So here I am again with another automatic transmission error. I have a 2004 L322 4.4 petrol which has started throwing up an intermitent Trans Failsafe Error which also leaves a P0720 (Output Speed Sensor Circuit) fault code. Here is what has been happening. The problem does not always happen and I can go a couple of days with no errors while doing the same trip to and from work. When I do get the error there is a 'Bong' along with the trans failsafe error message on the dash but nothing happens to the gearbox and I am able to drive normally, around 10 secs later another 'Bong' and trans failsafe error message but still no change in the car then a third 'Bong' and trans failsafe error message another 10 secs later but this time the car goes into limp home mode and stays in 4th gear. I can clear the fault code successfully while still driving but the car remains in limp mode. If i turn the engine of for a few mins and start up again the fault has gone and the car drives noramlly again with all gears available. This morning I had a slightly different experience, I still got the 'Bong' Trans failsafe errors every 10 secs or so but this time I had a forth 'Bong' and trans failsafe error message but the car carried on driving with no problem all gears available. There were no more bongs and the car continued to drive normally. Also during the initial bongs and messages there are no fault codes it is only when the car goes into limp mode that the error code appears. I know the obvious thing is to change the output speed sensor but I thought I would see if anyone has had the same issue before I do anything.
Before doing anything drastic you should check the condition of your battery. I experienced a similar condition until I replaced the battery installed by the PO that was at end of warranty at the time I bought the car a couple of months ago and undersized anyway. The MTP-49/H8 will not cut it. You need the MTX-49/H8. The former has 730 CCA while the latter has 900 CCA. This matters as on first start up the starter can pull the volts down, especially when we have the air compressor running, the Harmon Kardon cranking out Queen, A/C running, etc.
It would occur for me on first startup of the day or after sitting and cooling off so the automatic environmental systems would also be sucking volts running the fans. I used a plug in voltage meter in the cigarette lighter socket so I could compare the onboard diagnostic voltage display and could see differences at times. The diagnostics would show 11.5V while the socket displayed 12.0 - 13.8V. At that same time a direct reading with a calibrated Fluke DVM could show 13.8V across the battery terminals.
I read somewhere that the ECU provides the voltage threshold for the networked electronics but the diodes in the alternator voltage regulator can fail and give you erratic voltage to the ECU. Both high and low outputs are possible if any diodes fail. It's also possible that the TCU is being fed wrong voltage or just 'thinking' it's getting low voltage. Check all the battery connections, ground connections and plug connection in the E box.
After installing the new more powerful battery the condition has mostly gone away. But once everything os warmed up, say after 10-15 minutes driving it will remain okay all day. I'm about to flush the "sealed' transmission to see if clean fluid and a new filter might help too. The RR SC is around 95000 miles old and I believe it's recommended, unofficially, to service the transmission around 75000, so I'm late.
