Autoheat Debugging on an 89 Carrera Coupe

[T77911S]

awesome.
not sure if you were clear, but the fans DO NOT come on until the arm is in its full position which would be max heat from the engine.

about the flapper boxes.
with my flaps closed that is max heat. full open is no heat. this can get confusing. when my flaps are open the heat just blows to the out under the car.

I would just jumper the rear sensor so it thinks the engine air is hot all the time.

I cant wait to get my car back on the road to see how mine works.
I will test my sensors to see what they measure. I can also use a heat gun to heat up the rear sensor and test it.

[spyerx]

I was using term opposite. Open = heat. But I get what you mean.

I'll evaluate the motor switching again today.

It's just nice to have real variability on the heat and the "autoheat" part actually works.

[mysocal911]

The two temp sensor values and the preset potentiometer (under control knob) value (200 ohms)
is critical for proper operation. If either of the sensors is not connected or bad, the unit operates
open-loop, and the motor arm just moves from one limit point to another as the knob is turned
from off to the first position and then back to off. It's a very simple system that most mis-diagnose.

[spyerx]

Originally Posted by mysocal911

The two temp sensor values and the preset potentiometer (knob control) value (200 ohms) is critical
for proper operation. If either of the sensors is not connected or bad, the unit operates open-loop,
and the motor arm just moves from one limit point to another.

This isn't entirely accurate. BAD means its either OPEN ( no connection) or CLOSED (0 ohms).

In the testing I did with a variable resistor (potentiometer) wired to the rear sensor plug, at OPEN (which mine tested as, no resistance at all) the control arm DOES move to the full OPEN/full heat on setting 1. However, at any resistance below about 300 ohms, including to full short, the control arm is variable, and does vary based on input from the interior sensor and the top side knob.

So, if its not connected, or fully open: flaps go full heat on setting 1
If shorted or very low resistance, the flaps will be variable.

All I did was wire in a 200 ohm resistor at the sensor for now.

I suspect the majority of failures of the rear sensor result in an open circuit, therefore auto heat has no variation.

[mysocal911]

Originally Posted by spyerx

This isn't entirely accurate. BAD means its either OPEN ( no connection) or CLOSED (0 ohms).

Really? You found that definition doing a Google search, right? Keep "stabbing in the dark" as you
attempt to troubleshoot. You might eventually need to buy another unit.

[spyerx]

Originally Posted by mysocal911

Really? You found that definition doing a Google search, right? Keep "stabbing in the dark" as you
attempt to troubleshoot. You might eventually need to buy another unit.

Are you attempting to be an @ss or does this come naturally?

I was describing how I was using the term. If you need further reading comprehension help I'm happy to assist further.

There is no stabbing buddy. This system is not complex in the way it functions.

[Vereeken]

Great thread.
I am troubleshooting mine.

Can you explain to me how I measure the caloric feeler under the control knob?

The manual says turn knob to 5 remove knob and measure resistance, should be 950 plus minus 50.

But I can not see how I measure ohm on this knob as it is just a screw in a brass nipple surrounded by plastic. what am I missing?

[wrxnofx]

Originally Posted by Vereeken

Great thread.
I am troubleshooting mine.

Can you explain to me how I measure the caloric feeler under the control knob?

The manual says turn knob to 5 remove knob and measure resistance, should be 950 plus minus 50.

But I can not see how I measure ohm on this knob as it is just a screw in a brass nipple surrounded by plastic. what am I missing?

The terminals are inside the autoheat box. So you need to remove the box from the car (which means you need to remove the passenger seat to disconnect the servo rod) to get to them.

[spyerx]

Originally Posted by wrxnofx

The terminals are inside the autoheat box. So you need to remove the box from the car (which means you need to remove the passenger seat to disconnect the servo rod) to get to them.

this is what I did, measured it at the wires, you can seem them pretty clearly.

In my case, I just jumped the circuit as I didn't want to bother replacing the sensor in the exhaust.

It worked fine and regulated the heat well.

[Vereeken]

Thanks,

I got the factory specs when measuring except for the thermistor in the heatexchanger.
I got 350 ohm instead of 1500 at 20 degrees celcius.

Should that be replaced?

Michel

[spyerx]

Originally Posted by Vereeken

Thanks,

I got the factory specs when measuring except for the thermistor in the heatexchanger.
I got 350 ohm instead of 1500 at 20 degrees celcius.

Should that be replaced?

Michel

What symptom are you trying to fix? You really need to check all components. The rear caloric feeder simply allows the system to pass more heat until the car warms up then you have more valve regulation. If your rear sensor is at 350 ohms then you will likely get full control of auto heat. At least thst is what i found at 200 ohms the system worked like a warmed up csr and regulated temperature.

This was a few years ago and i don’t own the car any longer....