When we started our 2002 Chevrolet Silverado 2500 Duramax project a couple years ago, we set out with a few goals in mind. We wanted the truck to be reliable, first and foremost. We also wanted to put between 650 and 700 hp to the rear wheels, and we wanted the truck to ride on 37-inch tires. A full engine build got us to the power goal—in theory anyway—and a BDS coilover conversion lift kit got us rolling on 37-inch Nitto Ridge Grapplers. However, the combination was a lethal one for the truck's factory Allison transmission.
Knowing this was going to be an issue, we loaded the transmission up with a Banks billet torque converter as the engine went back in. Once the truck was running, we babied it around town until a plan came together to get a transmission in it that would hold up to the abuse. To keep us moving forward, no matter the situation, we turned to the Duramax specialists at Pacific Performance Engineering, aka PPE Diesel, in Montclair, California.
| The task of pulling the Allison transmission starts by draining the fluid. Because our transmission already had a PPE deep pan, we needed to drain 18 quarts from the system, plus or minus what was in the torque converter.
PPE Diesel has been in the business of hoping up Duramax trucks for decades, and their in-house knowledge of Allison transmissions is among the best in the business. With our power goals being sub-800 (we intended the project to be a hot street truck, not a racer) but still desiring the utmost in reliability, we skipped on the billet shafts but did go with PPE's top-of-the-line Stage 6 build.
In the next installment we'll dig deep into what exactly the Stage 6 is in comparison to the lesser transmission. For now, let's take a look at what was inside our transmission. When we bought the truck, it was claimed that the transmission had about 80,000 miles on a rebuild. Judging by the state of the rest of the truck, we doubted this. Interestingly, the transmission wasn't in as bad of shape as we had expected to see. Aside from a bit of clutch wear, it was actually quite clean inside.
The talented crew at PPE Diesel had the transmission out of the truck and on the bench in about an hour. And the tear down only took about an hour more. Check out the process below and be sure to check back for the next installment when we rebuild and reinstall our Allison.
| The driveshafts also need to be removed so the transmission can come out. To remove the rear driveshaft, start by loosening the nuts on the rear U-joint straps.
| Depending on cab and bed length, some trucks will have a shorter driveshaft (as seen here) and some will be longer with an intermediate shaft and carrier bearing.
| The front driveshaft on four-wheel-drive models attaches in a similar fashion as the rear. The front U-joint is held to the differential yoke with two straps, which need to be removed. Once the shaft is lowered, it can be pulled out of the transfer case. Depending on age, the splines have a tendency to get stuck in the transfer case. If it's bad enough, the transfer case can be removed with the shaft still attached.
| Accessing the starter and torque converter bolts can be quite tricky, due to their proximity to the engine mount and exhaust down pipe. The best option is a long extension and reaching the bolts from the front of the engine.
| With the transmission supported on a sturdy transmission jack, the support crossmember can be removed. You can start with the support bolts at the frame, as seen here, or at the rubber mount. Either way is acceptable.
| Speaking of the rubber transmission mount, the bolts to remove it from the crossmember are found in the center of the crossmember. This is a great time to replace the mount if yours is in need.
| Before removing the transmission from the truck, you'll also need to remove the two transmission cooler lines from their fittings along with the transmission's wire harness. The exhaust downpipe will also need to be separated from the tailpipe assembly.
| The transmission can be removed from the truck with the transfer case still installed. However, it's a lot easier with it removed.
| Because of its weight and size, it's best to have two sets of hands when it comes time to remove the transfer case. Also, be aware that tipping the case will cause its fluid to spill.
| With all of the bolts, hoses, wires, and associated parts out of the way, the transmission can be lowered out of the chassis. While this can be done on the ground, it is exponentially easier for a professional shop with a lift.
| Once the transmission is moved to a bench the tear down can begin. We started by first removing the torque converter. We had previously upgraded to a Banks billet converter, but if you have not yet upgraded be sure to save your stock converter to use as a core.
| An exceptionally clean working surface is needed for rebuilding an Allison transmission. There are lots of parts that need to be kept track of and you don't want any foreign objects entering the new transmission. In addition, transmission work is messy business, so having a place that's easy to clean is imperative.
| Speaking of small parts, one of the most often overlooked items during an Allison transmission service is the magnet that resides on top of the spin-on filter. Far too often people throw away the old filter and send this magnet to its demise with the old filter.
| The valve body is held in place with almost a dozen small bolts of varying lengths. It's important to keep the bolts organized and remember which one goes where, as using the wrong bolt when reinstalling the valve body can cause damage.
| After removing the transmission's tail housing, disassembly of the internals can begin. Pay attention to this shim washer, and the number of holes it has. The number of holes corresponds to the thickness of the shim.
| Inside the tail housing resides the C5 clutch piston. While building the transmission involves replacing the clutch assemblies, everything is removed to be cleaned and inspected.
| Seen here is the P3 planetary gear set. The large ring attached to this gear is what the parking pawl engages with to hold the transmission when shifted into the park position.
| In this photo, we see the Allison transmission's main shaft, also known as the intermediate shaft, along with the P2 sun gear. The factory main shaft is prone to breakage at horsepower levels above 800 when measured at the rear wheels. However, it's usually the input or output shafts that give first.
| Seen here is the P2 planetary gear set with the P3 ring gear. The set of three planetary gear sets are what create the five or six output gear ratios, and this is done by activating different combinations of the gears through the use of the clutch packs.
| There are five different sets of clutches found inside the Allison 1000 transmission. These are the C5 clutches and steels. The C5 clutch pack is engaged when the transmission is in first gear and reverse. Because of their physical size and infrequent use, the C5 clutch pack is rarely damaged and often reused in most transmission builds.
| There are two different types of large snap rings found inside the Allison transmission. Depending on year, you might find a spiral style like is seen here, or a larger solid unit. Some prefer the solid units, but both get the job done.
| Last up is the P1 planetary and ring gear. It's extraordinarily rare to break a planetary or ring gear, but it can happen with enough power or abuse.
| Seen here is the C4 clutch pack. The C4 clutch is active in second and sixth gear.
| Flipping the case over, there is a spacer plate that resides between the transmission case and the bell housing. The real pros can get the two gaskets off without tearing them, but we promised not to give away the secret. They still need to be replaced, no matter how nicely they are removed.
| The C1 and C2 clutch packs are housed inside the C1/C2 drum. The transmission's input shaft runs into this assembly, and it's connected to the rest of the gears by the main shaft. It's easiest to work on the drum by inverting it and inserting the input shaft into the torque converter. For those who might be wondering, the teeth on the edge of the drum are for the transmission's power take off (PTO).
| The C3 clutches are very similar to the C4 clutches in size and quantity. The C3 clutch pack is active when the transmission is in third gear, fifth gear, and reverse. Looking at the friction plates you can see a small amount of dark material that shows there had been a small amount of slip.
| There are a bunch of different size snap rings inside the Allison transmission. You're certain to be a pro at removing them by the end of your first build. Just be careful not to shoot them across the shop.
| The C2 clutch pack is active when the transmission is in fourth, fifth, and sixth gear. While some of the friction material is dark, it's not abnormally worn for the theoretical number of miles on the transmission.
| The smallest of the clutch packs is the C1 pack. Housed inside the C1/C2 drum, this set of gears is active most often. The C1 clutches engage when the transmission is in first, second, third, and fourth gears.