Diesel Bombers - Auto Transmissions Explained

Auto Transmissions Explained

Written By Den052

There seems to be quite a few problems with the Dodge transmission behind the diesel and I hope that this information will help with understanding the nature of the beast.

1. The transmission that is found behind the 5.9 Cummins is basically a “A727” transmission with an overdrive unit in the tail housing. There have been numerous designs that up the torque and horsepower ratings of these transmissions, but this is the basic design. The A727 was a “bullet proof” transmission from the mid 1960’s through the 1980’s and beyond. When they added the overdrive unit on, many transmission rebuilders say the unit does not get enough lubrication.

2. Automatics require 2 oil supplies. The first obvious supply is pressurized oil for the clutch applications. The second supply is the lubrication oil that keeps the bearings, bushings, and clutch plates wet. Lose either on, and the transmission burns up. These oil supplies are controlled by the transmission valve body and the oil filter. If the oil filter starts getting restricted, this affects both oil supplies.

3. Believe it or not, a properly operating transmission should not need a oil filter! There really is nothing in the transmission that has to be filtered. No impurities get into the oil and if the clutches are applying correctly, there is almost no wear to them. The only way the filter starts plugging up is because of clutch material, or bushing/metal failures.

4. A real good indication of how well your transmission operates is by looking in the pan. A properly operating transmission should have a clean pan (no metal or clutch material on the bottom of the pan). If you see metal or brass in the pan, the transmission fluid is not lubricating properly. It is becoming too hot and thin. Keep in mind that transmissions can operate at quite a hot temperature (200-300f), with no ill effects providing the oil doesn’t get too thin. The problem with regular ATF is that it is really thin at 200f. You basically have two choices. Either install an auxiliary oil cooler to lower the transmission fluid and provide better lubrication, or switch to synthetic transmission oil or alternative oil such as hydraulic oil instead of ATF. When you think of hydraulic oil in a transmission, it is exactly that. After all, an automatic transmission is operated hydraulically, so it is not so far fetched. I use it in my Dodge instead of an oil cooler, and my pan stays clean 24/7.

5. Diesels can be a little hard on the torque converters. Given a gasoline or a diesel engine, the torque converter has more stress on it behind a diesel. Diesels have a tendency to vibrate and can sometimes shake the converter to pieces. If you are installing a remanufactured, overhauled, or new transmission, put in the best heavy duty converter you can find.

6. When installing a transmission after a previous failure, be sure to flush out the oil cooler and lines real good. Previous debris can come back into a transmission and cause the filter to plug up again, resulting in another failure. It is real good practice to drop the pan and change the transmission filter about a month after the new transmission is installed even if you blow the oil cooler and lines out. Stuff can still work its way back into the new transmission.

7. A good indication of how well your replacement transmission is working is to pull the pan after 6 months or so and look for debris. There really should not be anything in the pan other than transmission fluid. The transmission fluid should be bright red and not dark or discolored. Dark, discolored, or yellow fluid is a real good indication that the fluid is operating too hot.

8. Believe it or not, an automatic transmission can pull extreme loads better than a stick. Three things create these advantages. One, the torque converter provides a smooth fluid coupling that acts a lot like a shock absorber between the engine and drive train. It is good on the drive train and rear ends. 2nd, The torque converter provides an additional gear ratio of about 2.5-1 for additional gearing beyond the straight 4 or 5 speed stick. 3rd, Planetary gear units can pull heavier loads than a main-shaft and counter gear stick transmission. Each pinion provides 3-4 teeth times 4 or more pinions, for a total of 12 teeth or more for pulling. Keep in mind that that is also in a perfect circle, so load and torque are distributed equally around. The stick transmission on the other hand, with a main-shaft and counter gear, only provides about 2-4 teeth at the most at any one time to transfer the torque and load from the main-shaft to the counter gear. Also extreme loads tend to cause these to separate, that can split a manual transmission case apart. Think about these things the next time you pull something down the road.

(Comments by a diesel and automatic transmission mechanic for over 30 years.)