Delivering the Goods - December - 2020

The ZF 8HP: It’s Shift Strategy, No Problems Here! Adaptive Strategies and How They Affect Your Diagnostic Process

The ZF 8-Speed transmission is a well-balanced unit that seamlessly fits into powertrain management systems. With a blend of shift quality, gear ratios, unit footprint, efficiency, and durability in one package, it produces stellar performance under every conceivable situation. However, the mechanical and hydraulic engineering only works so well due to the electronic powertrain management strategy.

Through many years of transmission development, ZF came up with a powertrain management programming strategy that not only delivers optimal performance but also ‘takes care of’ the engine, transmission, and drivetrain components. As a result, numerous OEM manufacturers choose the ZF8 speed as a part of their powertrain (figures 1a and 1b). As a technician, it is necessary to know that there is programming that is designed to prevent powertrain related issues and to know how it works. In this article, we will explore these built-in strategies that are used in vehicles equipped with the ZF8 speed units and how they can affect your diagnostics.

THE STRATEGY OVERVIEW

The ZF8 speed units come with cutting-edge computerized powertrain control technology. All OEM applications using this unit have a high speed, interactive control for the transmission shift scheduling (figure 2). The programming is designed to select the best gear given overall vehicle operating conditions. The gear selection is dependent on the selection of the driving program and the driver type assessment. The selection of the program is either indirectly or directly affected by driver input.

Components designated as direct input devices can immediately affect the gear selected. For direct inputs, OEM applications have select switches that the driver can use to change from the economy, normal, and/or sport modes. Of course, what each mode is called will vary from one manufacturer to another. Other direct inputs include:

  • Manual Gear Selector
  • Range Selector
  • Advanced Driver Assistance System (ADAS)
  • Electronic Skid Control (ESC)
  • Fast Off Throttle detection

Indirect inputs are split into two categories: Driver Type Assessment and Selection of Driving Program. The Driver Type Assessment is considered the ‘fuzzy logic’ that is used to tailor the Selection of Driving Program. The inertia sensor (sometimes referred to as a G-sensor), is the central component in this logic calculation. Engine torque demand, rates of acceleration and deceleration, and other items are stored and evaluated to produce a pattern of operation based solely on the driver’s inputs. This helps the computer to anticipate the proper driving program to select based on driver history.

Selection of Driving Program looks at preprogrammed shift patterns and assigns one to the choice of gears to select based on current calculated conditions. Here’s where knowledge of what the computer is doing based on calculated conditions is important. Inputs from other vehicle control systems as well as embedded driving condition detection programming, will change the selected driving program on-demand, as the computer sees fit! Here are the items the computer is evaluating:

  • Low Traction Detection
  • Warm-up Mode
  • High-Temperature Operation
  • Rolling Resistance
  • City/Heavy Traffic Driving
  • Cruise Control active
  • Electronic Skid Control active

The Driver Type Assessment will modify the selected program in most cases. However, during emergency management program selection, driver type assessment data is ignored.

FAILSAFE OPERATION

In the event of a loss of power to the transmission, the unit enters a default mode dependent on the current operating state of the vehicle. With no power going to the solenoids, line pressure defaults to high. If the vehicle is in a forward range when the fault occurs, the transmission defaults to 6th gear.

If the loss of power occurs when the vehicle is in park, reverse or neutral, the vehicle will remain in park or default to park. If power is not present at the TCM at startup, the vehicle will not be able to come out of park except by using the manual park pawl release lever.

If a fault occurs relative to an internal component or gear train failure, the TCM program will deselect the gears determined to be faulty and allow operation of the gears that can be obtained. Diagnostic trouble codes related to the determined failure will be stored in the TCM. High line pressure may or may not be commanded based on the failure.

THE STRATEGIES VS. YOUR DIAGNOSTIC PROCESS

The range of programmed strategies that comes with the ZF8 speed transmission is intimidating at a glance. However, many of these strategies were in the making for several decades and have slowly evolved into what we see now. Figure 3 is a summary of these strategies; note that the actual names may vary between OEM manufacturers, and this is a general overview of strategies used by various OEM manufacturers. There will be differences; however, the theme presented here is extremely useful. With the understanding that the powertrain management system can invoke one or more of these strategies at any given time, it is important to know how they affect the operation of the transmission when they are active. Qualifying a customer complaint relative to these strategies is mandatory. Let’s look at how to approach customer complaints relative to these strategies.

In light of all the possible functional issues that can cause a shifting concern, it is best to first identify and duplicate the customer’s complaint. If there are any codes in any modules, of course, this would be a welcomed starting point, but assuming there are none, focusing on the verified customer complaint is going to be your primary diagnostic tool. With the complaint verified, you then want to refer to figure 2 under ‘commanded transmission operation’ to identify each function that could cause the complaint.

For example, if there is a complaint of late, delayed shifting, we see by the reference chart that Warm-Up and Drag Recognition strategies can cause this. It should also be noted that the Low Traction Control and Stability Control strategies can cause this as well. Here is where things can get tricky if you don’t have a plan.

The best way to diagnose this issue is to check whether each function is activated or not based on vehicle operating or driving conditions. You can observe the Warm-Up function deactivate by watching the coolant temperature sensor data on the scanner go above 86°F. Also, verify that the Transmission Fluid Temperature sensor is responding the same.

The Drag Recognition function can be verified by comparing shift points going uphill versus downhill on the road with a steady, mild grade. Don’t select a hill with too steep of a grade because the grade speed control programming may command a lower gear for engine braking. There should be a noticeable difference in the shift points relative to the grade; (uphill = late shifts, downhill = early shifts).

If you suspect either the Stability Control or Low Traction Control functions are causing shift issues, disable the ABS/Traction Control system. Keep in mind that wheel speed sensors, inertia sensors, and steer wheel angle sensors all tie into these systems. Faulty sensors, poor signals, or calibration issues can falsely initialize functions that can cause shift issues. Either use a button in the vehicle or locate and pull the ABS fuse and retest for the shift complaint.

If your shift complaint goes away at any point, you will need to look at the related sensors and system operation to find your problem outside of the transmission. Always refer to factory OEM system operation and diagnosis.

New units and new operating strategies require a new diagnostic approach. Looking at the entire powertrain management system gives us the best vantage point to identify shift complaints and then determine if there is a problem or not. Equipped with the proper information, you can confidently identify an internal issue, or you may find yourself telling the customer, ‘It’s just the shift strategy. There’s no problem here!’