Centre Differential
 

1992 JDM model. I am getting a problem when the steering is on full lock. It's almost as if I had put the fuse in to lock the centre differential. But there is no fuse, so I think the centre differential is locking up. There are no symptoms at all when driving normally. Any other potential suggestions?
If that is the case, then is this a separate physical unit from the gearbox? Can I replace it without having to change the gearbox as well, as the gearbox appears to be OK.
If yes, has anyone got a spare?

The centre diff is part of the gearbox.

Please let me know if you find out what the problem is. Ours has been intermittently doing the same thing since it's recent rebuild. The gearbox shop don't believe me, so I'm hoping it's going to either fix itself or explode before the warranty runs out.

Phil,
That's interesting. I rang the local Subaru dealer and they said that they could obtain a replacement centre differntial for me for £800 plus fitting. But, I had always thought that the centre diff was part of the gearbox as you have confirmed. Perhaps they thought I was talking about a WRX and not an SVX.
I was thinking of putting the diff lock fuse in and seeing if that makes any difference. Perverse logic suggests that if this makes no difference then I do have a problem with the centre diff, but if the behaviour is different, then the problem could be somewhere else. What was the approx cost of having your gearbox reconditioned, including removal and fitting?
The other thought I had was to add a transmission additive as some of the guys in the US have done to cure a similar problem. But given that their transmissions are very differnt from ours, I am not sure that would work. But, I can't see how it could make things worse!
Dave

Dave,

I was going to suggest putting the fuse in to see what happens. I guess my logic is just as perverse as yours.

Our gearbox cost about 1650 in total. That includes everything: recovering the car, reconditioning the gearbox, new torque converter, cooler flush, labour, VAT. The lot.

Are you planning on coming to the JAE? It would be nice to see another version L there, given that there's probably only 4 or 5 in the country.

Phil.

Phil,
Thanks for the information - I will let you know what happens when I put the fuse in.
I can't come to JAE as we will be on holiday in Scandanavia at the time in my other toy - a 4-seater light plane.
Did you consider a second hand gearbox?
Dave

Hi Dave,

Given the compatibility problems that Joe and Tony found with fitting second hand gearboxes to their cars, I felt it was best to rebuild the original.

Have fun in scandinavia.

Phil.

Phil,
Some guys in the US suggest that my problem could be a CV joint. I am planning to replace the second one when the replacement arrives from Johannes in Latvia. I think I will hold off on any gearbox work until that has been replaced.
I will also do some tests with the diff lock fuse in place over the weekend and let you know the outcome.
If I do need to do anything with the gearbox, then I will have the old one
rebuilt - sounds like the best solution.
Dave

Gidáy Dave, it would seem that the Limited slip clutch is binding. This will cause the symptoms that you have. If there is no code stored in the TCU, the Transfer valve is causing the trouble. If it is the C solenoid there will be a code stored.
Does not matter which it is, the fix is the same, pull the back off the box(don't have to remove the box) and replace the C solenoid/transfer valve as a unit, and a set of new clutch plates. Have only had one other to go bad here in Australia, cost about A$250 to fix.

All the best,
Harvey. ;)

As I understand things, you are referring to the US model which has the limited slip clutch with Solenoid 'C'. The car I have is a Japanese Model with a proper differential and no solenoid 'C'.
But, thanks for offering some help.
Dave

No mate, I'm referring to the VTD trans that you and I both have. We have the same solenoid C valve assemble, it just does a different job. It applies the Limited Slip Clutch, to prevent wheel spin of the front/rear wheels.

If the C solenoid is faulty, it can either not work at all, allowing the rear wheels to spin. Or the valve can stick and will cause the clutch to stay applied, causing the front/rear to bind, just like when the Diff Lock Fuse is inserted.

The fix is as I have stated. Not expensive, don't have to remove the box, just the rear housing, to replace the bits.

Harvey. ;)

Harvey,
My mistake, I apologise.
If it is the C Solenoid you say there will be a code in the TCU. How do I access that code. I understand how to get at the codes for the climate control unit, the engine management system, but not the TCU.
Dave

No problem Dave, if there is a code stored, the power light will flash when you start the car. As yours is binding, I don't think it is the solenoid that is faulty. A faulty solenoid will flash the power light, and allow wheel spin.
Yours has the valve stuck, to cause the clutch to stay applied, so there will not be a code stored.

To check the codes:

1. Drive the car to warm the trans.
2.Turn the ign sw off.
3.Move the selector lever to D. and turn the Manual sw on.
4.Turn the Ign sw on.
5.Move the lever to 3 turn the manual sw off.
6.Move the lever to 2, turn the manual sw on.
7.Move the lever to 1,turn the manual sw off.
8.Partially depress accelerator pedal.

Power light will flash code. Solenoid C is 24 .Two long blinks, four short blinks.

Harvey. ;)

Harvey,

I had a thought on this. It's far-fetched and probably nonsense, but I'll run it past you anyway.

It's a common practice for JDM car importers to fit a "pulse converter chip", which slows down the pulses from the speed sensor to make the speedo read in mph rather than km/h.

Normally these are fitted at the speedo end, so they don't affect any other systems that feed off the speed sensor. But I have seen one fitted at the gearbox end. I can also imagine that someone might mistakenly wire one to the TCU rather than the ECU to try and overcome the 112mph speed limit.

So here's the question: What would you expect the TCU to do if the signal it was receiving from speed sensor 2 was slower than speed sensor 1? Would it interpret this as wheel spin and try to lock up the diff?

Phil.

Gid'ay Phil, If it was fitted on the sensor input to the TCU, it would. The two sensors are treated differently. The No.2 sensor is driven off the front drive parking gear, and is used for shift points, and speedo. The No,1 is driven off the rear sun gear, and is used to detect the difference in wheel speed, it's also used as a back up for No.2.

The No.2 runs to the TCU, then a wire from the TCU goes to the speedo. If the chip was fitted to the out put of the TCU, it would not affect the diff. only the speedo. If it was fitted to the sensor output wire, before the TCU, it would affect the change points, and cause the clutch to apply.

As far as I know, the TCU takes any difference in front/rear rotation over 20% to be wheel spin. Less than this, is turning, and will only cause a different torque to be applied to the front/rear drive, due to the difference in the f/r rotation. The clutch is not used to change the torque.

Harvey. ;)

Hi Harvey, thanks for the explanation.

My theory is good, the speed difference would be 37.5%. So an incorrectly fitted speed converter could cause the diff to bind up.

Dave tells me that his car is still metric and doesn't have a converter fitted. So it's not the answer in this particular case. :o

Phil.