Tech Stuff

ECU soft resets: Killing power to on-board modules does wonders in diagnosis and repair

Figure 1: This 2011 Buick Lucerne had an issue with its transmission control module (TCM) that was repaired with an engine control unit (ECU) soft reset.
<p>Figure 1: This 2011 Buick Lucerne had an issue with its transmission control module (TCM) that was repaired with an engine control unit (ECU) soft reset.</p>

Craig Truglia is an ASE A6 and A8 certified technician who presently works as a service writer for Patterson Auto Body, a repair facility in Patterson, N.Y. A former shop owner and editor of several automotive repair magazines, Truglia combines his Columbia University education with the real-world experience he sees daily in the automotive repair field. Technicians Truglia, Fred Byron and Cristian Cordova took part in diagnosing the different vehicles in this article.

When a smart phone is not working quite right or a PC gets frozen, what’s the easiest way to fix it? Turn it off, start it back up, and hope the problem is gone. Some PC users even wait a few seconds to turn the device back on, hoping it gives time for the computer chips to power down.

Without getting all scientific about why this works on computers, the principle is simple. When a computer acts screwy, sometimes it needs to be reset so it can work right (see Figure 1).

Being that today’s vehicle systems are computer-managed, it stands to reason that when they have weird sensor or computer-related driveability problems, resetting the vehicle’s computers may be the way to go.

Resetting on-board ECUs

Some technicians prefer removing the battery cables and jumping them for 30 minutes. Others like getting fancy and putting a one ohm 10 watt resistor in series and powering down the vehicle for a few minutes. The same is true of both strategies: A soft reset is known to work wonders.

Some high-end scan tools that enable reflashes, such as the Autologic, feature convenient help-lines. When calling the help desk after a failure reflashing, they will recommend that the technician perform a soft reset first. Further, many mobile diagnosticians will not even agree to take the time to look at a vehicle unless a soft reset was done first just to rule out a “computer glitch.”

Simply “deleting the DTCs” or “resetting adapts” does not solve the issue. Unlike computers, vehicles do not come with reset buttons. So, the soft reset remains the only way to get the job done.

It also does not hurt that doing a soft reset helps vehicles set their monitors quicker for inspection purposes, too. It should be noted that as more and more vehicles require idle, steering angle, and other relearns, we do not want to do soft resets haphazardly, especially if we do not have the proper scan tools to perform these relearn procedures.

Figure 2: Mitchell’s Prodemand’s color coded wiring diagrams helped speed the diagnosis, informing the technician that the direct inputs of the VSS were directly to the TCM, so if there was a sensor, as opposed to a module issue, the wiring up to the module should reflect a discrepancy on a labscope.
<p>Figure 2: Mitchell&rsquo;s Prodemand&rsquo;s color coded wiring diagrams helped speed the diagnosis, informing the technician that the direct inputs of the VSS were directly to the TCM, so if there was a sensor, as opposed to a module issue, the wiring up to the module should reflect a discrepancy on a labscope.</p>

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