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Transmission Fluid, Pressure and ProblemsPosted 1/16/2001
By Ed Anderson
Properly repairing a vehicle and satisfying the customer are top priorities for most shops. However, those two things are not necessarily the same. What the customer complains about is not always what is wrong with the vehicle.
More than a few shops have heard the customer say, This transmission slips really bad when I hit passing gear and sometimes when I'm going up a steep hill. A quick road test reveals that the vehicle does make a loud engine noise when the accelerator is pressed to transmission detent but there's no acceleration. In fact, you may slow down a little. Same thing on a steep hill. If you feather the accelerator, you can get over the hill OK or pick up speed on the highway just so long as you don't get in a hurry.
Many transmissions have been yanked out of the car and disassembled only to find absolutely nothing wrong; everything looks like new. It sure felt like a transmission problem. Two hours earlier, some good diagnosis might have revealed that nothing was wrong with the transmission. What do we do now?
The choices are simple...or maybe not so simple. Do we go ahead and do the usual rebuild things to the transmission, charge for a transmission rebuild, and try to fix the problem after the transmission is reinstalled? Or, do we put the transmission back together, install it back into the vehicle (no charge) and then try to figure out what's wrong with the vehicle? Good diagnosis should have caught a plugged fuel filter or a restricted catalytic converter before major work was begun. A seat-of-the-pants road test and an educated guess based on years of experience are not always enough for today's vehicles.
While on the topic of diagnosis, I would like to cover some important points on rear wheel drive Chrysler-type transmissions. They started out being called Torqueflite 6 and Torqueflite 8, or 604s and 727s. As time went by they were called by other names, such as A-500 and A-518 and some even more complicated designations, as they evolved into the new models being produced today. You may want to grab a circuit book for one of these transmissions and follow along with the figures in this article because for simplicity I have omitted some passages.
These are very good transmissions, but two things stand out that set them apart from other units. First, if they aren't driven for a few days, most of them have torque converter drainback. And second, after being rebuilt it is not uncommon to experience a broken reverse band within the first couple of thousand miles.
Part of what I'm going to say about these two problems is proven fact and part of it is personal theory that alleviated these two worries for me and many other rebuilders who took a few extra minutes when assembling a rear wheel drive Torqueflite.
Take a look at Figure 1, or in your circuit book look at a Torqueflite in PARK. Note the position of the manual valve. Pump pressure directed to the manual valve can exhaust out the end of the manual valve bore and back into the sump. For this reason there is virtually no mainline pressure in the PARK position. The torque converter can drain back into the sump also. Over the years, technicians, and even the factory, have installed check balls in the cooler lines and made other changes in an attempt to stop torque converter drainback. It takes two things to get drainback: a place for air to get into the torque converter and a place for the fluid to come out. One of those places is always there, at the manual valve.
Figure 2 shows the manual valve in the DRIVE position. A spool on the manual valve prevents pump pressure from exhausting out the manual valve bore. In fact, by checking your circuit chart you'll see that the manual valve prevents fluid loss in every position except PARK.
This brings us to one part of the story concerning broken reverse bands soon after the transmission is rebuilt. It's a proven fact that some transmissions have sticky valve concerns after being rebuilt. If the driver starts the engine in PARK with fluid exhausting out the manual valve bore there is no mainline pressure and the pressure regulator spring has the pressure regulator valve pushed to the bottom of its bore. The driver moves the manual lever to REVERSE and three things happen: 1) fluid is blocked from exhausting past the manual valve, 2) pressure boost is activated for reverse, and 3) the manual valve has to start regulating mainline pressure. At this point, if the pressure regulator should stick a little in its bore, it's anyone's guess how high mainline pressure could go.
You may ask why there's never a broken front band or bent linkage; there's no pressure boost in DRIVE. Hang a gauge on a Torqueflite and you'll see a lot of REVERSE pressure and you'll see it quickly when you move that selector. If you started the vehicle in NEUTRAL every time, it would eliminate most of the problems. If you started it in PARK and moved the selector to NEUTRAL for a few seconds before going to PARK it would help, but no one wants to do that.
Take a look at Figure 3. There's a kit available that contains a special design manual valve for Torqueflites. It has a very narrow full diameter spool in just the right place. The manual valve in Figure 3 is in PARK but pump pressure cannot exhaust out the manual valve bore. It is blocked by the thin spool. The pressure regulator begins regulating mainline pressure when you start the vehicle in PARK. This is one of the best things to ever happen to rear wheel drive Torqueflites. Most major transmission parts suppliers have these kits in stock. The kits contain other items to improve shift quality and transmission durability but the manual valve alone is well worth the price of the kit.
The new designed valve will allow you to check the fluid in PARK. You can also fill the transmission in PARK because the pressure regulator will be working, allowing fluid into the converter fill passage and mainline passage.
Let me share with you a couple of other recommendations to be done when rebuilding a rear wheel drive Torqueflite: When assembling the valve body, leave all the screws finger tight on the filter side of the valve body until you have the valve body bolted to the case, then tighten the screws on the filter side. If you tighten those screws first, it's possible the valve body may not lay flat against the case but will be pulled down when you tighten the bolts holding it to the case. This could cause some of the valves to bind in their bores.
The final phase of the reverse band saga has to do with the reverse servo. Some technicians have experienced a reverse servo piston cocked in its bore a few miles after a transmission rebuild. The final result is a burned reverse band. It is recommended that on every rebuild you refinish the reverse servo bore. Use a very fine sandpaper or crocus cloth and lightly sand the bore all the way around. Would a servo piston tip over if it wasn't dragging heavier on one side than the other? Do you think the factory had problems with them tipping over? Get that bore surface uniform all the way around.
Torqueflites are great transmissions and through the years numerous improvements have been made. The factory-recommended fluid has proven to be a plus for best performance.
The question often arises as to which is the best filter: screen or cloth. It's strictly a matter of individual preference. They were designed for a cloth filter. Hang a pressure gauge on a Torqueflite with a cloth filter and take it for a drive. Just as the transmission begins its shift, the gauge will dip about 10 or 15 psi, hang there a couple of seconds, and then jump back up. The gauge will drop before you can feel the shift begin. Notice the amount of time the pressure stays down during the shift. Install a screen-type filter on the same vehicle and take another drive with the gauge attached. When the transmission begins its shift, the gauge will just flick down and right back up again very quickly. That would tend to indicate the screen filter allows fluid to flow through more easily. Does the cloth filter catch more foreign material? It's just a matter of individual preference.
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