Before starting this tutorial on changing and installing a new differential for your MX5, have a look at this link if you are interested / unsure what a differential is or how it works, this provides a good explanation.
Diagnosing the problem
Diagnosing a problem is actually quite easy once you understand what’s going on. Anyone who drives will have at some point heard a funny noise or felt something wasn’t quite right. This is the first stage to fixing a problem- identify that one exists. I will now work through the logic of diagnosis.
In my case the interior of the car was filled with an awful whining sound. I know it can’t be a wheel bearing, experience tells me they make a lower “wub wub wub” noise. Secondly this is usually only heard when the car is rolling, (regardless of acceleration, coasting etc). Also, expect for wheel bearing noise to increase or decrease depending on cornering loads. In my case the sound went away when i rolled along in neutral.
We now know that by removing the power the noise goes, hence, it must be ‘powertrain’ . Powertrain components essentially consist of engine and gearbox. It can’t be the engine, the noise was not changing with engine speed. Stop and think, if revving the engine does not immediately speed up the whine, it can’t be connected to the engine- and my sound wasn’t. In fact, if the diff is failing, the frequency and pitch of the noise must be directly linked to road speed. Road speed is associated with gearbox and in particular the output of the gearbox, consisting of the layshaft, final drive and differential.
Identifying the component making a loud whining noise is not easy, the noise fills the cabin. In the end I made an informed guess! With some input from my Dad and a work colleague, it was decided that the most likely culprit on my car- a Jap import , was the early spec viscous limited slip diff (LSD).
So I left work early on Friday and nursed it 250 miles in the slow lane, hanging out with the lorries. Next morning, I headed to MX-5 Heaven, to collect my reconditioned diff. It just had to be another LSD, I love the way the car handled with the viscous diff, a generous helping of opposite lock mid corner on the power was commonplace!
The upgrade employs a torque sensing design, ‘torsen’ diff. This was originally fitted to a later model 1800cc NA MX-5. It is actually a nice upgrade for those considering fitting a turbo- A far more robust and consequently larger form factor.
To upgrade an NA MX-5 (Miata) diff you must have the following:
- Replacement diff (obviously)
- Replacement drive shafts- shortened torsen spec (due to larger size of the torsen replacement)
- Replacement prop shaft- shortened torsen specific
- 0.65 litres API GL-5 Differential/gearbox oil
- Goggles (I had to have rust removed from my eye, by the eye hospital during the job!)
- 23mm socket (Diff filler plug)
- 24mm socket (Diff drain plug)
- 14mm- removal of rear wheels & drive shaft to diff flange
- Hammer and chisel or equivalent to remove staking from hub nut
- 29mm socket (remove hub nut)
- Breaker bar and almost certainly some kind of extension
- Jack & axle stands
- Wire brush for cleaning duties
- Assortment of ‘standard’ spanners and sockets for other bits and piece
- Replacement exhaust gasket (as you will be splitting the exhaust)
- Long flat screw driver or equivalent (to remove rubber exhaust mounts)
The night before starting, it might be helpful to spray the areas you need to loosen with some penetrating oil. Once equipment is available you are ready to begin.
Start by simply slackening the hub nut (using your 29 mm socket), here is a picture of me in the process. I applied so much torque, the wheel was rotating on the driveway despite the car being in gear with handbrake on! Have someone stand on the foot brake to prevent this.
Once those nuts have been cracked, get the wheels off and car in the end. Then remove the driveshafts from the diff using your 14mm socket.
Gain access to the diff for it’s removal. This requires splitting the exhaust, up near the catalyst and dropping it off it’s rubber mounts (I used a long flat blade screw driver to lever it off). Also, don’t forget to unplug the small plastic sensor found in this area. My Jap import has a metal bar connecting the lower wishbones together for added rigidity. Here I am working it loose with Dan’s help, the bolts were all corroded.
Disconnect the driveshaft from the diff flange, in preparation for their removal as shown (below).
Now to get the drive shafts out disconnect the top wishbone from the upright (below), then drift the splined shafts out allowing them to drop onto the waiting ground below.
The final problem is the Mazda power-plant frame (PPF)- this is designed to give good throttle response, by ensuring that the gearbox and the diff are rigidly connected. I found that by loosening it, it is possible to slide it off the diff and with some string tie it out of the way. Don’t be afraid to get rough with this sturdy old bit of girder!
Now you have access to the diff, it is simply a case of disconnecting the propshaft from the diff and removing the old unit. Take note here, as far as I can remember the propshaft just slides into the gearbox. Expect gearbox oil to seep out, when you pull it. To avoid any further mess I would recommend supporting the car at an angle, (one hole higher on the axle stands at the rear) this helps keep the gearbox oil in the gearbox when you do remove the prop shaft.
The time had come and because Dan wanted to be a hero he bench pressed it out. We loosened the single 17mm and two 12mm bolts attaching the diff ‘arms’ to the vehicle. Then, getting underneath it and undoing the final bit lowering it out by hand. It’s fairly awkward but gravity is working with you.
To replace is the reverse of above, this time I would suggest utilising a jack so that you can line up the bolts without having to hold the heavier diff in the air. Prepare for some swearing as you try and mate the PPF with the diff, whilst simultaneously offering up to the car for attacment. Once up get it tightened up quickly, you don’t want to be looking for your 12mm bolts when its in position!
Now would be a good opportunity to replace the diff oil, for peace of mind. I used 0.65 litres of API GL-5. Unscrew both plugs and allow to drain. Replace the lower plug and with the car level fill to the top of the filler (do not over fill). Now replace the top plug, torque to approx 45Nm.
Fix the shorter driveshafts back on. Smear the upright to top wishbone swivel nut in copper grease and reattach. Reattach exhaust and all other things which are lying around on the floor! Put the wheels back on and put the car back on planet earth.
I kept my old driveshaft nut, it is advised to replace however. Now torque up the driveshafts to 260Nm and re-stake them.
The scary thing now, was driving the car in hope that the noise did not persit- thankfully it didn’t, all was well in the MX-5 and the new diff gave the car a slightly different handling characteristic. I have found that turning through a chicane has the effect of locking the outside wheel, this is great, except when you wish to finish the chicace, turning in the other direction. It is impossible without lifting off. Without lifting, the diff remains locked up and is effectively preventing the wheels from taking alternative radii, if this happens to you, you will feel the car’s front wanting to push on forward and the steering wheel becomes incredibly difficult to turn.
Hopefully this tutorial has been helpful.