ZF 6HP26: A torque calculation error - Transmission Digest

ZF 6HP26: A torque calculation error

A 2008 Lincoln Navigator with a ZF 6HP26 transmission arrives at Holcomb’s Transmissions in limp. When checked for codes, a P1719 comes up for “Inaccurate Engine Torque Signal.” The TCM receives this over the CAN, causing it to initiate failsafe.

The information Ford provides for this code is vague. The Action step provided is to simply monitor the appropriate PID to validate that an error is present. The PID chart provided for this step consists of transmission-related information followed by instructions to clear all DTCs, and once cleared, test the system for normal operation. If the DTC returns, the tech is to refer to Computers and Control Systems.

This leaves many scratching their heads, not sure what they are meant to be looking for. In this case, the tech did clear P1719 which allowed the transmission to begin shifting again—but for just as few drive cycles before the code set and the transmission went into failsafe mode.

With engine torque reduction being used for many years now for a variety of reasons, when there is a problem with this computer strategy, one of the first things to look for is what other codes may be lurking in the system. Additionally, you should look at sensors related to engine load such as air filters, MAF sensors, throttle bodies, TPS, APPS, etc. Sometimes a new air filter, cleaning of the MAF and/or throttle body resolves the issue.

In this case, the tech working on the vehicle only pulled P1719. ATSG tech Al Sanchez suggested scanning for OBDII generic to see what codes might be found there. When he did, two codes came up that offered a clue to the problem: Code P0171 for fuel system too lean (Bank 1), and P0174 for fuel system too lean (Bank 2). Al immediately asked the tech to examine that air intake system to see if there was a vacuum leak allowing excessive air to be sucked into the system. In doing so, the tech discovered a vacuum leak at the throttle body where the bellows hose was not fully seated. After coupling it back into place (see the image above), codes erased, and a road test confirmed that the problem was resolved. It was quick calculating of a torque calculation error once the lean codes became exposed.

Read more stories from our Technically Speaking column series here.

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