Key to the ECM lock

Jan. 1, 2020
Many shops will attempt to save the owner of an older vehicle a few dollars by installing Engine Control Modules (ECMs) acquired at a salvage yard or rebuilder. The unknown history of these parts can present challenges to the repair shop. A difficult

This article was originally published April 1, 2011. Some of the information may no longer be relevant, so please use it at your discretion.

The Body Control Module (BCM) supports and controls the VTSS system. The BCM receives hard-wired inputs from the door lock and lift gate cylinder switches if the vehicle is VTSS equipped. The option programming in the BCM allows it to process the lock or unlock information from these lock cylinder switch inputs.

When the BCM sees an unlock switch input from one of the cylinder switches, the BCM then creates a password message it broadcasts on the communication network. The ECM then knows it is okay to allow starter actuation and fuel injection.

The challenging part of this ECM/BCM based system is that the ECM will learn whether a particular vehicle currently is equipped with VTSS based on whether the ECM receives a VTSS password from the BCM when the vehicle start is attempted. This presents a problem when installing remanufactured or salvage ECMs. There is no way to tell by its external part number if a particular ECM has come from a vehicle that was equipped with VTSS.

This can result in a situation where once the ECM has learned it needs a VTSS password from the BCM it will not allow the engine to run without the password. The ECM cannot unlearn this password requirement by simply installing it on a vehicle without VTSS. The work around for this is to turn a non-VTSS vehicle into a VTSS vehicle in order to satisfy the ECMs need of a BCM password.

The methods and details to do this vary a bit depending on the vehicle model. We will explain the methods we have learned.

1996 and newer jeeps

In order to get non-VTSS equipped Jeeps to start with an installed ECM that has learned a VTSS password requirement, you need to use a DRB3 factory scan tool or similarly equipped scanner to go into the BCM options screen and turn on the VTSS option. Once this option is turned on, it cannot be turned off.

Because our subject Jeep did not actually come from the factory with the VTSS option, it will be missing the installed VTSS switches on all the lock cylinders. However, there is only one vehicle harness made for these Jeeps, and all harnesses have the VTSS wiring already installed. All that needs to be done to get the vehicle to start is to permanently ground the harness for the missing cylinder switches at the BCM connector.

This tells the BCM that an unlock command has been sent from a cylinder switch. This ground easily can be installed at the BCM connector under the left bottom edge of the dash kneepad shown below.

1994-97 LH body cars

These vehicles can be doubly insidious, as both the ECM and the BCM can learn to have VTSS simply by having them installed on a vehicle that has VTSS in the BCM or ECM already. Therefore, you potentially can infect not only the ECM but also the BCM on a non-VTSS vehicle. The good news is if you install a VTSS learned ECM on an LH car without VTSS in the BCM, the BCM learns to have VTSS without having to use a scan tool to turn on this option.

Unlike the door cylinder switches, which have two resistors to either designate an arm or disarm command, the trunk lock cylinder switch is a simple ground to disarm only position switch. On the LH type vehicles, I use this easily accessible trunk cylinder switch harness wire to permanently disable the VTSS system.

Caravans, Durangos, and others

To enable the VTSS in the BCM on 1996 to 2000 minivans, you must open the hood and operate the lift gate cylinder switch, which has an internal resistor that drops a reference voltage from the BCM down to about 2V to accomplish a disarm input. After you enable the VTSS, use this operation to disable the system permanently. On the Durango and some others, you might have to cycle the key cylinders switches during the scan tool VTSS enable process.

In the cases where the switch inputs are not simply open or short to ground, you must look on the wiring diagram for the vehicle in question to find the ohm value of the resistor used for the disarm position. If the diagram does not list this resistor value, you usually can find the proper disarm voltage level listed in the troubleshooting section for that particular cylinder switch.

Use a variable resistor to obtain that proper voltage to simulate the enable/disarm command to the BCM. Then, wire in a permanent resistor to ground to disarm the system.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.

About the Author

Jim Garrido

Jim Garrido of “Have Scanner Will Travel” is an on-site mobile diagnostics expert for hire and president of the Mobile Diagnostics Group. He has over 23 years of experience as a GM technician and is considered one of the best techs in the country. Garrido is an avid participant on iATN and was a board member for STS. He has written programs for GM and many aftermarket groups including some research on the GM CSI ignition system. Garrido is an ASE Certified Master Technician with L1 and currently takes care of CARQUEST customers in Western North Carolina.

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