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Originally Posted by Bill
Ray -
I don't disagree with anything you have said. HOWEVER, I have been unable to find anyone that I consider reputable (including two Ford dealers and at least two transmission shops) that is willing to drill a hole in the transmission pan and install the fitting for the sensor. Some of them have simply declined without explanation, some have told me that they are afraid of the liability issue if there is a tranny failure in the future.
As I recall, you had to jump through some hoops to get yours installed.
By the way, some of these guys have also pointed out that a pan-mounted sensor measures the temp of the fluid in the pan (duh!), which is only loosely related to the temp of the fluid in the torque converter. And it is the temp in the torque converter that matters. I'm not sure what to make of that. I suppose any indication is better than none, which is what I have now. But let's agree that pan temp probably WON'T tell you if the fluid is getting so hot that it is starting to degrade - because the degradation is happening in the torque converter.
So that begs the next question. Where does the OBD-II system measure temperature? I believe that it is measured somewhere in the guts of the tranny, not in the pan. I say this based on the 600-page book of Explorer wiring diagrams that I bought from Ford. It shows the sensor as mounted inside the tranny, with the shift solenoids and the torque converter lock solenoid.
Finally, there is a subtle but interesting implication to the discussion of OBD temperature readouts. The fact that there is a temperature readout means that the vehicle already has a temperature sensor. You don't actually have to buy and install one! And in my Ford, anyway, it is analog to boot! I have been thinking about tapping into the wire that leads to the sensor - it is readily accessible in a connector on the firewall - and using it to drive an analog meter. I may not be able to screw up my courage, since the consequences of messing it up could be severe. But it is interesting to know that the sensor is already there.
Bill
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As Frank notes, there's a lot of disagreement about where the best place is to measure transmission temperature.
Some people (like the mechanics you talked to) think the temperature of the fluid in the torque converter is what you should measure since that's where the hottest fluid is. The easiest way to measure that is to put a sensor in the line from the transmission to the cooler(s) since that line comes directly out of the TC.
Other people (me included) take note of the fact that fluid degradation is a function of both elevated temperature
and time spent at the elevated temperature. Yes, the fluid in the TC is hotter than in the pan but (1) only when the TC clutch unlocks, (2) is only about 20 degrees hotter when the clutch does unlock, and (3) it stays hotter than the pan for only a matter of seconds before it goes through the cooler(s). By comparison, if the fluid in the pan is too hot, it will be too hot for a period of minutes or even hours.
Furthermore, damage to a transmission from overheated fluid is primarily to the seals (and somewhat because of warpage in the clutches/bands)...and the fluid that's going past those internal components is coming directly out of the pan so they are, in essence, immersed in pan temperature fluid. Accordingly, if you look at the footnote of
this chart of transmission lifespan vs fluid temperature, you'll see it notes "transmission temperature reading taken in the pan". Other charts I've found that relate transmission fluid temperature to either fluid degradation or transmission lifespan have also used the temperature of the fluid in the pan.
Finally, there are some people who recognize the difficulty (and expense) of installing a sensor in the pan and thus install the sensor in the return line from the coolers to the pan. For example, the owner of Import Performance Transmissions in NJ (they do performance mods on imports) recommends putting the sensor in the return line from the cooler(s). In most cases (all steady state cases), the temperature in the return line will be very close to pan temperature since that line is the feed source for the plan fluid. The exception is during a long climb where the temperatures are steadily rising...or during a descent after a long climb where temperatures are dropping. In these situations, the gallon or so of fluid in the pan acts as a thermal buffer...it both heats and cools more slowly than fluid coming back from the coolers.
It sounds to me that the sensor in your Ford's transmission is just downstream from the pump...which takes fluid from the pan...so it is basically measuring pan temperature. Some domestic transmissions have a "test port" downstream from the pump and upstream from the valves, actuators and torque converter. It's possible on these transmissions to craft a fitting for this test port to measure the temperature there without all the issues of measuring pan temperature.
Bottom line is folks who want to measure TC fluid temperature don't understand the chemistry of degradation requires the fluid be too hot for an extended period. I've also noted that these folks are the ones who seldomly tow and who install a transmission temperature gauge primarily for gee-whiz and entertainment reasons. Putting the sensor in the output line from the TC will produce a much spikier temperature display (it will rise and fall everytime the TC clutch locks/unlocks) so it increases the entertainment factor. But those spikes are really false alarms unless the pan temperature is also getting too hot. And finally, the fluid that goes back into the transmission's guts is the fluid that's in the pan...if this fluid is too hot, the transmission can be damaged; if it's not, the transmission won't be damaged...even if the fluid going to the coolers seems too hot.