In an email exchange with Martin I mis-spoke about Transbrake and Manual Valve Body, Martin then took the time to explain them to me and educate me on this series of transmission and capabilities. I haven't bought anything from Martin, but you can bet with this level of support I will.
Hey Gregg,
Let me see if I can assist with some understanding on the 518 trans operation in regards to "Trans Loc" (I'll assume we're talking about a TransBrake??), and Manual VB.
First off, some very simplified (for the sake of time and writing!) trans operational theory.
The following will apply to:
904,
727,
999 (bit of a rare bird),
AND
Overdrive units,
A-500 (overdrive equipped 904, mechanical governor circuit),
A-518 (overdrive equipped 727, mechanical governor circuit).
42RH, 44RH (mechanical equivalent to the A-500)
46RH (mechanical equivalent to the A-518)
42, 44, 46RE, (evolution of the A-500 and A-518. Uses electronic governor pressure solenoid to generate governor pressure rather than the mechanical governors in the earlier units)
A-618, 47RH, 47RE (Diesel and V-10 engines only. Different bellhousing pattern, interchangable internals with other cases, provided you know the parts and pieces to keep everything operating together. Followed same evolutionary pattern of mechanical gov, to electronic gov). Early 618's had OD, but no TCC. The cases were the same, but VB is obviously different, and the block adapter from Cummins engine to Dodge transmission is shallower on a non-lock application. Can swap over to lock-up style, but need an additional 3/4" clearance from block to bellhousing, or later style block adapter.
These transmissions all have three forward gears that operate via hydraulic controls in the VB. Overdrive units have a fourth forward gear OD operating a planetary gear set via hydraulic controls, but triggered by electronic signals from the PCM. Take away the PCM or other extraneous electric circuit and you will have no fourth gear or TCC engagement.
I listed them all together because for all intents and purposes the valvebodies and components work basically the same in all the listed units in regards to 1-2 and 2-3 shifts in operation stretching all the way back to '63 in general operational significance.
The primary circuits we will be discussing are:
Line Pressure. (I'll call this apply pressure in various places for understanding purposes). This is the hydraulic pressure generated by the pump as it is supplied to the various clutches and bands to create the shifts and maintain clamping loads to prevent slipping. Line pressure will increase with rpm and is ultimately controlled by the line pressure spring and valve, as well as the Boost Valve where pressure is increased when in OD and with TCC engaged. Also indirectly affected by Throttle Pressure as it rises and falls.
Throttle Pressure. Throttle pressure is controlled by a mechanical connection to the carb or throttle body on the vehicle. As you apply more throttle, the TV is forced inward, generating more throttle pressure in the trans. I'll explain how this comes into affect later..
The exception to this is the 48RE which uses a TVA (throttle valve actuator) to move the lever up and down to raise or lower the throttle pressure.
Governor Pressure. Governor pressure is either generated by a mechanical governor assembly or through a pulse solenoid in the RE units to provide 1 PSI Gov pressure per 1 mph. Obviously this means your gov pressure should rise in lock step with your road speed.
Now for some operational application.
Imagine a cylindrical valve with three separate hydraulic zones.
One end has Governor pressure acting against it pushing it from left to right, the other end has Throttle Pressure acting against it pushing it from right to left with a spring holding it in the left position, and the middle zone has Line Pressure waiting to be released into a port when the valve slides to the RIGHT.
We'll call this the 1-2 shift valve.
When at full left position, Line pressure is available to the Forward clutches which give First Gear, but the Line pressure cannot get into the circuits that apply Second gear because the valve blocks the port when at the full Left position, so all you have is First Gear.
As road speed increases, so does Governor Pressure, which is acting on the left end of the valve. When Governor pressure is adequate to override the spring on the right end of the valve, the valve moves over to the right, opening the port for Line Pressure to apply the Intermediate Band, which stops the Direct Drum, causing the planetary gear set to provide Second Gear.
As we discussed earlier, Throttle Pressure increases with throttle application, so as the Governor Pressure is rising on the left end of the 1-2 shift valve, Throttle Pressure is rising on the Right end. This is why shifts occur at lower rpms with light throttle than with WOT. The Governor pressure has to rise enough to overcome the static load of the spring AND whatever load is being generated by the Throttle Pressure circuit. More Throttle pressure generated, more gov pressure is required, thus more road speed must be attained before the Gov circuit can override the spring and TV circuit and push the valve to the right and open the port exposing Line Pressure to operate Second Gear.
OK, now that we have shifted into Second Gear, forget the 1-2 shift valve and assume it will stay in it's fully open position for the remainder of our discussion.
Line Pressure is now going through the 1-2 shift valve to the "middle port" of a similar valve: the 2-3 shift valve.
The 2-3 shift valve has Governor Pressure and Throttle Pressure and static spring forces acting against it, just like the 1-2 valve.
Thus, to generate a 2-3 shift, Governor Pressure must rise enough to override the spring and applied Throttle Pressure and push the valve to the right side, exposing the third gear port to Line Pressure. Third gear shift is a bit more complex in that the Intermediate Band must release and the Direct Clutch pack engage simultaneously! Mess up the timing on this shift and you will have burnt clutches and bands in short order!
So the Third gear shift is accomplished by releasing the Intermediate Band and engaging the clutches in the Direct Drum, which causes the Direct Drum to go from ZERO rpms, to Engine rpms, INSTANTLY! This causes the planetary set to go static and provides a direct power flow path from the input shaft straight through the Intermediate shaft into the OD unit.
OK, so now you're in Third gear, Line Pressure is now feeding into the circuits that allow both OD and TCC apply via PCM engagement of the OD and TCC solenoids, which then operate valves to send hyd pressure to the various elements required to shift into OD and lock up the converter clutch.
Involved in all this is a host of additional valves which control flow, pressure, timing, exhaust, lubrication, etc. Apply pressure is rising and falling with Throttle Pressure to produce soft shifts at light throttle, getting progressively firmer the more throttle is applied. Granted, this is all a bit of oversimplification, but not writing a novel here! LOL!
The net result of a properly built Auto shift VB is that your shift firmness can be tailored to be light at part throttle, getting firmer as you increase the throttle input. A properly built performance automatic transmission should provide lightning quick and firm shifts at all times without being overly harsh to upset the chassis or passengers!
The thread is too long and will be continued in next posting.
