The PCM will defeat OD until the fluid reaches 60*, and TC clutch below 70*......I'm not going to argue about it. The sending unit is in the hot line at the trans, and doesn't take long to reach 59*. Wanna defeat it? put a 1K ohm resistor between the two terminals in the plug and tape it up. If you neysayers don't believe me, your loss. And no, shorting the two terminals together won't do the trick. The sending unit generates a resistive value depending on teperature. The PCM must see a certain ohm value from the sending unit. When installing an aftermarket gauge and sending unit in the stock location, a resistor must also be placed in the OEM plug to trick the PCM.

Chris

 

okay if i was one of those naysayers who might have the dreaded foot in mouth disease cummins express what does 12v1911 need to know to fix his problem if the sending unit failed it wouldnt fail so his od would work? and for my own knowledge if it has such a low temp to work what is its purpose?

 

The sending unit values are two fold. At low temp, high viscosity characteristics, DC chose to defeat OD and TC lock-up to protect against inadequate pressures and flow rates to effectively hold those two selections, and preserve transmission life. Additionally, the PCM will also fail TC and OD if fluid temps rise above 270*-275*, and subsequently illuminate the indicator below the heater controls showing trans temp.

I'm assuming the 140* he's talking about is engine temperature, and I'll concede that by the time the engine is at this temp, the trans fluid should be well beyond it's lower threshold to allow OD/TC. Fluid shear within the TC raises temperature pretty rapidly (providing it's not locked), another reason DC chose to fail lock-up at low temps...to let the trans heat up it's fluid better. So yes, my gut instinct is that there is indeed an issue.

The reason could be two-fold. Bad connections affect continuity, aka resistance, which is the very heart of this circuit's operation. Though the plug uses a weatherpack plug, it's not impossible for water intrusion, and hence corrosion. Secondly, the sending unit is a potentiometer which changes it's value according to it's surrounding environment's temperature. It can fail without completely failing...kinda like a fuel level sending unit. Same principle, different environment. Now I'll admit you caught me with my pants down, as I can't find the chart I used to have to correlate temperature to ohms for that sending unit. So....using a VOM on it and looking at ohms to it's current temp is out of the question. So what I would do is visit Radio Shack and buy a 1000 ohm resistor...all of a few cents....the type you would see on a board, with multi-colored stripes. Then, place each end of the resistor into each terminal on the plug (harness side) with it unhooked from the sending unit. Go for a drive and see what the trans does. Then he should let us know here what happened, and we'll go from there.

BTW...if installing an aftermarket gauge/sending unit, the front servo port is a terrible place to place the sending unit as prescribed by most instructions supplied with the gauge. The pan is a terrible close second. Neither of these places truly reflects an accurate temperature. You want to read the fluid's peak temperature, and the best place to do this is immediately after leaving the transmission, and in the hot line going to the coolers. The stock location is the best....for anything up to 95 that is. The RE's read it internally I believe, but I'm a RH 12 valve guy. So to leave the factory sending unit intact, you'll have to tee the line right after it and place your new unit there. Otherwise, by installing a 1/4 NPT x 3/8 NPT adapter onto your new sender, it'll screw in right in place of the stock one. BUT, you'll have to trick the PCM or else guess what...yup, no OD or TC. Here again, you'll permanently install that 1000 ohm resistor in the harness and not look back. Anyway, for what it's worth...

Chris

 

Great info guys, my RH does the same thing, but it usually only takes 1 or 2 miles if its real cold outside (20 deg. or less).