Dodge 47/48 series Diesel Transmissions
There are 3 basic places to install a temp sender for the transmission; the transmission pan, the front cooler line, or one of the transmission case pressure tap ports. There are pros and cons to each location.
Many people like to install the sender at the front cooler line. This location will give you the temp of the fluid coming out of the converter and will be the hottest temp you will see from the trans while in the fluid coupling mode (converter clutch unlocked).
On the ’94-’95 transmission there is a temp sender already in the front line that gives info to the transmission computer so it will not let the trans go to 4th or lock the converter clutch until the transmission and engine are warmed up. At prox 70° F it will take approximately ½ mile of driving before the computer will command a 3-4 or lock up shift. At approximately 30° F it will take about 3 to 4 miles before it is warm enough for the 3-4 / lock up and if the ambient temp is below 0 it can take 5 miles or more.
Bottom line, if you have a ’94-’95 this sender must stay in the line. If you are going to install a gauge sender in this line you must make some sort of manifold to install the second sender. The original sender MUST be in contact with the fluid - taped to the line is not good enough. One problem with a manifold set up for the sender is that it adds weight and the extra weight coupled with vibration could crack the line and cause a leak that you would only see when the engine is running.
90% of the heat in a properly operating automatic transmission comes from the torque converter fluid coupling. The fluid coupling is when the drive fan (connected to the engine) of the converter is blowing oil at the driven fan (fan connected to the trans input shaft). Once the converter clutch (single disc or triple disc) locks the fans together, all of the power is transferred to the input shaft. Because the fluid is not being used to transfer the power while in “lockup”, no heat is transferred to the fluid. The more efficient the fluid coupling is, the cooler it will run WHILE YOU ARE MOVING. An efficient converter (fluid coupling) will run hotter when you are in gear but not moving. This is because when you are not moving, the engine fan of the converter (impeller) is still moving with the engine and trying to transfer the power (heat) to the driven fan (turbine). If the driven fan is stationary, as would be while at a stop sign, the power has no place to go but into the fluid and out to the cooler as heat. On the other hand, when you are up to speed and the converter clutch locks the 2 fans together, you are now transferring 100% of the power to the rear wheels and the power (heat) will be transferred to the rear wheels. When the lock up occurs you would see an immediate drop in fluid temp of about 20° F.
Normal temps of the transmission fluid will fall in the area of about 100 to 280° F depending on where you check it.
Pros for the cooler line location:
You will only see the full range of temperature at the front cooler line.
Normal temps when monitored at the front cooler line would fall in the range of about 140° F to 280° F
While watching the temp at the front cooler line you can instantly see the temp climb if you are pulling a heavy load. You can also see the temp fall almost instantly when you back off the throttle and you can also tell that the converter clutch was COMMANDED to lock, because the temp will drop instantly even when under heavy load when the converter clutch is COMMANDED to lock up.
The cons of this location, and one of the reasons I prefer the sender in the pan is because if we are monitoring the gauge this close your eyes are not on the road. This is a very active gauge when the sender is in the front cooler line. The sudden and extreme range of temps you will see may make one nervous even though they are well within the norm at this location.
Just the opposite is true if you install the gauge in one of the pressure ports of the transmission case. The fluid at any of these test ports is stagnant oil, once the oil gets to the test port it is at a dead end and is no longer circulating. At one of these locations we will really be reading the temp of the transmission case. At these locations it will take the longest to get a reading, and by the time you see a reading above 200° the converter temp was probably around 250° - 270° for quite some time because it takes a while for the heat to radiate into the case and once the fluid cools it will also take a while for the case temp to drop, back to that radiation thing again. At this location expect to see normal temps range from about 140° to 190° - if the temperature reaches above 200° I would find a place to pull over.
Pros for the pressure test port location would be ease of install and multiple locations to use.
Cons for these locations would be slow gauge reaction time and also we need to make sure the sensing tip of the sensor is not too long and bottoms out before you have the sensor tight. WE DO NOT RECOMMEND THESE LOCATIONS
We like to install the temp sender in the trans pan. The gauge will react quick enough to save the trans form overheating and yet the gauge won’t be so active that it would make one nervous about the temp extremes. The normal temp range you would see will be about 140° to 200°. If the trans temp gets above 200° F we would want to get the engine rpm above 1500, I will explain why. Fluid that is cold does not move very quickly through small passages, like the small passages in the valve body. Fluid that is too hot is hard to pump because it is too thin.
Fluid at 230° does not hurt the seals, gaskets, or clutches, but, because it is so thin it is hard for the pump to maintain enough flow so the valve body can maintain enough pressure in all the circuits. At approximately 200° in the trans pan even a good pump will have a hard time flowing enough fluid to satisfy all the circuits when the pump rpm (same as engine rpm) is below about 1,300 rpm. If the pump can’t maintain the volume of oil and the pressure regulator valve cannot maintain good pressure, the clutches and bands will slip. The cooler flow and pressure will also be lower and this will escalate the heating problem. This can easily be seen on the transmission dyno where we can monitor trans temp, clutch pressures, cooler pressures and volumes. Even with hot fluid above 200° these pressures and volumes come back to normal when we bring the rpm close to 1,500.
With 4.10 gears this is not really a problem because the engine rpm will not be around 1,500, but, with 3.54 gears you can easily be at 60 mph or lower with the converter locked up and the engine rpm could be around 1,500. All depending on tire diameter, of course.
For the above reasons alone we do not like to get the converter stall too low, lets say someone wants an extremely low stall converter and they are going to do a lot of snow plowing, the engine and customer may like the low rpm but if you are working things especially with the converter clutch unlocked at low rpm the pressure and cooler flow may suffer.
Hope this clears things up for you as far as sender locations are concerned
If you have any questions just call Dave @ 563-778-2719