Since the procedure for checking the automatic transmission fluid in NAG-1’s requires correlating to temperature, I thought of a way for folks to accomplish this without the need of your local service department. The cost of an application-specific dipstick from industrial suppliers like Miller is in my opinion, ridiculous.
For a few bucks more one can not only develop their own way of accurately measuring NAG1 fluid levels, but also measure temperatures of other stuff. Add to this some of you will already possess a digital volt meter (DVM), which will further reduce cash outlay.
First I went to a local wrecker’s and chose a nifty transmission dipstick out of a Ford minivan. The minimum length required is ~40”, so FWD vehicles are convenient as generally their length is longer. The one I chose has a locking feature (expanding rubber plug), but after a little more thought I elected to leave the dipstick longer and actually attach the thermocouple to the dipstick shaft itself.
I cut down the donor disptick to a suitable length, then using a Dremel scored a line every 10mm (up to 10cm) along the bottom:
The Type J thermocouple used to measure automatic transmission fluid (ATF) temperature is very common. To read more, go here: http://en.wikipedia.org/wiki/Thermocouple This particular unit is from Fluke, actually it came with the thermocouple module. The PN for just the thermocouple is 80PK-1.
One either requires a DVM “with” thermocouple capabilities (a separate receptacle to plug in the unique spade-type connector), or a DVM that measures millivolts (mV) or volts (V) and a thermocouple module (Fluke 80TK is what I have – with this unit I can employ different types of thermocouples to read temps in excess of 4000F).
Using a long length if heatshrink, I sandwiched the thermocouple lead to the dipstick itself. When I insert the dipstick, I get the ATF level and the temperature in one operation. This is not a requirement, in fact you could insert the thermocouple lead into the transmission’s dipstick tube until it bottoms out on the oil pan, observe the temperature, then insert the dipstick and note the fluid depth (in cm) and compare to the graph in the service manual.
I have included a representation of the graph to save folks hunting the service manual or TSB’s:
One bit of advice; add a small amount of ATF (say 1/3 of a litre), then wait 5min while the dipstick tube drains sufficiently to prevent erroneous readings as the dipstick picks up remnants off the inside of the dipstick tube during insertion/removal. Being out a couple of mm either side for a given temperature is not critical, as the descrepencies of the measuring equipment exceed this tolerance. I also confirmed that the onboard thermocouple sensor employed by the PCM measured within 2C of the J-type thermocouple (I have the Aeroforce dual gauge set which shows amongst other parameters transmission temperature, TCC lock up, slip, etc through the OBD II port – the same sensor is used by StarSCAN/MOBILE).
NOTE: Those of you who use devices like Aeroforce or DashHawk which display transmission temperature by way of the OBDII port; When the vehicle is in "PARK", the OBDII port output for transmission temperature is not correct, and is actually displaying coolant temperature! Therefore, to observe transmission temperature, engage emergency brake and shift into "DRIVE" or "REVERSE" prior to adding fluid!