vehicles with trans temp readout

tluxon wrote:
Hey BurbMan, I know you know your stuff about Burb's, so I'll take your word for it. I think you might be onto something regarding a rolling average, because that would definitely appear like a "lag" in comparison to "instant" temps. Instant temps are not always the most accurate because the reading can be reporting sensor 'spikes'. I also wonder if some analog gauges have their needles zeroed in better than others.

Thanks for the clarification on the sensor location.

15 litres of trans fluid will not overheat in a flash, it takes quite a while to raise it by 30 degrees. I can vouch for the F150 gauge moving. 230F got mine to move up 1 notch, 240F another. I'm guessing 250F is in the yellow. 1st gear WOT up the side of a mountain GCWR maxed.

Fwiw some vehicles have multiple sensors. My 2011 Tacoma had two trans temp sensors, one in the pan and one at the outlet of the torque converter. Monitoring with a scan gauge, you could tell which was which because the torque converter sensor would fluctuate wildly depending on conditions.

Marsland wrote:
There are trans temp charts on the internet that indicate your danger zones regarding lifetime of fluid.

I've seen this chart. It appears to be made by some transmission rebuilder, not any of the engineers or manufacturers of transmissions. I'm inclined to trust the info in my owners manual, which indicates that I don't need to worry unless I'm at or over 255*.

Days are gone where 200* was considered to be a dangerously high transmission temp.

Like someone said, there is only one stock trans temp sensor. It isn't like the dash gauge connects to one sensor, and the OBDII port connects to another. Same one.

I, for one, trust that the mfgr installed the sensor in the location that is optimal for reading transmission temps, and calibrated the vehicle warning system according to temps that would be found at that location. As such, I do not know or do not care where the sensor was placed by GM. People a lot smarter than me know that readings of 255* at that location indicate a problem, and I'm willing to take that advice over someone who was working on 3-spd units under a shade tree back in 1975 (no offense intended to anyone on this board, but these new 6 spd units are different animals).

Re: different readouts, it would have to be due to the gauges. In my '06, it had an analog needle gauge that was quite small. So obviously it would have been hard to know if it was showing 197* or 203* or whatever. In my '10, the readout is digital, so obviously easier to read and get specific numbers. I would be curious to know if the scangauge gives different numbers than the digital dash readout.

I've always understood that the location of the sensor makes a big difference in the data received. Is the sensor on the outlet of the trans to the cooler or in the pan? Not sure on my F150 but the readings are very stable without wide swings. On my '01 F150 I had a greater variance. There are trans temp charts on the internet that indicate your danger zones regarding lifetime of fluid.

Tim the other possibility is that temps need to be sustained for x seconds or minutes before the PCM moves the dash gauge to be sure it's not a spike, this would keep the gauge from bouncing all over the place. Next time you're towing see if you can make out any correlation between dash and scan gauges, would be interested to see what the relationship is. Is there ever a time when the scan gauge and dash gauge read the same?

I have an OBD diagnostic tool that communicates via Bluetooth and is run by an app on my phone. It also has a realtime dashboard function, and after scrolling through hundreds of variables including minutia on O2 sensor voltages, there is no trans temp option!

One thing to note is that mfrs seem to be regulating the trans temp higher than in the past. My F150 runs 190-197 when driving normally unloaded as told by my digital readout. When towing I see 195-205 whether on flats or hills. Maybe it's for efficiency? The needle never moves except during warm up.

Hey BurbMan, I know you know your stuff about Burb's, so I'll take your word for it. I think you might be onto something regarding a rolling average, because that would definitely appear like a "lag" in comparison to "instant" temps. Instant temps are not always the most accurate because the reading can be reporting sensor 'spikes'. I also wonder if some analog gauges have their needles zeroed in better than others.

Thanks for the clarification on the sensor location.

There's only one sensor for trans temp...and only one PCM that it reports to. Unless you installed a new trans temp sensor with the scan gauge. The scan gauge and dash gauge get the data from the same PCM, so not sure why the scan gauge would read higher than the dash gauge. Clearly the dash gauge works so I wouldn't automatically assume that the scan gauge is correct and the dash gauge isn't.

It's possible that the scan gauge is reading instant real time temps and the PCM sends rolling average data to the dash gauge. That's how the fuel gauge works....two fuel tanks with two sending units, and the PCM averages the readings and tells the gas gauge how much fuel to indicate is left.

rexlion wrote:
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The Jeep GC showed 170 degrees on the digital readout, just test driving around town. But you think that the on-board readout will not be accurate? I wonder how and why the mfrs would set it up to take measurements that are not accurate?
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Question: has anyone ever compared their on-board tranny temp readout to a Scangauge or similar, to quantify how accurate or inaccurate the onboard readout was?

I don't know how accurate or responsive the readouts are on other vehicles as I only have experience with my 2002 Burb. With the Burb, there is a very real difference between the instrument panel gauge (analog) and the ScanGauge (digital). Based on the temperatures being reported, I'm guessing that the analog gauge is not as well calibrated as it could be, and there appears to be a lag between the digital temperature and the analog (needle) temperature. Sensor location may be different for the analog dash panel gauge than it is for the OBD II port's output, which would be an additional factor to consider.

Bottom line, though, is that any readout at all is going to be helpful and more useful than relying only on a warning/indicator light.