Wanted to get some thoughts and see if this has ever popped on for others. I was sitting in stop and go traffic tonight by the Holland Tunnel and the trans temp light came on. Nothing I could do about it at that point as I was sitting in the approach for the tunnel and then in the tunnel. If you know the area, it is two lanes with no shoulder on either side...so no chance to pull off. Temps were in the low 80's out, and the transmission was not really under a load (no towing, hills, etc).
As far as the transmission, the fluid is likely as close to 100% new as you can get, as it has been changed a few times over the last 3,000 miles. In addition, both trans cooler lines are new about 2K ago, and I had the trans cooler out and about 2K ago and it is clean. Also, the fan/fan clutch are brand new and the electric fan also works fine. At the last fill of the fluid, it seemed I got the right amount in, and did the proper procedure while adding it (truck running, level, friend shifting thru gears while I pumped it in until it flowed out). I'm using the recommended Dex/Merc high mileage stuff and maybe 1/2 bottle of Lucas trans stuff as well.
Once I got out of the tunnel I only had maybe 1/2 mile to where I was going, and the light went out just about as soon as I got out. On the drive home later on it never came on, and all shifts (even with the light on) were fine.
As a side note, this didn't set off any type of chime on the dashboard, and it did not throw a warning or code on the Ultra Gauge.
Anyone else ever have the light come on? Any ideas or things to check? I will take a look at the fluid level tomorrow, but I don't think it is low and there are no leaks in the system. Considering the transmission was not under a heavy load and it does not set off warnings besides the one little idiot light, is this something to really sweat should it happen again? I'm thinking maybe the sensor is getting hot, but I don't see why the transmission being hot would affect it all that much, especially seeing as the fluid is good and is brand new.
Would love to hear thoughts and if you have seen this too. Also, for those that followed the cooling issues I had...I'm happy to say that even sitting in that traffic for a long time tonight, the temps never went high. Stayed perfect. The soft spring stat is working wonders.

 

There are 2 sensors on that circuit. A tbox temp switch (on the left upper corner of the tbox) and a trans temp switch on the trans cooler (lower right corner of the trans cooler) This can be accessed by removing the plastic frame cover in the r/f wheel well. You can find the connector for the trans temp sensor under the radiator cover. While you are standing there looking a the radiator, it will be the only connector attached to the radiator at the upper left corner. the circuit is normally open, if one contact in eiher switch closes or shorts internally the light will come on. More than likely its the trans sensor. You can disconnect either sensor, if the light still comes on it will be the other sensor that is still connected that is faulty. Not a big deal.

 

The temp sensors go bad on the DII all the time, lots of posts about this.

 

It may be interesting to put a scanner onto the OBD2 connector and note your running engine temp.

Also-
If you had the A/C on, this would force hot air thru the transmission cooler - raising the ambient temperatures making it harder for the cooler to work.

And -
When that transmission light is on - if you put the truck into neutral and take the engine RPM to 2,000 I would think the mechanical fan would roar as it is pulling air through all three coolers in line - the condenser for the A/C
the trans cooler and the radiator in turn.
If that fan is not engaging via the fan clutch, then you are not getting max cooling efficiency and the trans fluid could be running too hot.

OK, and here is a guess. If you have overcharged your A/C system with freon, I am not sure if that will cause more heat to come off of the condenser.

All of these things combined could trip the temp switch.
Or, as Spike noted, the switch could just be bad too.

Too bad you can't get a thermocouple in there and look at the trans temp.



From Ashcroft's site

300° F=
The maximum allowable temperature. This is the recommended place to install a temperature gauge or sensor. Do not allow the converter outlet temperature to exceed 300° F. The temperature at this location will vary significantly because of load, hill climbing, etceteras. If the temperature reaches 300° F, reduce throttle. To lower transmission temperature with transmission in neutral, run the engine at 1,200 RPM for at least 2 - 3 minutes while keeping a close check for signs of engine overheating. A transmission in a heavy throttle, stall condition (a typical situation is rocking a vehicle stuck in mud, sand, or snow), may increase temperature at a rate of 1° degree per second of stall.
150° F=
The minimum operating temperature. Note: It is possible in low ambient temperatures to overcool the transmission with auxiliary oil to air coolers. Oil to water coolers in standard factory radiators will normally not overcool a transmission.

175-200° F=
Normal pan oil temperature operating range.

275° F=
Maximum allowable oil pan temperature for short durations during long hill climbs.

300° F=
Damage occurs to internal transmission parts, including warpage of metal parts, degradation of clutches, and melting of seals. Transmission oil oxidizes, (forming varnish-like substances causing further clutch slippage and compounding heat build up) and transmission oil life is extremely short.



Automatic transmission oil can provide up to 100,000 miles of service before oxidation requires replacement, at an operating temperature of 175° F. Above this temperature, the oil oxidation rate doubles with each 20° degrees F increase in oil temperature. See chart.
Note: Transmission oil and filter should always be changed more frequently when driving with heavy loads (RVs), typically at least every 25,000 mile intervals, since converter outlet temperatures then normally far exceed the ideal 175° F.