Photos by Jason Carson
Unlike the earlier “slushbox” automatic transmissions that soaked up massive amounts of power and left you with sloppy shifts and slower ETs, the modern electronically-controlled transmission operates with much tighter parameters. Considering that 0-60 times for most high-performance vehicles are now spec’d with automatics instead of their manual-shift counterparts, the reality is that the automatic transmission is the better solution. That also means that you need an electronic control unit. For LS swaps, this is often taken care of when you use a factory GM ECM, because most of them have the trans controller built in. If you are using an aftermarket controller, carbureted LS, or one of the new LT-series engines, or if you simply want better control of your automatic transmission, you need a separate controller. Yes, you read that correctly, the LT-series engine ECM does not have transmission controls built in, it is a separate controller. A laptop is required for tuning the controller.
Painless Performance has the solution for all of these situations, it is called Perfect Torc. Designed to manage GM 4LXX transmissions from 1995-up (95-up 4L80, 96-up 4L60), this little unit packs a lot of features. Not only does it give you tuning control of the transmission, including shift timing and firmness, but also comes with pushbuttons for paddle-shifting as well. For the most basic installation, there are only four wires to connect: ignition (handles all power functions), two grounds, and the TPS sensor input. The rest of the wires are secondary controls, including: button-shift, speedometer output, and a 5-volt TPS power feed for vehicles that require a stand-alone TPS unit (carbureted engines). The other wires are all bundled together in two plugs that plug directly into the ports on the transmission.
The installation is quite simple. Under the vehicle, route the transmission harness to the passenger side of the transmission case. Plug the large round plug into the port on the shelf of the case. There is a second two-wire plug that connects to the rear speed sensor port. Make sure that the wires are routed away from the exhaust and driveshaft. The main harness is long enough to reach the firewall and into the vehicle for most cars and trucks. You could also run the harness through the floor, just be sure to use a grommet to protect the wires from chaffing. This harness has 3 separate molex connectors. The controller itself should be mounted inside the vehicle, the glovebox or center console are great locations because the controller has a diagnostic and tuning function for on-the-fly tests and tuning.
The controller harness has eight wires in one molex connector. These are the power, grounds, sensors and shift button wires. As previously stated, only four wires are required for operation. The power wire terminates to a 12-volt source when the ignition is on. This must have power only when the ignition switch is in the run/start position. The ignition switch is the best source for this wire.
The grounds must be terminated as close to the ECM ground as possible. Most LS engine harnesses have multiple ground points, so locate the closest set and run the ground wires to that point. On our vehicle, the grounds were made to the driver-side valve cover with the ECM grounds, this was directly in front of the large factory grommet in the center of the firewall.
The TPS signal wire can be tricky. If you do not have a pinout for your ECM, you will need to trace this wire. Our install is using an LT1 ECM, according to the GM ECM wiring diagram, port 70 on the C2 or J2 plug is the TPS signal. LS engines typically have two TPS signals (redundant systems), you need to connect to the actual signal side. You can check this with a voltmeter; the wire that changes voltage with moving the throttle open and closed is the correct signal wire.
If you do not have a TPS sensor, you will have to install one. This will be required for carbureted LS swaps. You will need to purchase a TPS unit and bracket. The sensor should be labeled with the proper terminals, but the Perfect Torc install manual outlines the procedure for determining the ports of unknown function, so that you can get the TPS wired correctly. The harness for the Perfect Torc includes the ground, signal, and 5-volt feed wires required for connection.
The speedometer output can be calibrated to operate any electronic speedometer. The calibrations are all managed in the laptop software that is included with the system. Mechanical speedometers are not supported by GM 4LXX transmissions. If you must retain the mechanical speedometer, then an electric to mechanical conversion unit is required. The vehicle speed calibration can be done with a GPS unit, with a pace vehicle, or through an adjustable speedometer.
If you want the ability to paddle-shift your vehicle, there are two wires on the harness for that purpose. The kit comes with several pushbuttons for dash or console mounting, or you can purchase a steering wheel adapter with paddles. 3 buttons are required- one wired to each shift wire (turns on/off manual mode), and then each shift wire is connected to a separate switch for shifting. The other side of the up/down buttons goes to ground. Each press of the buttons shifts the transmission up or down one gear.
The last wire on the controller harness is for table selection. The Perfect Torc unit can manage two shift tables, allowing the user to set up a performance mode and a race mode, or street and off-road shifting tables. This uses a toggle switch (included), when this wire is grounded, the table 2 is used, when open, table 1 is used. While not required, this is very useful for dual-purpose vehicles.
Once the wiring is completed, you simply plug in your laptop and run the software. The program has several base tunes that you can choose from and then tweak from there. Load your tables, tune, and then enjoy your freshly-tuned transmission.