Bench testing the vacuum ATC interior temp sensor

[kishy]

If you'd rather just watch a video... https://www.youtube.com/watch?v=erItV2X1XKg



I have been trying to figure out a reliable way to determine if a junkyard car has a good vacuum ATC interior temp sensor (used up to 89). As we know, there is a common failure mode where the car no longer has hot heat, because the interior temp sensor is passing vacuum when it should not be. The full heat position for the blend door servo is zero vacuum; if any comes through to it, the blend door will mix cool air and give you lackluster or totally inoperative heat in the winter, which is not a desirable condition.

If you have a hand vacuum pump and some line, you can test this part against that common failure.

Hook up the sensor and servo as they are in the car, and connect the vacuum pump to the black line going to the sensor. With the temperature set at the full heat position, pump the vacuum pump. There is continuous vacuum leakdown, which seems to be normal, but the important part is making sure the servo does not move at all even after pumping for a while (for the most accurate test, do this for a few minutes).

If the servo does not move, then the sensor is able to provide full heat. The next test would be to make sure it begins moving the blend door to somewhere cooler when the sensor is adjusted to a cooler setting. Since this will vary by the ambient temperature when you are testing the sensor, the best test would be to just set it to the coldest setting and make sure the servo can move the blend door all the way until it's fully pulled in.

Finally, make sure the sensor will release/vent the vacuum by moving it back to full heat (while it still has vacuum applied in the full cold position), and make sure it allows the servo to fully expand back to the full heat position.

This does not allow you to test the accuracy of the sensor (to numeric temperature values), but at least you will know if it functions correctly.

[87gtVIC]

Yes. Yes. and Yes. Stuck ya.


More of these please. I love it!

[gadget73]

I took one of these apart, and to be honest I'm not quite certain what actually fails in there. There is very little going on inside, so unless its just dirt gumming up the works I don't quite know why they fail, only that they do. Next time I find one that isn't working, I'm tempted to try throwing it in an ultrasonic bath for a deep clean. At worst, it still will not work.

Unfortunately the way they are constructed internally makes disassembly near impossible to do without wrecking it. I have some pics though if anyone wants to know what goes on inside of the sensor.

[kishy]

It occurred to me after recording that I missed a detail and conveyed it wrong. Adjusting the temperature setting does not proportionately adjust how far the blend door servo will retract.

Adjust temperature to a setting other than the current interior temp > blend door servo will gradually adjust in the required direction (but may continue to an extreme beyond the required mix) > once interior air reaches desired temp, sensor closes vacuum valve causing servo to back off > air temp differs from specified value eventually, so sensor reopens valve > desired temp is achieved, sensor recloses valve (ongoing temperature regulation continues in this fashion)

This means the blend door will "average" to "somewhere in the middle" but may not stay at that exact location due to continually receiving more or less vacuum based on what the current temperature is.

In practice these would be small adjustments, not wildly flapping the blend door from hot to cold.

This doesn't even begin to touch on the fan speed control built into the "blend door servo programmer" which is a whole 'nother can of worms (essentially fan speed is highest when blend door is positioned correctly for the desired temp - and I'm not sure how the heck it knows that since temperature regulation is 100% vacuum).

Lightbulb moment - maybe if the servo is at an extreme in either direction (hot or cold), fan speed is high, and if the servo is closer to dead centre the fan speed comes down. I dunno. Makes sense in my head. Some manual reading is in my future.

Anyway, I think what this thread establishes is a functional basic test to figure out if a particular temp sensor is worth buying at the junkyard. The vacuum pump is a tool we really all should have given the ages of our cars but I'm sure many lack it. On sale they don't break the bank and they have far more uses than just this exact one.

[gadget73]

Friction on that blend door also affects how gradually it moves. I suspect that when the crank arm gets rusty it doesn't move as smooth as it did when new, contributing to some amount of overshoot in both directions from desired temperature. When the sensors get crappy and don't allow the servo to return correctly it also makes temperature regulation suck.

and yeah, thats exactly how the fan speed works. As it moves towards the middle, the fan speed is decreased. It also shuts the recirc door. There are two extra linkages that connect to the blend door arm to make all that work. That is all that the "auto" position does. It routes blower speed control through the switch mounted on the blend door instead of using the switch on the climate control unit.

If you don't have a spare servo around, a vacuum gauge will stand in for it. Thats how I test them.