This is how to convert your three speed transmission to a 1995 back A518 overdrive transmission with a three wire connector (gas). A 518 transmission has a 31% overdrive and will produce a 30 to 40% increase in gas mileage on the road over a three speed transmission. Remember overdrive is only 3rd gear high range anyway. There are two wire 518 transmissions out there, these have a non-lock-up torque converter. With this three wire setup you have a four speed transmission plus a torque converter lock-up feature. The extra gear is 3rd high range. We've been asked this question for years, so I decided to put it down in black and white. What we're actually going to do here is control the overdrive and torque converter lock-up normally controlled by the computer using one vacuum and two oil pressure switch. Instead of using an electronic speed sensor and throttle position sensor we will use a mechanical / hydraulic / electronic speed sensor and a vacuum / electronic throttle position sensor. This will be accomplished with the use of three GM parts and some trick wiring. The Chevy and Ford guys make the swap to overdrive all the time, it's time for the Dodge people to have the same option. Lets say you drive an average of 400 highway miles per week, at that rate you could save as much as $1000.00 per year on gas. This can be made to work with a diesel motor also with or without a vacuum pump and vacuum regulator#1. The center wire on the transmission is the hot wire and the two other wires are ground wires for overdrive and the lock-up torque converter. The front wire is the overdrive ground and the rear wire is the torque converter lock-up ground. You can get a three wire female connector from a junk yard or buy one from us.
#2. Run a 12 volt fused wire that turns on and off with the key to the vacuum switch, then to the center wire on the transmission connector. This is a normally open vacuum switch that takes 7 pounds or more of vacuum to shift into overdrive and lock-up the torque converter and 3 pounds or less of vacuum to down shift and unlock the torque converter. Lets say you're in overdrive going 54 miles per hour at very light throttle, when you press on the throttle and the vacuum drops below 3 pounds the transmission will immediately shift back to 3rd gear low range without lock-up or even a lower gear depending on the throttle position.
#3. At the governor pressure tap on the transmission install a tee fitting for two oil pressure switches. Say a 44 PSI oil pressure switch for overdrive and a 52 PSI oil pressure switch for the torque converter lock-up. Or any other combination of switches you may want. At light throttle the transmission will shift into overdrive at 44 MPH and the torque converter will lock-up at 52 MPH. The pressure switches go on the outside of the transmission case in the governor pressure port. It's located on the right side, at the upper rear of the main transmission case. The figures on the pressure switch will only be correct if you have a stock tire size and rear end ratio.
#4. Run a wire from the front pin on the transmission connector to the 44 PSI oil pressure switch and a second wire from the rear pin on the transmission connector to the 52 PSI oil pressure switch. Now at 52 MPH when you step on the gas the transmission will shift back to third gear and the torque converter will unlock at the same time. It can't get any easier than this.
#5. Install a toggle switch in the overdrive ground wire. When the switch is turned off you will have 1st, 2nd and 3rd low range, exactly like a 727 transmission with the same ratios. When the switch is turned on you will have 1st, 2nd and 3rd gear low range plus 3rd high range. When the toggle switch is on the transmission will immediately shift into overdrive at light throttle over 44 miles per hour whenever itís in drive range on the gear selector. Once in overdrive the transmission will not come out of overdrive until the overdrive solenoid is turned off, or the vehicle speed goes below 44 miles per hour, or the vacuum drops below 3 pounds. The only electronic parts on a 518 transmission are the overdrive and torque converter lock-up solenoids.
NOTE: People make this swap all the time, but I've never heard of anyone doing it correctly. They drive down the street flipping a toggle switch on and off at every stop. This setup works automatically.
NOTE: The way governor pressure works is 1 pound of pressure per square inch equals 1 mile per hour unless someone has changed the tire size, or rear end ratio, or governor springs and weights. The vacuum switch acts as a throttle position sensor and the oil pressure switch acts as a speed sensor.
NOTE: Use a lock-up type torque converter if possible, this could get you as much as 5% better gas mileage and the transmission will run much cooler. Running cooler will make the transmission last longer.
NOTE: You can also run the torque converter ground wire through the brake switch if needed.
Throttle Valve Adjustment
#6. A 518 transmission has a throttle valve cable (pressure cable), it must be hooked up and adjusted correctly. You can get the cable and brackets from a junk yard. If you have a 727 transmission now this linkage may work. The correct adjustment on the T V cable or linkage is minimum throttle pressure at minimum throttle. In other words no slack in the cable at idle and it must start to pull immediately off of idle.
#7. When going from a 727 transmission to a 518 transmission the driveshaft will need to be cut about 3 and 1/2 inches. The swap may require some driveshaft tunnel modification on some cars. You will have to fabricate a new crossmember using the old crossmember ends because the transmission mount is lower on a 518 transmission than a 727 transmission. The stock Dodge truck mount is very tall, some extra room can be gained by using a biscuit-type Camaro or Mustang mount. See pictures below. In the 1st picture notice the 518 transmission has a larger tail area. The second picture is the crossmember ends cut off ready to be reused. In the 3rd picture notice the transmission mount area has been lowered.