How are the temperatures different? The unit listed earlier uses a sensor bolted to the block as opposed to a traditional temp probe.
In my opinion you can use the above mentioned style digital temp gauges with an external sensor (thermistor) bolted to the block or head of the engine, or bolted to some suitable part on the transmission as long as you treat the readings as a guide only as they will be somewhat inaccurate. Essentially they tell you the temperature of the piece of metal they are bolted to and can be a reasonable guide while all systems are functioning normally.
I have experimented with and tested similar units on my last and current D4D and can easily get a 30C-40C difference in engine temp reading just by moving the sensor around on the cylinder head.
Also consider this example, imagine a sensor bolted to some external part of the head or the block and the cold engine you are about to start has lost its coolant overnight thru a split in the bottom radiator hose. Without the coolant to conduct the heat from the internal parts of the engine to the external part where the sensor is located, you will most likely find any damage will be done before you are alerted via the sensor and the gauge. (And your OEM temperature gauge probably wont be of much use in these circumstance either.) None the less it may explain why manufacturers use the coolant temperature for their ECU's as the preferred way to monitor engine temperature.
On the auto trans you would need to mount this type of sensor close to the fluid output line to the cooler as this is where the heated fluid exits the trans and is usually the vicinity where the manufacturers monitor trans fluid temp. It needs to be a good metal to metal contact that is not influenced by exhaust temps or airflow. It is not much use attaching it to the trans sump or thereabouts as "cooled" fluid from the cooler is deposited back in the sump and would influence readings.
Even if fitted as above it would only be a guide as the fluid can heat up quickly under certain circumstances and before the metal body you have the sensor on, registers the change.
While a wonderful bit of kit it is unfortunate I reckon, the manufacturers trans overtemp warning light which is in the fluid and set to trigger at a critical point, it is reactive device that only allows for you to pull up and let the trans cool down.
The best way to monitor trans temperature is with a good quality aftermarket gauge/kit that has the sensor in the output fluid line of the trans.
For the record I have a combined low coolant and temperature alarm on my engine and a separate temp sensor only on my auto trans. I consider the temp sensor on the engine as a cosmetic device of little real value and the one on the trans as a guide that reads approx 10C below the actual trans temp as verified by my dealer on a 40km test run with his laptop plugged into the ECU.
As I have a thermostat controlled aftermarket trans cooler on my vehicle I am not too worried about trans temp but if I had to do it all again I would start with the aftermarket gauge/kit and take it from there.
Apologies for wandering off topic.
