Illustrated Guide to the Mitsubishi 3000GT AWD System
by Jeff Lucius
These pages are still in development. I will add more pictures as I am able to disassemble components or as others send them (pictures or components) to me. If you have comments about this web page or corrections for its contents, please send an email to jlucius@stealth316.com.
A transaxle is a transmission and differential combined into one unit. Transaxles are common in front-engine, front-wheel drive cars, but they are also found in front-engine, rear-wheel drive cars like the C5 Corvette. There were two manual transaxle models installed over the 9 years of production of the 3000GT VR4 and Stealth Twin Turbo models. A 5-speed transaxle (W5MG1) was installed in the 1991 to 1993 model years. From 1994 till the end of production (which was 1996 for the Stealth and 1999 for the VR4), a 6-speed transaxle (W6MG1) was installed. Getrag (of Germany) exclusively manufactured the AWD transaxles for Mitsubishi at their plant in Newton, North Carolina, USA.
In both models, a dry, single-disc, diaphragm-spring clutch transmits engine torque to the input shaft of the transaxle. The input shaft spins at the same RPM as the engine when the clutch is engaged. The intermediate shaft and two concentric output shafts are connected to the driveline and spin whenever a wheel is turning.
The drive gears are paired. There are two gears for each forward speed and reverse. In addition, reverse requires a third gear (called an idler) to reverse its rotation direction. For each gear pair, one is mounted on the input shaft and one is on the intermediate shaft. One gear of the pair "floats" around the shaft on a needle bearing. The other gear in the pair is solidly fixed to its shaft. All gears are fully meshed at all times.
The synchronizers (synchros) spin with the shaft they are spline-coupled to and are used to engage a drive gear (the one that floats on a needle bearing) to the shaft. Mitsubishi uses a double-cone type synchro for the 1st and 2nd speed gears and a single-cone type for the other synchros. The shift lever and cables control three shift rods in the 5-speed transaxle and four shift rods in the 6-speed transaxle. Each shift rod controls one fork that engages one of the synchros into either of its gears (or just the reverse gear in the W6MG1). If no synchro engages a gear, the transaxle is in neutral and engine torque is disconnected from the driveline, even if the clutch is engaged. The grinding sound heard during a shifting mistake is the failure of a synchro to properly engage a gear. Drive gears are always fully meshed with their corresponding gears on the shafts.
The center differential is placed on the concentric output shafts and torque is transmitted to the front differential and the transfer case by means of a planetary gear assembly. Differential limiting takes place when there is a difference in rotational speed between the outer shaft (which connects to the front differential and the sun gear) and the inner shaft (which connects to the transfer case and the planetary carrier) by means of a viscous coupling unit (VCU) mounted co-axial on the dual shafts within the center differential.
Inside the VCU, plates are alternately attached to the inner and outer output shafts and rotate in a silicone fluid. When there's a difference in rotational speed between the two shafts, the plates try to shear the fluid, causing the fluid to heat and expand and "lock" the plates together. This causes a torque transfer from the wheels that are slipping (connected to the faster-rotating shaft and plates) to those that are rotating slower (with the better traction). The dual output shafts must spin at close to the same rate. However, depending on the design of the VCU, some slippage can occur before full lockup. If both shafts are spinning at the same rate then engine torque is split 50/50 (front/rear) within the CD/VCU.
Because the front and rear differentials have different gear reduction ratios, the CD/VCU, the transfer case, and both differentials are not interchangable between the two transaxle models. To complicate things just a little, Mitsubishi changed the center output shaft design during the 5-speed production cycle from 18 splines (early units) to 25 splines (later units). There are even two versions of the sleeve used to carry the 18-spline output shaft (one is slightly larger). All 6-speed transaxle output shafts have 25 splines.
Frank Martin (aka KORMEX) has posted the following information to the 3SI message board. He says these are the problems that he encounters most often with our manual AWD Getrag transaxles, in order of frequency.
1. Output shaft splines (stock material is not hard enough); symptoms include a clunking noise when splines are worn and "grumbling (like rocks in a tumbler)" when splines are completely stripped.
2. Leaks.
3. Synchros (1st-2nd most commonly); symptoms include grinding when shifting.
4. Reverse gear broken.
5. Complete meltdown (no oil).
6. Broken case.
Frank also posted this advice for those of us shopping for a "rebuilt trans". Here are the questions he suggested asking.
"What's new on the rebuilt trans?"
"Do you replace all the synchros with new ones?"
"If so, then would you sell the new synchros to me?"
"If not, then why not?"
"Would you put down in writing everything that you claim is new on the trans?"
In most states a rebuilt transmission must have at a minimum all new bearings, synchros, and seals. Reconditioned transmissions (such as those sold by Kormex and others) have some new parts and some rebuilt or reconditioned parts. New parts usually include the center output shaft, center output shaft sleeve, 1st-2nd synchro, bearings, and seals. New synchros other than 1st-2nd are not yet available at the time of this writing.
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