1227730/1227727 Coolant Temp Sensor circuit for $8D

Hi all,

I am trying to edit the lookup tables to get the '7727 to use a Toyota CTS which is close, but towards the higher temps, the resistances are off enough to throw off the ADC counts alot. 212*F water reads right around 240*F on ADX.

So, in my quest to understand the dual tables, and see if I can actually do the A/D count math close enough to hand tweak, I looked at the following drawing from Ludis's schematics.

There is a "348 ohm" table and a "4k ohm" table in the $8D disassemblies... What is weird is the diagram from Ludis. I am NOT an EE and maybe I just don't get what is going on, but it looks to me that this diagram has the two resistors (348 ohm and 3.65k ohm) backwards?

If not, how does the 348 ohm pull up actually occur? It seems like it would be a 3.65k ohm pull up for CTSHI, or a 4k ohm pull up for CTS?

Last question: If I have some temp/resistance numbers for the toyota sensor, how do I back into the AD Counts to populate the table.

Thanks for the help and advice...

-Brett

GM coolant sensors are completely non-liner. To get higher resolution accross the range, the pcm switches internally between 2RC networks for pull up. And you can have the same voltage output for different temperature.
The threshold for switch is around 40*C if I remember correctly. And is hard coded.

It will be much easier to retrofit gm sensor than to try to recalibrate the one you have. It is possible but lots of work will be needed. First a chart of temp vs resistance, second the threshold for the RC network switch, And third calibration of output voltage vs temperature.