I have an intermittent problem with the 6 speed gearbox on my 2006 SC which I am hoping someone might be able to provide some input on.

The problem generally only happens when the vehicle is cold and after having not used the car for a couple of days. When setting off from home I can get to the other side of town (about 3-4 mins drive), then the road continues at 30mph and then increases to 40mph. When driving through the 30 and 40mph zone at a constant speed, if the problem occurs you can feel the revs hunting slightly (only between 100 revs or so) as if the gearbox is getting conflicting messages and opening closing valves, this can be accompanied by a light shunt or two, followed by the 'transmission fault, limited gears' message pinging up on the dash. After that the gearbox will be locked in 4th gear until the car is turned off and restarted.

As I say this intermittently happens and I think it is more likely to happen when I haven't used the car for a few days.

When warm the gearbox seems to shift smoothly and operate as it should.

The car is high mileage but it looks like the gearbox has been replaced before, potentially quite recently, as the casing looks very clean. Unfortunately I have not a lot of history with the car so cannot be sure if this is the case.

I guess my first port of call will be to change the fluid and filter, but I dont really want to invest the money doing that if I am going to end up opening the box shortly afterwards, hence posting up here incase anyone has any pearls of wisdom!

Sounds like the usual Torque Converter issue! Best to seek help from a gearbox specialist sir

In my opinion it would be unusual for the transmission to go into limp-home mode just because the torque converter lock-up clutch was worn. Solenoid issues are very common on early 6HP26s and if the previous owner had a similar issue and the transmission was replaced it’s not uncommon for the mechatronic unit (which houses the solenoids and the transmission ECU) to be transferred from the old ‘box to the new one if the garage doing the work does not have the necessary diagnostic kit, as the transmission ECU is paired with the vehicle. Hence the solenoid problem is transferred across to the 'new' transmission.

It would probably be sensible to get the fault code(s) read and report back. I agree that there’s no point in spending money on new fluid and a filter until the source of the problem is known (assuming of course that the fluid and filter aren’t the source of the problem ).

Phil

Thanks for your replies. Scanning with my iid tool there are no fault codes coming up, despite the gearbox fault occurring as recently as yesterday. Are there any tests I can do?

The same thing was happening on mine every time I drove it after around 2 miles of driving. It turned out to be a sticking rear brake caliper causing the car to go into safe mode for some reason, had this sorted and hasn't thrown the error up since. Mine is a TD6 however.

So I spent some time looking over the car today. There are no brakes dragging or anything like that. One of the rear tyres was a few psi underinflated, no sure that would cause the problem though and even when the tyre was inflated the gearbox still felt like it was going to enter limp mode when driving from cold - slight rev fluctuating when holding constant speed.

The box isn't low on fluid, it ran out when I undid the drain plug and the oil looked quite a nice dull yellow / honey colour.

No faults recorded when scanned with iid.

So.... what next. I wonder if it is solenoid issues? Is there a way to test this or fault find with the iid live values? Or would I be best dropping the mechatronics unit and send it somewhere for testing / repair?

Any help much appreciated.

Quote:

The box isn't low on fluid, it ran out when I undid the drain plug

Good shout, engine was off. I'll have to check the fluid again I think!

You need the gearbox oil heated to the correct temperature (do a search on the forum for temp) then engine running with gearbox in park or neutral remove the plug. Oil should be right on the edge of starting to drip out to be at the correct level

The oil should be checked between 30-35 degrees on the 6HP box