Other Articles - August - 2020

Slowing Down: Diagnosing a Grade Braking Issue

Grade braking was introduced in the 2006 Allison applications. It’s a feature that many customers have learned to value, especially for those that use their vehicles to tow or carry heavy loads.

There are two types of grade braking used today: normal grade braking and cruise control grade braking (which was introduced in 2009). And like regular grade braking, it’s transparent to the customer.

2009-2011 applications required the customer to engage tow/haul for the feature to operate. However, beginning in 2012, grade braking can engage in either the tow/haul or normal mode when the cruise control is engaged.

The grade braking feature is transparent to the customer but, if for some reason, the customer no longer wants the feature, it can be turned off for the current key cycle. To disable the grade braking feature, depress the tow/haul button for three or more seconds. On some applications, the feature may be turned off with a switch mounted in the switch pod below the climate control panel.

Remember, the grade braking feature is designed to use the transmission and engine to provide engine braking during deceleration when the proper conditions are met. This feature improves vehicle control while reducing wear and tear on the vehicle brakes. The feature automatically downshifts the transmission to provide the needed vehicle deceleration. Cruise-control grade braking is available anytime the cruise control is set. If the brake is applied, the normal grade braking program becomes active.

With grade braking active, the customer will typically see a message displayed on the vehicle’s driver information center showing that grade braking is in operation (Figure 1).

Several messages may be displayed depending on the year of the vehicle, including:

  • Grade braking ON or Grade braking ACTIVE: Grade brake is operating.
  • Grade braking OFF or Grade braking DISABLED: Typically, this message occurs when the tow/haul button is no longer in tow haul mode.
  • Grade braking ENABLED: This message is typically displayed when the tow/haul feature is active.

When active, the controller will command the needed downshifts to maintain engine braking. Multiple downshifts may be commanded depending on the change in vehicle speed during deceleration. If the controller feels that the engine may overspeed or if it determines that grade braking is no longer needed, it may command the vehicle to upshift.

With an understanding of its basic operation, what do you do if grade braking is active all of the time? Lots of different symptoms may arise with a grade braking issue, these include:

  • Grade braking is active all the time, even when you are not operating on a grade or stepping on the brake.
  • Reduced power with no driver information messages.
  • The transmission may be hunting between gears, shift busyness.
  • DTCs P057B, P057C, and/or P057D may or may not be set.

If you are faced with any of the issues listed, a good starting point is the brake switch.

Starting in 2012, the switch design was updated. Referred to as the BPP (brake pedal position) sensor, it operates much like an old-time TPS. The BPP consists of one or two separate BPP sensors mounted in the same housing, each having a 5V reference, signal, and a ground circuit (Figure 2). If the BPP uses two sensors, one sensor is typically connected to the BCM (Body Control Module) and one to the ECM (Engine Control Module). Your scan tool will usually display the data for both sensors as:

BPP %: 0-100% (0% represents your foot off the brake)

BPP Voltage: Since it’s a potentiometer, its output will vary depending on your brake pedal position. With your foot off the brake, it usually reads around 1 volt +- .3 volts.

BPP learned value: This value should equal the value for the sensor signal circuit when it is in the released position. This PID represents the lowest voltage that was present from the BPP signal circuit. Once learned, this value should not change if the vehicle is operating correctly. The controller uses the BPP learned value as its baseline value telling it that the brake switch is released when that voltage is achieved.

The controllers constantly monitor the BPP values during vehicle operation. The controller compares the learned value to the current lowest value it sees during that key cycle. If it sees the voltage lower than the current learned value, it will relearn the new value as the updated BPP learned value.

REAL WORLD EXAMPLE

Let’s say the BPP learned value was 1 volt. During that key cycle, the value drops to .5 volts, due to a loose wire connection. The controllers will now learn that as the lowest value, and the BPP learned value will be updated to .5 volts. Now let’s say the loose-wire problem goes away. The controller sees the BPP return to 1 volt. The controllers will compare the current value of 1 volt to the learned value of .5 volts, and conclude that the customer has their foot on the brake. As soon as the other criteria are met, normal grade braking will be enabled. The complaint may be no upshift or a downshift for no apparent reason. Other symptoms may occur when the other grade braking criteria are not met, leading to low power and shift busyness complaints.

Typically issues like this are not caused by a faulty BPP sensor but rather by a wiring-connection issue such as:

  • Inline connector X115
  • Inline connector X119 or X319
  • Inline connector X205 (older trucks)

If a problem exists, you will need to correct the issue, which is typically related to an open/high resistance or short to ground issue in one of the BPP circuits. Your scan tool should be equipped with a BPP learn feature, which will erase the value stored, allowing you to learn the new value after your repair. The important point is that you’ll want to look for inconsistent BPP values when you have a shifting concern. Shift busyness, late upshifts, or downshifting for no reason may be the result of a BPP connection problem that’s simple to fix.

If your scan tool does not have the feature, you may be forced to reprogram the controllers. Doing this will erase the stored value, allowing it to learn the new value.

Well, that’s about all the time we have for now. Until next time remember: “Wasting time is the most extravagant and costly of all expenses.”