Trans Temp - Easy to read with a multimeter

[civdiv99]

As part of a much larger project, I wanted to be able to monitor my transmission temperature electronically. While eventually this will be part of a much more comprehensive project of monitoring (and changing) various parameters, I thought I'd post this piece. This article discusses how we can check the trans temp using a quality digital multimeter, and discusses how I went about my set-up, with some pics. This is on my 2002 X5 4.4i with the ZF 5HP24 trans. Other models will be similar, but actual values and resulting table may not.

Most of us know the transmission uses a resistor, integrated into the transmission harness, that varies it's resistance by temperature - a thermistor. Well, if the TCM can read it, so can we. Tables of resistance vs. temperature for this thermistor are readily available. I've included in the series of pictures a shot of one of those tables. Knowing the resistance vs. temperature, all we need to do now is translate that into a voltage. And what better way to do that, then measure it. I started with the car not having been started for 2 days, and the temperature has been hovering around 20 degrees F, maybe teens overnight. With the outside thermometer on the house reading about 20, and the IR thermometer on the trans pan showing 23, that'll be plenty consistent enough for a baseline; we're not designing the next gen Mars rover here. Refer to the pictures as necessary. Note - it was dropping into the teens out, so playing with the camera wasn't high on the list of things to do whilst gloveless and working, but I gave it a shot.

1. Pop the hood and take off the plastic part at the back - the flat housing surrounding the cabin air filter. Just twist the knob thingys and work it out. Then remove the passenger side end piece of the air-intake duct. REALOEM labels it air inlet housing - right. Lift it straight up.
2. Locate the housing which holds the engine management and transmission management computers, which you gained access to once the passenger side air duct part was removed.
3. There are 4 5mm Allen headed screws. Remove them, and using the tab provided on the cover, lift it up about 1 to 2 inches, and then wiggle it forward. There are tabs in the back, thus come forward with it and don't try to just lift it straight out. You'll feel when the tabs are free.
4. There are a couple of computers in here, and some relays and such. The engine management unit is the silver one - if you like you can ease the tabs holding in and slide it up to see the label, and record that info. Or not. The rearmost computer which is using the BLUE connectors is what we're interested in.
5. The sensors from the transmission are at connector X7004, pins 12 and 22. Pin 22 is the control - and pin 12 is just through an internal resistor in the computer to ground. I'll show how to access both - but depending on what you want to do, pin 22 is really the point of all this.
6. The connectors are numbered X7001 through X7005, and if you are standing at the passenger side fender, the numbering is such that number 1 is furthest away from you. Only 1, 3, and 4 are used. The plug closest to the passenger side is X7004. Just rock the black strap down and the plug pops up. The black strap hinges and is used to insert and remove the plug, so no gorilla moves here. Refer to the pictures.
7. Once the plug is out, you can see these are just Amphenol connectors, 2 of them in this one plug, one is grey, and one is black. The blue is just a housing, and since the connectors only go into the blue housing one way, pry back the little tab visible on the blue housing and slide out the black and grey connectors.
8. The connectors you just removed are clearly labeled at the end pins. There are 4 rows of ten. So find the number 11 at one end, and the one next to that will be number 12. Ditto the number 21, the one next to it is number 22.
9. If you like, read across between number 12 and number 22. Using the chart of resistance vs. temperature, you can confirm your choices as well as check out the temp you expect based on the reading. In my case, the resistance was 774, and from my WAG of temps in the trans in the 20 to 22 range, that was just right.
10. To do this, you want to backprobe the pins you are interested in (12 & 22). I used 18 gauge solid core wires. The gauge is heavier than necessary for the reading, but fits into the back of the connector nice and snug for a good connection. Cut a couple of pieces in your choice of colors and strip off maybe one 1/4 inch, and slide the stripped end of the wire into the back of the connector alongside the factory wires. 18 gauge solid (stranded will just piss you off here - obviously) will slide in easily but you'll be able to fell the snug fit. I suggest about 2 feet each color. You can always cut it shorter, and is much easier to work with than wires that barely clear the cover when you reattach that.
11. I used a couple of small zip-ties to fasten the wires I just inserted to the adjacent factory wires so maneuvering the plug back into place didn't cause the backprobed wires to slide out. Test your job by taking a resistance reading off of the other ends of your wires. Should match your direct reading previously from the pins.
12. Reassemble the connectors into the blue housing. They only go one way, so just hold the grey and black connectors together with the tabs that fit into the blue housing notches outward, and slide them into the housing. Put the blue housing back in place and help it wiggle down as you rotate the black strap which levers the assembly back down into place. You more than likely had the half-moon rubber grommet behind this computer pop out during your work, so put it back in place.
13. I used one more zip-tie before feeding the wires out so I didn't accidently put strain on them. At the front of the computer housing is a half-moon rubber grommet. I took a tiny snip out of the top lip to accommodate the 2 wires I just inserted.
14. Refit the cover - slide the tabs into place and push back and down. It's kind of a tight fit - you'll know you got it when it rocks down into place and the screw holes line up.
15. Reassemble everything. Since I am going to be making use of these wires later in a larger project which will include an LCD trans temp reading, etc, I terminated the 2 wires in a female connector. You can tuck in, tape it against the computer housing, whatever you prefer. Put the air intake ducting back, and fire it up.

Now what to do with this. Since the tech documents gave us resistance vs temperature, and while not absolutely linear, the table is close enough to linear for our purposes (Mars Rover analogy here), we just need to convert that to voltage vs. temperature. So, mine at about 775 ohms cold was 1.315v across the wires. Good start. (You can also go just from pin 22 to ground. Different voltage, but will follow the same progression) Driving around and taking voltage readings, I recorded the voltages, and hit the pan with an IR thermometer. Back checking the results - voltage and temp should follow the same rate of change as the resistance vs temperature table in the technical data, I was satisfied.

Pictures follow.

CivDiv99