MACS Monthly Newsletter - 2021 - MAY2

on the vehicle may set this DTC or it may occur after a compressor replacement, especially if the technician disconnects the compressor's low voltage power, ground, and CAN data connector WHILE the ignition is still on.

/ transient that caused the software corruption / module lock up in the first
place is NOT repaired by this power reboot, so when all the right conditions
reoccur, you're right back to where you started. This now leads to the need
for improved software (reprogramming) to address the logic lock up condition, so it doesn't happen again.

That last statement should set an alarm bell in most technicians' heads
who know anything about automotive electronics. Electronic devices can
sometimes be disconnected while powered up (without damage) and
sometimes they can't. Using an analogy from the personal computing
world, you may likely have experienced a portable hard drive failing because you didn't properly shut it down prior to pulling its USB cable from
the PC it was connected to. Personally, I've even experienced portable
drive failures occur on solid state (no moving parts) versions such as those
USB " jump drives " because I didn't " eject " the jump drive the correct way.
This especially applies to jump drive removals when the drive is working
on moving a file on the PC it is inserted in.

Delphi Technologies

Module Programming " 1st Commandment "
GM makes a special mention of what I'll refer to as the " 1st Commandment " that applies to any module flash / reprogramming procedure. That
commandment is, " Thou shall not program modules with low voltage or
noisy battery power. "

GM's TSB states, " The technician may find DTC U016B Lost
Communication with Electric A/C compressor Control Module
set. This may occur when the device is disconnected while
the communication circuit is still awake. " The TSB describes
the correction as, " Perform the Service Procedure completely. "
So, what exactly is GM referring to as, " completely " ?

Figure 2: Battery State of Charge Chart: SOC (state of charge) needs
to be as close to 100% as possible for a successful module programming procedure to happen. Note: OCV stands for open circuit voltage. That means you're measuring the battery's voltage
without any loads. The lower the battery's temperature, the lower the voltage level to declare " 100% SOC " but you'll note even
at a chilly 32°F temperature, the battery is still only at a 40% SOC
when it's exactly 12.0 volts. AGM (absorbent glass mat) batteries
have SOC specs slightly higher than the traditional wet cell lead
acid battery SOC specs, but are close. Basically put, 12-volt batteries are not supposed to ever be at only 12-volts!

Dave Hobbs

1. Ignition ON / Vehicle in Service Mode
2. Perform Diagnostic System Check (Pull all DTCs in all
modules)
3. Verify the DTC U016B is set:
* If only DTC U016B is set, go to step 4.
* If other DTCs are set, follow published diagnostics.
4. Allow the vehicle to power down for 3-5 minutes, then
disconnect the negative battery cable for 3 minutes.
5. Reconnect the negative battery cable.
6. Use the scan tool to verify the A/C compressor control
module is communicating.
7. Reprogram the A/C compressor control module.
8. Perform the Diagnostic Repair Verification after
completing the repair.
Draining oﬀ Capacitance (Battery Power / Computer Reboot)
Obviously, Step 4 is an old trick we've all used to straighten out (at least
temporarily) various gremlins that arise within software on vehicles. The
" Global Battery Reset " (as a GM hotline colleague once referred to it) is
not only removing the negative battery cable, but also touching that same
disconnected cable to the positive cable / post. Some techs will hold the
two cables together for a few minutes. The idea is to drain off any residual
capacitance within various modules on the vehicle which might be keeping a module from doing the equivalent of a PC's reboot. The latest method
shared is to use a 1 ohm / 10-watt resistor in series with the cables touching
together, in order to induce a more gradual (thus more effective) power
down of all modules.

Figure 3: Battery Chargers are Created Equal? NOT! Many conventional battery chargers emit noise into the battery
that is then passed on to the electronic modules on the
vehicle. Most of us have heard the terrible noises coming
from a vehicle's radio that has a low battery and is being
charged (fast) by a conventional battery charger. Note:
If the battery charger says " smart " on it, that term may
only apply to the charger's ability to back off the charge
rate (float) once the battery is at 100% SOC. The term
may also mean it can charge AGM batteries correctly as
well. But " smart " chargers aren't always clean!

I've spoken with automotive electrical and software engineers over the
years about this procedure and received some skeptical responses regarding the validity of such a procedure. However, most of us who have tried the
procedure can recount the numerous cases where the battery cable touch
back saved the day. On the other hand, the root cause of the voltage spike

May 2021

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MACS Monthly Newsletter - 2021

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