Blowin' Smoke

An Editorial by dieselmann
Volume 13; July 02: 2002 Factory Transmission Temperature Gauge

I was curious to see how active the factory ATF gauge was, and what temperature a particular gauge position represented. First, let me relate how the ATF temp gauge works. The powertrain control computer (PCM) monitors the transmission fluid temperature (TFT or TOT) sensor in the transmission pan. The TFT sensor is a thermistor that varys in resistance with temperature changes. The PCM sends 5 volts to the TFT, which is then grounded through the PCM on a sensor ground wire. As the ATF fluid temperature increases the sensor resistance decreases, which in turn causes the voltage reading on the TFT sensor wire to decrease. The PCM uses the TFT signal for shift timing.

The instrument cluster is actually a micro prossesor or module that is networked to the PCM. On this network the PCM sends information from the cam position sensor for tach display (which is why the 02 tach does not register during crank), the water in fuel sensor for that warning light, TFT sensor for the ATF gauge, and also a signal to turn on the service engine and wait to start lights. It does this over two wires instead of circuits for each function.

To check the ATF gauge I disconnected the solenoid connector from the transmission and hooked up a gauge tester to the TFT sensor wires. The gauge tester allows adjustments of resisance over a wide range-- the TFT sensor's normal operating range is 800-100,000 ohms. I also hooked up a scan tool to the diagnostic connector to see what temperature the PCM was reading at a particular gauge position.

At a reading of 50 degrees the needle started moving and came up to the top of the cold range. At 100 degrees the needle read just below the center position of the gauge. The needle would creep up as the temperature reading increased, and at 70 degrees was near this position. I was disappointed to see the gauge would continue to stay at this position all the way up to 220 degrees. As the temperature reading was increased, the gauge rose to just above halfway at 230 degrees. The gauge rose rapidly to the yellow portion of the gauge when the temperature was increased to 250. At 280 degrees the gauge read in the red.

I can see Ford wanting to dampen down the movements of the gauge so it is not constantly sweeping across the normal range, but I would have preferred a smoother display. As it is, the gauge stays the same though both normal operating temperatures, and those that arre generally considered excessive. Basically, if your gauge has a sustained reading over half way for any length of time the trans is overheating and the fluid's service life has been reduced to approximately 25000 miles. If it gets into the yellow, plan on having the fluid changed soon.

I'm sure not every gauge is going to read at the exact same position, just as not every fuel gauge reads the same, but this will give you a general idea of what temperatures your transmission is beign subjected to while the gauge sit there near the middle of the scale. I hope this helps explain why the gauge always seems to read the same, and prepares you for when the gauge reading starts to increase, instead of rising gradually as it did on the bottom half of the scale, it's going to come up fast.

Charles David Ledger; dieselmann1999

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