The approach to diagnosing a transmission problem isn’t always clear. It can be due to a lack of information or that the problem is intermittent, making it hard to duplicate.
Well, this 2004 Range Rover with a 5HP24 transmission had both conditions. It came to us from a local shop; they’d rebuilt the transmission and now they couldn’t get rid of code P0720, which was putting the transmission into failsafe. They’d replaced the output speed sensor twice.
Code P0720 is supposed to indicate an output shaft speed sensor circuit problem. Since they’d already tried replacing the sensor, it more than likely indicated a circuit problem, rather than the sensor itself. To make diagnosis more difficult, you’d have to drive the vehicle a while before the code would set; sometimes as long as 30 minutes.
We performed a scan of all the vehicle’s modules. We found these codes in the transfer case module:
0x5223 — noise on actuator signal
0x5221 — sensor short circuit to battery
We couldn’t find any information on these codes; even the factory web site offered no information on them.
During our various road tests we observed that sometimes when we’d select low range on the transfer case, the mountain light that indicates low range would flash (figure 1). Then we couldn’t shift it back into high range until we cycled the ignition off and on.
One time the information board chimed and informed us that we were exceeding the 30 MPH limit. That indicated the transfer case module thought the transfer case was in low range, while we were actually in high range.
The first thing we did was to drive the vehicle with a scan tool connected to record a movie when the transmission went in failsafe mode. We noticed that, when code P0720 set (figure 2; around frame 920), there were no problems with the output shaft PID.
Next we decided to check the output shaft signal for electrical interference. So we backprobed pins 3 and 13 of the C1835 connector at the transmission control module.
Channel 1 (yellow) shows the signal from pin 3 and channel 2 shows pin 13 (figure 3). The scope is grounded directly to the battery negative terminal. We connected the scope this way to check for any electrical interference that could be causing noise in the signal.
When the transmission went to failsafe, the signal was clear and didn’t indicate any problems.
Since there was no real indication of a problem in the output speed sensor, we decided to concentrate on the transfer case codes. We took a movie from the transfer case module PIDs. The potentiometer signal was noisy when the vehicle went in failsafe, but the output shaft signal remained normal (figure 4).
Now we had something to go on. We backprobed the potentiometer to monitor its signals, while watching the PID on the scan tool. The potentiometer is the only sensor the transfer case module has to indicate whether the transfer case is in low, neutral, or high range.
Normally we like to backprobe as close to the module as possible, but in this case the transfer case module is behind the battery and very inaccessible. So we removed the cover from the harness at connector C1858 (figure 5) to get access to the potentiometer pins. This made it easy to backprobe the connector at the transfer case motor (figure 6).
Channel 1 (yellow) is connected to pin 2, the potentiometer signal; channel 2 (green) is connected to pin 1, the potentiometer ground provided by the transfer case module (figure 7). Pin 4 is the 5-volt reference provided by the module and the scope is grounded to the battery negative terminal. This configuration allowed us to watch both the potentiometer signal and ground at the same time.
As soon as we started to drive the vehicle, the potentiometer signal (yellow) became unstable and the transmission went into failsafe (figure 8).
Now we knew where the problem was: a bad potentiometer. But the potentiometer isn’t available separately; to get a new one, you’d have to buy the whole motor for about $3,200!
So we purchased a used motor (figure 9) and replaced only the potentiometer (figure 10) to confirm our findings. Sure enough, after one week and 320 miles of driving, the problem was fixed.
This also explained why the transfer case sometimes wouldn’t go in or out of low gear unless we cycled the ignition. Turns out there was noise present in channel 1 (yellow) from the bad potentiometer when the transfer case motor shifted from high range to low (figure 11).
After replacing the potentiometer, we rechecked the signal as the transfer case motor shifted from high to low and back again (figure 12). There was no more noise in the signal.
Our conclusion is that the P0720 was set by the transfer case module to force the transmission into failsafe, since the transfer case module can’t place the transmission in failsafe itself. The output speed sensor harness and the transfer case harness are nowhere near one another, so we can’t blame this code on electrical interference.
This is just one more example of the lack of information that manufacturers provide; information that could really make a difference for those of us in the auto repair business. And it’s one more reason why GEARS is such a valuable resource for us all.
