EVAPORATOR TEMPERATURE

[ptseng]

Hi 

It has been a while since I installed the AC for my 1990 964.

My center vent temperature was always at around 17-18 degree since the beginning.

Recently I tried to check if my evaporator was cold enough with a THERMAL IMAGE INFRARED thermometer from the cabin inlet flap under the dash.

Here was what I found, with OAT at around 30 degree, my evaporator temperature between 17-20 with the thermometer, wouldn't go lower than 17, at this point my ETC was set to -5.

Next day, I checked the temperature at idle with AC OFF, and hooked Putty to check the ETC reading compare with infrared thermometer.

Evaporator temperature was showing 3 degree and the evaporator with thermometer reading at 26 degree, around 23 degree of difference.

Then I change ETC value to -23 and went for test run.

OAT was about 24 degree, I can see ETC from putty reading kept going down to around -12 and doesn't go lower, then I pulled over to use thermometer to check the evaporator temperature with the reading of 12 degree, vent temperature went down to 9 degree.

The delta temperature is around 23-25 in my case between the installed evaporator temperature sensor and infrared temperature thermometer.

Is it something wrong with the evaporator sensor signal to the ECU?

Thanks

 

Parker

 

 

[Jonny Retrofit]

I would say it is more likely the sensor itself.    You could test this by putting the sensor in some ice water and looking at the ECU reading.  If it is more than a few degrees out then it can be 'tweaked' by changing the 'KBET" value in the ECU settings.  This value changes the calculation 'curve' for the sensor.   If you change the KBET value, you must type SAVE afterwards of the change will be lost on reset.

[Jonny Retrofit]

If you need to source a new sensor, they are readily available online - google NTC thermistor.   You need one where the Beta value is stated, usually it is around 3950 which has to match the KBET value in the software. 

[ptseng]

Thanks Jonny, I have google  NTC thermistor 3950, but few results with different resistance value like 100K.

Does the resistance value matter in my case?

Btw, I am not able to pull the sensor out of the suit case and test it.

Might have to cut the sensor wire, how to wire new sensor from that 4 wires harness to temperature sensor?

Thanks

 

[Jonny Retrofit]

On 12/12/2024 at 9:47 AM, ptseng said:

Thanks Jonny, I have google  NTC thermistor 3950, but few results with different resistance value like 100K.

Does the resistance value matter in my case?

Btw, I am not able to pull the sensor out of the suit case and test it.

Might have to cut the sensor wire, how to wire new sensor from that 4 wires harness to temperature sensor?

Thanks

 

You need a 10K. 
 

the four wire harness is for two sensors.   The sensors are not polarised.  You should see how they pair simply by looking at the plug.  

[ptseng]

Thanks

What about if I don't replace the sensor and use the temperature reading difference as a constant to correct the reading for ECU?

Thanks

 

[Jonny Retrofit]

Since the main task of the sensor is to prevent freeze, yes you can adjust the KBET value so that the sensor is accurate about the freeze point (0 deg C).  Set the ETC value a couple of degrees higher. 

[ptseng]

Thanks Jonny,

Currently KBET is 3950 as default value. 

Can you explain more about this value?

Do you have any related link to explain this?

What is the KBET value suitable in my case?

 

[Jonny Retrofit]

KBET is a correction constant used for linearisation of NTC thermistors.  
 

NTC thermistors are non linear, meaning that their resistance vs temperature graph is a curve.  The standard ‘correction’ uses a formula which is coded into our software, leaving the beta value configurable so we can use different sensors.  More info in the article below:

https://product.tdk.com/system/files/dam/doc/product/sensor/ntc/ntc_element/general_tech_info/ntc_thermistor_general_technical_information_digest_en.pdf

In the case of the evap sensor, place it in Ice water and adjust the KBET value up or down by increments of 10 until the temp reads 0 degrees.  Note that you must type SAVE after changing KBET for the value to be remembered after power off. 

[ptseng]

Thanks Jonny,

that means i have to remove the AC suitcase again just to test the sensor.😅😅😅

I was hope to minimize the work to solve the issue.

Btw, what is the price if I order an additional temperature sensor harness from you with longer harness from you?

Thanks and happy new year.