MACS Monthly Newsletter - 2017 - JUL2

BEATING THE DEAD J2788 HORSE? COMMON SCENARIO / CASE STUDY
I'm still amazed how many professional shops are
using the older SAE J2210 standard RRR machines
on newer vehicles with low refrigerant capacities.
A few months ago while hanging out at a friend's
shop in Fort Myers, Florida I had the chance to work
on a 2009 Nissan. The car had warm air out the
ducts, high pressures on the gages, a blistering hot
and slightly noisy compressor, a hefty A/C repair
bill receipt in the glove box and a very hot owner.
My friend knew the shop's owner where the original A/C repairs had been performed and called the

owner up. Same old story. "We charged her like the
sticker says. What's that? SAE standard? Where do
I find that on my machine? How old is it? I think
maybe 15 . . . no, I don't remember. It still works
great. Why do you ask?"
The refrigerant was recovered with my friend's
J2788 machine confirming a significantly overcharged system. A proper charge was then applied
to the vehicle and away went a happy customer
with cold A/C in hot, muggy Florida! ■

BEST MECHANICAL AND ELECTRICAL A/C PRACTICES
Unless you've been out of touch for several years,
those aforementioned best practices should have
you nodding in 100 % agreement. In my response
to Andrew I highlighted a few of those best A/C
practices to pass along to his repair shop so to encourage them to do the job by the book. But thinking about his A/C compressor replacement question, it occurred to me that one of the best A/C
practices is to simply make sure that the diagnosis
of a faulty compressor is an accurate diagnosis to

begin with. Now we are talking "best diagnostic
practices" which would include the electrical side
of the A/C system. How many times have we had
a customer bring in a vehicle with a compressor
condemned by another repair shop only to find a
clutch circuit activation problem, bad battery, control head issue or other electrical fault causing the
compressor to be inoperative? I'll bet your answer
to that is "plenty!" ■

Auto Enginuity scan capture - Dave Hobbs

WORLD OF COMPRESSOR ELECTRICAL WOES

Figures 2: Compressor electrical diagnostics include verifying there is a duty cycle signal to the refrigerant control solenoid if the compressor is variable
displacement (1) along with the usual A/C request to the PCM (3) and PCM commanded status of the clutch (4 - if equipped). Verifying there is at least
some refrigerant charge in the system can be performed very quickly by viewing the A/C high pressure sensor (2) and evaporator or duct temperatures (6).
The PCM, BCM, HVAC head / programmer all need a consistent and clean B+ power and ignition supplies (5 & 9) This means no excessive voltage spikes
and AC ripple under 0.5 volts AC. Scrambled or grossly inaccurate data inputs to the PCM / ECM can mean no clutch and / or displacement solenoid
operation. If ECT (8) or IAT (7) are above or below a specified value the PCM will assume the vehicle is too cold for A/C operation or the cooling system is
overheating and A/C needs to be shut down. Note: Sometimes enhanced data will show a failsoft (default) value that looks normal. Always compare to the
values from the same PIDs in OBD II global.

July 2017

MACS Service Reports

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