

 

Thanks for the write up.

 

Thanks for the write-up and the pics.

Something that greatly improved the performance of mine was a quick inspection of the ambient outside temperature sensor located underneath the vehicle on the passenger side in front of the lower part of the A/C condenser.

The air flowing to the sensor was partially blocked by some sort of insect cocoon. I removed the sensor, took a can of electrical contact cleaner, and skeeted the unit out real good. The difference in overall system performance was immediate and noticeable.