TFT logged voltage vs temperature

[dhs_42]

Hi all,
I'm logging the TFT voltage using the same sensor as is used in the ECT. All logging well.
I know these sensors have a change in resistance to changes in temp.
All good.
What I'm wondering is the relationship between the voltage I am logging & temperature. Anyone have a formula for this?
(I believe the range is something like 0V = 151°C, 5V=-40°C. Not sure if its linear.)
Presumably the same as for ECT...

I'm using V2 & a 0411 computer.

Thanks

[joecar]

The pid GM.TFT can be displayed in degrees C or F.

[EagleMark]

It's not linear. We did a lot of work on this in OBDI ECM that go through a 1k pull up and it uses AD counts inverted to C or F. The equation that worked accurately was:

(-1*.0315*X + 4.5)*(-1*.0315*X + 4.5)*(-1*.0315*X + 4.5)+(X*X*-1*.00045)-(X*.135)+73.00000

Not sure if that helps your issue or with newer PCMs...

More info here, long thread, diffearnt subject but same scenario:
http://www.gearhead-efi.com/Fuel-Injection/showthread.php?1697-Code-EGR-and-CTS-conversions

[dhs_42]

Hi Joe - thanks. I forgot to mention.... I am also logging TFT in °C, but all I'm getting back is 0. So I logged the TFTS (voltage) to check, & the voltage is changing - so thought I might try converting the TFTS into °C.
(Maybe I should be finding out why TFT is just returning 0°C?).

Hi EagleMark - Thanks, I'll give that a turn (& have a read). Thanks for that.

[joecar]

I reread post #1: oh, so you have an ECT sensor connected to the TFT wiring, ah I see... like EM pointed out the ECT voltage function is polynomial (hmmm, I have to now dive into the service manual to see the voltage charts for ECT and TFT, I can't remember what I previously read there).

[joecar]

You can make a calc pid from EM's polynomial equation (let me know if you need help writing the calc pid).

[joecar]

I looked at some charts showing resistance-vs-temperature for various ECT and TFT, and they were the same...

:nixweiss:

[dhs_42]

Hi Joe - thanks. Yeah I was hoping to come up with a calc pid. I'll have a play & see how I get on. I'll put up what I come up with. Thanks

[dhs_42]

I've had a bit of a play with the equation. Have substituted X for X*51 (so I can use X as 0-5v). All good (I think).
The TFTS range I have logged (from a short run) is 4.4 - 3.5 volts. Using the equation that gives me about 3°C to about 33°C. Seems too cold, I was expecting something like 20ish°C to 80-90ish°C.
Not sure why the voltage is high(er) than expected.... any thoughts? (I guess I can alter the 73 in the equation to move the poly over a bit)?
Thanks

[joecar]

Calibration points:

If your trans is using the in-radiator trans cooler, then let engine idle for 30 mins, you should see TFT come up to same as ECT.

Also, when cold, before startup, in the morning, TFT and ECT would be pretty much the same.

[dhs_42]

Hi Joe,
Yeah - a few more details I left out. The engine is in a boat, I've got the sensor in the dry sump tank.
Yep, I reckon the cold temp would be about the same for ECT and TFT (ECT was about 22°C cold - about right). After running, I can "feel" the oil is up to some temp (hot too touch, guessing 80ish°C). I have a heat measuring gun coming to get a rough idea, hasn't turned up yet.
Thanks