1991 Chevy Truck Transfer Case - Trans-Action

The Compushift controller is super easy to install and hook up. All the wiring is labeled and any questions can be answered by the knowledgeable staff just a phone call away. To run you simply need a clean, keyed 12-volt power supply, a throttle position sensor (TPS) signal, an output speed-sensor signal, and the main transmission plug attached.

This '91 GM truck has a Powertrain Control Module (PCM) that runs the fuel injection on our TBI 454, and controls the 4L80E transmission. Since we were swapping to the Compushift controller from B&M we no longer were going to be plugging the PCM into our transmission. However, we did need to get a signal from the TPS to help tell the transmission when to shift. If your 4x4 is carbureted you can use Compushift's cable-activated TPS. We, on the other hand, simply tapped into the TPS wires that plug into the PCM.

Our Red Sled project is a wiring nightmare. It's old enough that no dealership still has a full harness for it, yet new enough that no one is restoring them so the aftermarket hasn't started making the harnesses. We got this used but priceless (to us) factory wiring diagram book on eBay and have found it helpful when hooking up the Compushift controller to the factory truck PCM and harness. Another option is Helm Inc., which sells many different service manuals.

For a speed-sensor signal you have a few options. Some automatics have a transmission output speed sensor (TOSS) and others rely on an output speed sensor (OSS) in the transfer case. We had our transmission rebuilt and made sure it had a TOSS installed, and we also installed this Advance Adapters' speed sensor onto the input of the STAK case. 3 The AA piece has the pickup threaded into the transmission-to-transfer-case adapter, so all speed-sensor signals are recorded prior to transfer-case low-range gearing. Thus the tranny always shifts like it is in two-wheel drive. At first we assumed we would need the TOSS to run the Compushift and the AA unit for the factory speedometer, but we found that the Compushift has a reproduced TOSS signal that can be wired into the factory harness to control the speedo.

This truck came with an NV241 chaindrive transfer case (left) from the factory. It's plenty strong but only has a 2.72 low range and we wanted more. We haven't had a chance to test the new STAK 4x4 Monster Box (right) yet so we ordered one up. The STAK is a 150-pound brute of a case with dual low ranges and front or rear low-range capabilities, which should make for some interesting abusive testing of the front end when the time comes. Ours came with a driver-side front output and 1350 yokes front and rear.	Though roughly the same length and width as the NV241, the STAK is a bit larger in height. To deal with this we had to lay a beating onto the truck's floorboard with our biggest hammer. This allowed for about 1/2 inch of clearance.	In addition to the floor massage, we also trimmed the factory crossmember and lowered it about an inch, welded in a new plate, and bolted on a new transmission mount from Daystar Products. We were able to then bolt back on the factory transfer-case skidplate.
Even though we were told that the transfer case would be fine bolted to the back of the 4L80, we opted for the additional transfer-case mount from STAK to assure that the 150-pound three-speed doesn't break the tailhousing off the tranny. The mount comes with a slotted half-moon bracket and a bushing with tabs. We mounted the tabs as close to the Daystar tranny mount as possible to keep the drivetrain mounting points in somewhat of a triangle. We then welded a short tube to the bushing to hold the back of the case in place. This also reduced the shifter vibration in-cab as a bonus.	With the swapping of transfer cases, we also had to address driveshaft length. We decided to upgrade to all 1350 1-ton U-joints both front and rear and headed to JE Reel to have the upgrades done. JE built us an entirely new front shaft and lengthened and rebuilt our two-piece rear shaft with new joints, a fresh carrier bearing, and a longer front section to reach the STAK case. If you own a Jeep you should check out JE Reel's waterproof U-joints made for deep mud and water use, but sorry to say they're not yet available for the big-truck crowd yet.	The display/programmer is where all trans data is displayed and adjusted with easy pushbutton steps. This controller tells you what gear you're in, tranny temp, battery voltage, engine rpm (when tach wire is installed), speed, line pressure, throttle position, and when the torque converter is locked up. Additionally it can be used for all types of performance testing such as accelerometer, dynamometer, and stopwatch.

Here is the new view from the cockpit. There is a B&M temp gauge on the dash keeping track of the 4L80E, the new three sticks controlling the STAK coming up through the floor. The left shifter is for the front output. Push it forward and you have low range, middle is neutral, and back is high. The middle shifter chooses between 5:1 or 3:1 low range, and the right shifter engages the rear output. Unfortunately we got a little excited when cutting the hole for the shifters and had to then go back in and plate over the giant opening we cut. Finally we added a new triple-shifter boot from JB Custom Fab that greatly eliminates heat and noise intrusion to the cab. Also on the floor there is the Compushift display programmer within easy reach and in sight to keep track of the data arriving from the fresh tranny. The red knob on the dash below the gauge is the front axle Posi-Lok activator; see the complete install of that elsewhere in this issue.

We also returned to Race Trans to make sure we had all the settings perfect on our programmer. This controller allows quick and easy adjustment of shift speed, line pressure, gearing and tire size figures, and when the torque converter will lock up. This ability to adjust numbers is a bonus the factory PCM never offered and great whether you're using it in the factory application like us or swapping an electronically controlled overdrive auto into any of your 4x4 projects.

Here is one problem we encountered that we hadn't considered. Because we lowered the transfer case slightly from stock (maybe an inch or two) the front of the engine was rotated upwards just a hair. Though it ran fine, when in a hard turn or under throttle, the engine mounts would flex a little and send the fan clattering into the fan shroud. We pulled the shroud, trimmed an inch off, and haven't had a problem since.