Welding a Center Diff - How and Why

The 1G and 2G Center Diffs are the same as far as operation and welding goes so this mod works for both.

Welding the Center Differential can give you a couple of advantages on a DSM. It effectively locks the power transfer at 50/50 to the front and rear. It also eliminates some inherent design flaws that could lead to failures otherwise.

All operation of the Viscous Coupling is eliminated along with any chance of it wearing out or slipping. So you can cut down the VC or install a VC Eliminator and remove a big chunk of rotating mass. YOU CANNOT REMOVE THE VC COMPLETELY! You must have something on the shaft to retain the output shaft or it can slide out.

The original design of the Center Diff uses soft thrust washers behind the side gears which fail over time and allow too much lateral play of the output shaft which leads to premature spline failure. Have you ever seen worn down splines on the output shaft? That's not from rust, or too much power as some have suggested. That's the thrust washers failing, and rebuilding the Center Diff is the only solution. And that won't guarantee that it won't happen again. Welding the Diff is a permanent and cheap solution.

Here's why cars with a welded Center Diff has "tire skipping":

The welded Center Diff effectively locks the front and rear shafts together but you still have a rear LSD that allows one side to turn free of the other, to a point. The rear LSD requires a certain amount of torque difference to cause it to differentiate. If it takes LESS torque to break the tire loose then the tire will "skip." My '91 GSX with Welded Diff and street tires chirps on slow sharp turns. Slap on my sticky track tires and no tire slip at all. Why? Because it takes more torque to skip the sticky track tire than it does to differentiate the LSD. That's also why it only does this at slow speeds and not while highway driving. It is not hurting anything or over-stressing the axles.

So now that we know WHY we want to weld the Center Diff, HOW do we do it? Refer to the diagram below copied from the factory shop manual.

Welding the differential effectively locks it together by attaching the side gears (6) & (10) to the differential case halves (4) & (12) and eliminates any output shaft end play.

The actual welding must be done by a professional as the gears are hardened and will crack if not welded properly. They must be brought up to a specific temperature before welding then cooled slowly to prevent breaking. Do not attempt to do this at home if you don't have the proper equipment or are not familiar with the process.

Be sure to bring the Viscous Coupling along because you will need it to align the splines of the side gear (6) & case half (4) so the VC can be reinstalled.

1) Remove and discard the parts marked with an "X" as they are not used. Only parts (4) (6) (10) and (12) will be used.

2) Weld side gear (6) to case half (4) using the Viscous Coupling to align the splines.

3) Weld side gear (10) to case half (12)

4) Clean out all weld spatter and reassemble the case halves and transmission. You're good to go!

 

[jtmcinder]

The reason that you get "skipping" after welding the center diff is the rear wheels (on average) track inside the front wheels. Therefore, the rear wheels need to turn slower (again, on average) than the fronts, but the welded center won't let them.

One way to show that the rear diff is not playing an important role is to see what happens in a car without a rear LSD when you weld the center: you still get skipping. Another way to show that the rear diff is unimportant is to make an extremey slow (as in under 5 mph) tight turn. This is too slow to allow the rear VC to get involved, but you still get skipping.

Edit: so why do the rears skip and not the fronts? Because the rear of the car weighs less, so the rear tires have less grip.

One last quibble: welding the center diff does not mean the "power" distribution will be 50/50. It only guarantees that the front and rear wheels will turn the same speed (on average). If the fronts are off the ground -- yes, I know our cars never pull wheelies, so take this as a thought experiment -- you do not lose half your torque; under these conditions all of the torque is used by the rears. That is why I use the term 'bias' for the effects of limited-slip devices.

- Jtoby