MACS Monthly Newsletter - 2017 - JUL4

GLOBAL BATTERY RESETS
Many techs I run into know the procedure but
may not know the unofficial name for removing the
process of disconnecting B+ power by pulling the
negative cable from the battery post (to interrupt
power) and touching that loose battery cable back
to the positive cable / post for a few seconds. I
first heard of the time honored procedure by that
title when talking with a few tech advisors at a particular OEM's hotline. The procedure has the intention of draining off any capacitive effect of storing
a tiny amount of current at 5 volts or more in one or
more modules. The capacitive effect can on occasion cause a module to "hold onto" enough power
to keep a microprocessor or RAM chip alive and
still in the dreaded software logic lockup condition.
Touching both cables together (when the battery is

DISCONNECTED (Figure 3) in theory provides a
shorting condition for the tiny bit of capacitance to
bleed off. A module absolutely without power is
more likely to reset and perform normally again.
Just like rebooting a PC. Automotive electrical engineers are for the most part, skeptical of any 'capacitors' within electronic modules that don't simply discharge / bleed off immediately after losing
12-volt power. However, the thousands of techs
who work on high electronic content vehicles (i.e.
luxury models) have advised me that indeed this
'global battery reset' procedure works with enough
reliability for them to include holding the disconnected battery cables together on every battery
power reset. ■

Drawing - Dave Hobbs

The negative battery cable REMOVED
from the negative post and touching the
positive post / terminal will NOT cause
any problems with shorts or any other
adverse condition. It may, however, fix a
phantom electrical problem.

Figures 3: Removing the negative battery cable "should" allow all modules to reset and start fresh when the cable is reconnected. However, in some cases,
there are capacitors internal to various modules that may not bleed off enough keep-alive memory for that module to fully power down. This means phantom
electrical conditions such as logic lock up and data corruption can continue when the battery is reconnected. In such cases, touching the loose negative cable
to the positive post should do the trick to bleed off any capacitors that don't fully power down. Hold the negative cable against the positive post / terminal for
a few seconds to ensure you've got everything drained. If the cable won't reach to the positive post, simply employ a jumper wire between the removed negative cable and positive post. I emphasize that the negative cable is removed from the post prior to shorting to the positive post with a jumper wire. Failure
to remove the negative cable from its post will, obviously, be a dead short resulting in shower of sparks and a very hot jumper wire!

LOGIC LOCK UP CAUSES / CORRECTIONS
"Messing up F.R.E.D.'s (Frustrating Ridiculous
Electronic Device) head" can be caused from something as common as an excessive voltage drop on
power or grounds to a module. Go over / clean
& tighten power and ground connections when

July 2017

you discover the voltage drop culprit. Add a redundant ground if necessary. Electronic modules
also can lock up due to a voltage transient (spike).
Voltage transients are a form of EMI - Electromagnetic Interference. Using a scope connected to the

MACS Service Reports

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2017