Without question, the 4L60E automatic transmission is the weakest link in the LS-powered full-size trucks. While the Gen. III motors seem to make more horsepower every year, improvements in the 4L60E's ability to harness this power have barely kept up. Installed in a 3,500-pound F-body or GTO, the 4L60E or 65E makes for an adequate match to the Gen III's ever-increasing power levels. Now, consider that the lightest full-size trucks start at 2 tons and SUVs approach 3, and the 4L60E becomes the weakest link in the drivetrain. GM heavily torque-manages the powertrain to address this mismatch, resulting in an unsatisfying driving experience.
In the next step of our Radix-powered 4x4 Silverado build I had planned a systematic series of 4L60E tweaks, culminating in a complete transmission upgrade. Unfortunately, a botched Corvette servo upgrade by this writer resulted in a cracked transmission case, requiring I jump straight to a complete swap.
Since I would not be able to do the systematic upgrades I had hoped, I had to find somebody to build a complete transmission to replace the one I broke. After viewing Jim Neuenfeldt, alias Parish8's, regular cab short-bed 4x4 Silverado make 1,011 hp at the tires on www.1320video.com, I noticed Jim was running an FLT transmission. Jim has been running low-10-second passes with an FLT-prepped 4L80E. If FLT could build an 80E to take 1,000-plus hp, I figured they could help me. I called FLT and told them I had a Radix-powered 5.3 4x4 that I wanted to autocross, drag race, and use as a tow vehicle. FLT's Chuck Johnson told me its Level 5 4L65E combined with a Vigilante torque converter from Precision would be perfect for my needs. Since FLT is located conveniently close to O'Hare airport, I flew to the Windy City for a closer look at FLT's shop and an education on what it takes to make a 4L65E handle the abuse a supercharged 4x4 would give it. Since transmission overhaul and installation could easily fill a book, I am going to cover some of the common 4L60E failure modes and how FLT corrects them. In the next installment I will give a rundown on how well this combo performs under towing, drag racing, and autocross conditions, along with tuning and details.
FLT uses a state-of-the-art electric dyno to ensure every transmission receives a virtual road test in a controlled manner. The dyno also allows controlled research and development testing. In addition to the dyno, FLT uses modern environmentally friendly cleaning equipment to assure cleanliness. | 
Based on FLT's recommendation, I contacted Precision Industries for a torque converter upgrade that would match my combination. After discussing my plans to tow, drag race, and even autocross, Precision supplied its Vigilante 10.5-inch, 2,800-stall, multi-clutch, billet torque converter. Because torque converters are shipped dry, it is imperative to fill the converter with a 1/2-quart of trans fluid before startup. I will go into further detail about the performance of this piece in the next installment. | 
Here is the Corvette servo on the left as compared to the stock truck servo on the right. The Vette part has a smaller bore diameter, allowing for more surface on the apply area for fluid to push against, consequently giving the Vette part greater clamping for a firm, but not harsh, 1-2 shift. For truck owners, this is an economical, proven upgrade--just don't screw it up like I did. |
The forward sprag assembly, output shaft, input housing and input shaft assembly, lower clutch race, and reaction shaft are all cryo-treated. Cryo-treatment is a heat cycling process that takes the parts down to minus-310 degrees F, significantly reducing internal stresses and dramatically improving wear resistance. | 
FLT custom fits this reinforcing sleeve to the splined section of the input housing, seen lying on its side in the middle of the previous photo. | 
The Electronic Pressure Control (EPC) solenoid dictates shift characteristics by changing line pressure. The EPC receives orders from the VCM, which is continuously comparing sensor inputs against its stored algorithms and tables. In theory, this marriage of computer and transmission should provide the perfect line pressure for any situation; FLT has found in practice this is not always the case. The sophistication of the system is its Achilles' heel. An improperly oiled air filter is capable of bringing this system to its knees by oiling down the MAF wires and providing the wrong signal to the VCM, which can result in complete transmission failure. While FLT's approach may seem a step backwards, it removes several potential failure points and replaces them with the proven and dead-simple operation and reliability of a vacuum modulator. This is not to say the EPC solenoid should never be used; it will depend on your application. |
FLT relocates and leaves the factory EPC solenoid connected so the computer doesn't throw trouble codes. The vacuum modulator takes up the space vacated by the OEM solenoid. | 
FLT custom machines pass through fittings for the vacuum modulator. The fitting is securely tapped into the case, allowing for a secure leak-free vacuum port. | 
One of the most common 4L60E trouble codes is the dreaded 1870: Transmission Slippage. This occurs when the OEM Torque Converter Clutch (TCC) regulator valve's bore wears out in the valve body. As the TCC regulates torque converter clutch pressure, a worn bore causes a leak in the apply pressure circuit and allows the clutch to start slipping, ultimately resulting in converter clutch failure. FLT replaces the OEM TCC regulator (top) with the TransGo part on the bottom, designed to prevent and compensate for bore wear. |
Cracks in the plastic OEM forward (right) and 1-2 (left) accumulator pistons are common. The pistons on the top are the stock parts and on the bottom are upgraded aluminum GM parts. | 
A cracked 1-2 accumulator piston creates a leak in the 1-2 gear circuit, which causes band slippage, and eventually, complete transmission failure. | 
These are check balls that interface with feed holes in the valve body spacer plate. Deformation of these holes is typical, with balls actually pushing through or getting stuck in the worn hole, causing harsh shifting. FLT replaces all of the steel check balls (left) with composite plastic balls to eliminate this problem. |