Overdrive Automatic

A-500 and A-518 Four Speed Automatic Transmission Swap

The Chrysler overdrive (OD) transmission is four-speed unit based on the three-speed Torqueflite with a .69:1 overdrive (engine speed reduction) unit located in the tailshaft.
The OverDrive transmission was introduced in 1988 in Dodge trucks and vans.   All A-500 and A-518 series transmissions share the same post-62 A-LA bellhousing bolt pattern.
It is important to remember that the 904 series mounts the starter about ½” higher than does the 727 and therefore cannot be used on EarlyHemi engines due to interference with the engine block.
The A-904-based overdrive is called the A-500, and the A-727-based overdrive is called the A-518.  In 1992, Chrysler changed the transmission nomenclature in order to provide more information. The new nomenclature indicates the number of forward gears, service/duty level, front drive vs rear drive and the method of control. 
  • First Character = Number of forward speeds: “3” = 3 speed, “4” = 4-speed
  • Second Character = Type of duty: “0” to “9”, with 9 being the strongest
  • Third Character = Type of drive: “R” = Rear wheel drive
  • Fourth Character = Type of shift control: “H” = Hydraulic control, or “E” = Electric control
 “42RH”  Four speed, light duty, Rear wheel drive transmission with Hydraulic control.  (This would be an ideal candidate for a flathead six package. )
 “42RE”   Four speed, light duty, Rear wheel drive transmission with computer control. Unless you are an electrical whiz it may be best to avoid all ‘E’ transmissions.
There are three basic OD trans designs:
A-500, 40RH, 40RE, 42RH and 42RE, 44RE 
Yes, MotherMopar has actually labeled transmissions with a ‘0’ duty/service level… Not too sure what they had in mind with that…
These are based on the A-904 design.  Gear ratios are the same as 1980-up A-904- transmissions. They share many of the same internal parts with A-904 series transmissions except for the OD tailshafts. The transmission pan has an identical shape to the A-904 series transmission pan, but deeper.  The tailshaft mounted OD units do not interchange with tailshafts from A-904 series transmissions. The A-904-style slip yokes are smaller and do NOT interchange.
A-518, 46RH, 46RE 
These are based on the A-727 design and the gear ratios are the same as the 727 transmissions. They share many of the same internal parts with A-727 series transmissions except for the OD tailshafts. The transmission pan has an identical shape to the A-727 series transmission pan, but it is deeper.  The tailshaft mounted OD units do not interchange with tailshafts from A-727 series transmissions
A-618, 47RH, 47RE 
These are the heavy-duty transmissions used with the Cummins Diesel and V10 truck engines.  Although designed for much heavier duty service, the internal parts will interchange with A-518 parts.
The transmission case, however, has a Cummins specific pattern bellhousing as is not compatible with the A-LA engines.
Gear Ratios:
Transmission A-500 A-518 A-618
First 2.74 2.45 2.45
Second 1.54 1.45 1.45
Third 1.00 1.00 1.00
Fourth 0.69 0.69 0.69
Reverse 2.21 2.21 2.21
Overdrive Units:
A-500 and A-518 tailshafts both contain the same .69:1 overdrive unit, and they will interchange with each other.  Keep mind that there are variations in the number of clutches between the various applications and some swapping can be done for your specific needs.
As mentioned before, the OD tailshafts do not interchange with the earlier non-OD transmissions due to different bolt patterns and needed oil passages. The OD tailshaft is much bulkier than A-904 or A-727 tailshafts, and the location of the mounting pad is farther back than A-904 or A-727 transmissions. Small details perhaps, but must be accommodated in swaps.
Lock-up Converters
The OverDrive transmissions were available with both lockup and non-lockup torque converters.  The non-lockup version of the A-518 was available through 1991.  The A-500 transmissions were only offered with lockup.
  • The early 1991 and older non-lockup A-518s will have two pins at the back of the main case adjacent to the rear oil line port. The input shaft will be splined all the way to the end.
  • 1995 and older lockup A-518s and all A-500s will have three pins at the back of the main case adjacent to the rear oil line port. The last inch or so of the input shaft will also be smooth with no splines.
So, you still want to adapt an OD trans to your EarlyHemi or Flathead…
  • Best Bet –  1995 and earlier
  • The 2-pin A-518 unit only requires one fuse protected hot wire and one switch controlled ground wire to toggle the OD.
  • The three pin A-500 and A-518 units only require one fuse protected hot wire, one switch controlled ground wire to toggle the OD and one switch controlled ground wire for the converter lockup function.
  • These can either be manually operated switches or set up to operate automatically with a control kit from Performance Automotive and Trans Center. (see Useful Links)
  • Unless you are a computer/electrical wizard it might be best to avoid the 1996 and newer ‘E’ labeled transmissions.
Other useful details:
  • Part throttle Kickdown
  • The stock 904 kickdown can be used with the A-500.  Depending on the year of the transmission used, you might be limited to the older one piece kickdown linkage design or an aftermarket cable setup due to the lack of a mounting pivot required for the three-piece setup. It is also possible to fabricate a new pivot if needed.
  • Trans Fluid Lines
  • OD transmissions have larger 3/8″ cooling lines vs. 5/16″ lines for the A-904 and A-727 transmissions. This is helpful only if your cooler is also equipped with the larger line size.
  • Slip Yoke
  • The A-500 and A-518 use the larger A-727 style slip-yoke.
  • Torque Converters
  •  A-518s and A-500s do not use the same torque converters. The A-500s use A-904-style torque converters; The A-518s use the A-727-style torque converters. Make sure that a non-lockup torque converter is not mixed with a lockup transmission and vice-versa.
  • Additionally, remember that there is a balance issue when sourcing converters from cast crank engines vs forged crank engines.
  • Lockup and OD Control
  • There are a few ways to control lockup and OD:
    If you control the lockup function with a manual switch and then fail to turn the switch off and slow down to the point where the transmission downshifts on its own to second, the torque converter will shudder severely as the clutches try to stay engaged against the imposed load. This can eventually damage the transmission and/or torque converter. 
  • Overdrive, or 4th gear, is a separate function from the converter lockup.  Since OD is electrically controlled by a 12v source, if it is left on, the transmission will immediately shift to 4th once it shifts into 3rd, but, if OD is left on, it will not down shift via the kickdown.  It will only downshift when slowing down. 
  • Following the instructions on the PATC website you can run a 3/16″ line off of the governor port to a tee fitting and install a 48PSI and 50 PSI switch. Run the OD control to the 48PSI switch and the lockup ground to the 50PSI switch. (These switches are used in GM TH350 applications).  
  • A vacuum switch (Standard Motor Products P/N VX2) can be installed in the fused key-on 12-volt supply to the center terminal (on the 3-prong plug) of the transmission; this automatically kicks OD and lockup off when you ‘get into the throttle’. The ground sides (the front and rear terminals) can have the lockup wire going through a normally closed relay hooked up to the brake light switch that opens (unlocks converter clutch) when the brakes are applied.