TFT 850F

[joshg678]

So unfortunately I've been having to park outside the garage for a while, and I've noticed some odd behavior with the ScanGauge.  Not sure if its the SG or the car, but when its about 32F or less (and the ICE has to start) the TFT will show 849-850F for a few seconds, then switch to a realistic number, and then back again a few times.  No CEL or anything.  Mostly just wanting to report to see if anyone else have seen this.

Picture was after a minute of running, WT was up to 69F(nice) from 32F at startup.

[ptjones]

I don't think I have seen this with mine.

 

Paul

[plus 3 golfer]

I don't monitor TFT a lot but have never seen this.  Does the Scan Gauge read both the PCM and SOBDMC for trouble codes as the transmission TFT codes are in the SOBDMC?

If TFT is above 643.5 * F for more than 2.5 seconds, there should be a DTC (likely P0710:11, P0711, P0712) that the sensor voltage failed low - generally a short to ground.  This monitor runs continuously.   Since the TFT apparently cycles between bad / good, the DTC may be a pending DTC and then cleared when the reading is good for so many monitoring cycles. 

Possible Sources

• Connectors damaged or pushed-out terminals, corrosion, loose wires and missing or damaged seals
• TFT sensor
• SOBDMC / TCM

Also, the above may not be emissions related and may not trigger a CEL unless the following range threshold shown below fails as this DTC is stored in both the PCM and SOBDMC.  This check is only performed when the Motor, Generator, and TFT sensors check OK.  In essence, the absolute value of the difference in temperatures between TFT for both the Generator Coil AND also the Motor coil exceeds 30 C while at the same time the absolute value of the difference between motor and generator coil temps is less than 10 C.   So, although the first two checks may fail when TFT shows as 850 F, the third check does not fail the test and hence no range fail DTC.

 

This is where FORScan could be used to record appropriate data while this issue is happening.  Also, if the TFT sensor needs replaced or the internal transmission wiring harness is at fault for the DTC, the transmission must be removed and opened up as the sensor is internal to the transmission.

 

 

Edited March 3, 2020 by Plus 3 Golfer

[joshg678]

That is Exciting information (some what sarcastic).  On a serious note, thank you for it.

 

IT seems to only happen when the outside temperate is at or around 32F.  I'll see if it has any pending codes next time.

 

I'm tempted to see if i can hook up multiple Scan gauges, so i could have one showing my TPMS, and one showing Temperatures.  It looks like you can connect a second one (daisy chain) but I've never tried.

Edited March 3, 2020 by joshg678

[plus 3 golfer]

So, if ambient is 32 F but transmission is warmed up and you restart the car, does it happen? 

 

A failing thermistor (TFT sensor) usually fails open which would result in a low voltage (no voltage) from the sensor.  So, if the TF is cold and the thermistor is beginning to fail, it is more likely that the internal connection between the thermistor material and metal lead will begin to separate.  Once the TF warms up, the thermistor may seem to work fine but it's reading could be off.  The SM gives the thermistor resistance vs temperature values that one should see if one pulled the connector and measured resistance of the thermistor.  

[joshg678]

Its only happened when cold.

[plus 3 golfer]

So, evidently TF has to be around 32F or lower.  What is the "realistic" TFT you get when WT shows 69F.   What happens if you don't start car but put in ACC mode?  You should still be able to scan PIDs.  Does the TFT flop between realistic / bad reading?  I can't see how starting ICE would affect the readings flip flopping , unless ICE vibration affects the resistance of the TFT sensor (or path to ground connection).

 

I guess the bottom line is unless you get a CEL or critical display message, I wouldn't worry about it.