Other Articles - December - 2020

Electrical Headaches Looking For The Cause of a DTC P0707/P0708

I hear it at every seminar, and from most of the shops I visit inside and outside of North America, “this stuff is getting harder and harder to fix!”. It does not differ whether I am at a dealer or an independent shop; I hear the same concern. Technicians are frustrated with the lack of quality and accurate information in the shop manuals, the lack of technical support available from the manufacturers. I get it, and I agree. But that does not change the fact that you have a customer’s vehicle setting in your stall. You would love to see it go away as it has been taking up valuable shop space and causing far too many sleepless nights for too long.

An example I have seen a few times as of late is a P0707 or P0708 DTC that may be set on late model GM 8 speeds and GM 9 and 10-speed applications. P0707 and P0708 DTC’s are related to issues with the range sensor or its circuits. You may find a P2802, P2803, or P2805 set with or without the P0707 or P0708.

These range sensor-related DTC’s are referencing the following conditions which may/may not be currently present on your vehicle:

  • P0707: Transmission Range Sensor 1 Circuit Low Voltage
  • P0708: Transmission Range Sensor 1 Circuit High Voltage
  • P02802: Transmission Range Sensor 2 Circuit Low Voltage
  • P02803: Transmission Range Sensor 2 Circuit High Voltage
  • P02805: Transmission Range Sensor 1 and Sensor 2 Voltage not Plausible/Possible

The range sensor on many of the new GM and Ford transmission applications are now PWM control devices (figure 1). The circuit no longer a high or low voltage but rather Pulse Width Modulate and the signal voltage based is based on the range you have selected.

If your monitoring range sensor signal circuits 1 or 2 with a meter or with your scan tool, you will see the duty cycle change as you move the shifter between the different ranges. So, what should the duty cycle values be on this design sensor? On the GM applications, you will typically see values like the chart in figure 2.

So, what will you see on your scan tool? As you change ranges, the duty cycle displayed on your scan tool will also change. A trick on these design systems is to add the values from range sensor 1 and range sensor 2 together; they should add up to 100%. If not, you will need to make some further diagnosis.

The range sensor circuits are actually connected to both the TCM and the ECM in these applications. The ECM supplies the 5-volt reference circuit and the ground for the sensor. Each sensor is then connected to its appropriate module (TCM or ECM) through its signal circuits. Sensor 1 is used by the TCM, while sensor 2 is used by the ECM (figure 3).

So what does it take to set the range sensor DTC’s?

  • Ignition in the on position, Ignition voltage greater than 9 volts.
  • Fault condition present for more than 1 second.

P0707/P2802: Sensor signal duty cycle less than the minimum for the range selected (typically less than 7%).

P0708/P2803: The sensor signal duty cycle greater than the maximum allowed for the range selected (typically greater than 96%).

P2805: Sensor duty cycle values are not within the excepted combined value.

At this point, you are probably thinking, “OMG, this is really complex!” Really it is not, so here are some shortcuts to give you a clue as to where to go (figures 4 & 5).

So what happens if some of these DTC’s are set? Many default actions can occur depending on which circuit has failed and what the failure mode is. Default actions include:

  • Starter Motor = Disabled
  • Autostart/Autostop = Disabled — If equipped
  • Grade Braking = Disabled
  • High Side Driver 1, 2 = Disabled
  • Line Pressure = Maximum
  • Manual Shift Control = Disabled
  • Neutral Idle Mode = Disabled
  • Tap Up & Tap Down = Disabled
  • TCC = Disabled
  • Transmission Adaptive Pressure Control = Freeze adapts

TIPS:

A couple of tips that may help in the diagnosis of these concerns include: Does the engine crank? Remember the circuit between the range sensor and the ECM is used for P/N input, so if it does not crank, it may be a fault in the sensor 2 circuit or the sensor.

Connection issues with the harness connector do occur. Inspect the pin tension for the female pins as well as inspect the pins for fretting.

We have seen internal harness issues on some applications, so it may take an internal transmission harness to address your concern.

As we have discussed for years, having a signal generator in your shop is invaluable. If you have a signal generator (which is capable of producing a duty cycle signal), you can unplug the harness and send a signal to the TCM or ECM circuit while you monitor your scan tool values. This will help you isolate the external harness from a transmission related issue. If the issue appears to be internal, you can use your signal generator to isolate internal harness issues from sensor failures.

Well, that is about all the time we have for now, until next time, remember to “Keep calm and carry on.”