MACS Monthly Newsletter - 2017 - JUL3

Sure, most any tech can put a meter or test light at the A/C
clutch to see if the relay is providing a clutch engagement
signal. Most can also swap the horn relay for the clutch relay
to quickly check for a faulty relay if there's no voltage to the
clutch. But what if the relay just isn't getting the trigger from
the PCM? Logic tells the professional that there needs to be:
1. Scan tool PID confirmation that the PCM is seeing
an A/C request from the HVAC or BCM. (Figure 2)
2. A PCM relay control circuit free of opens or short
which includes BCMs that contain relays and fuses
such as the Chrysler TIPM (Totally Integrated Power
Module), Ford SJBs (Smart Junction Box), and their
equivalents on other models.
3. A good power supply which includes the battery
and alternator.

4. Accurate pressure and evap temp switches.
5. Properly functioning HVAC control head / module to make the A/C request possible and a fully
functional BCM to pass that request on to the PCM.
6. PCM with accurate engine management inputs
telling it that A/C compressor engagement is fine
to happen. Inputs would include the ECT / IAT /
OAT. Without these inputs in proper range, the compressor may not get activated.
7. The absence of a high engine load detected from
inputs like RPMs, MAF, MAP and TPS.
8. Functional variable displacement solenoid along
with the associated module controls and wiring for
vehicles thus equipped. ■

LOGIC LOCKUP - CORRUPTIBLE SOFTWARE
All of those topics get decent service manual and
training coverage from the OEMs and aftermarket
providers. But does that wrap up the compressor
diagnosis? One possible source of an inoperative
compressor is often overlooked in both the manual
and the classroom. What might that be? Software
and logic. How can software go bad? Well unlike moving parts like compressor clutches that can
mechanically wear, the average lines of code in a
module that make the A/C relay engagement decision contain no components that can wear out. 1's
and 0's burned into the Flash or EEPROM memory
in various automotive modules keep working for as
long as the vehicle is on the road 98 % of the time.
If your shop seems be 'lucky' enough to get more
than its share of that remaining 2 %, you might
want to know how software can get messed up. It's
simple. Just ask my nephew Andrew, the computer
expert. He'll tell you what some MACS members
including this author have learned the hard way -
software can become like many politicians. They
start out great and then later become corrupted! In
the IT support world where my nephew works, it's
not that uncommon to have to reimage a computer.
What is that? Basically, reload all the software with
a fresh batch that presumably is void of viruses and
other performance killing glitches that can occur
over time. In the automotive world, we haven't
had to deal with internet borne viruses in the past.

July 2017

However, even without that PCM's trip to a 'bad'
website containing ransom ware or viruses, it can
still become corrupted. One such corrupted condition is commonly referred to as "software logic
lock up." When a module's logic is locked up it
can cause intermittent unwanted conditions ranging from a radio that won't respond its own buttons
to an HVAC head that can't seem to convince the
PCM to activate the clutch relay. Think of the times
you had a personal computer that had a screen /
mouse / keyboard that just froze up. Vehicle electronics can experience a few of the same causes and
effects as with those common to PCs. The fix can
sometimes be temporarily performed via the vehicle's equivalent to a PC's warm boot (Control-AltDelete); a simple ignition cycle from on to off and
back on. When switching ignition to off and back
on doesn't afford the temporary logic lock up fix,
the equivalent of the PC's "cold boot" is the next
step. That "cold boot" would be removing the battery cable. While you may erase DTCs, and reset
many adaptive learns, the module suffering from
the software logic lock up may lose whatever freeze
up symptom it was experiencing. C.O.P. (Computer
Operating Pulses) reset and the module's operation
may well return to complete normalcy. The symptom could return in 5 minutes, 5 hours, 5 months or
never!! ■

MACS Service Reports

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