MACS Service Reports - 2016 - SEP5

HEY, THIS ONE IS APPARENT, OR IS IT?
Most of the repair problems a shop has to deal with
are straightforward issues, that is, you don't have to
diagnose them - or maybe you really do. A car comes
in with bald tires - replace them, but make sure the
wear is balanced and the mileage appropriate, or
there's possibly a suspension issue, wheel alignment
at least. Or screeching brakes with no lining left on the
shoes - also look for a normal wear pattern on all the
linings...well you get the point.
But as we've often said, in addition to root causes
you have to make an overall check of the vehicle
system. We don't mean a bumper-to-bumper inspection, although for a regular customer that's a good
way to identify any other needs. But with trouble
codes and PIDs (parameter ID "data items"), and so
many that can cause the A/C to cut out, you can reduce the chance of a comeback with an OBD II scan
check at least. Let's look at a couple of examples.
First, a 2008 Honda CR-V with inoperative A/C.
Technician lifts the hood, sees the belt broken and
the clutch hub loose. Pretty straightforward: install
a new compressor and belt, making sure the belt
runs true, no noises, no wobbling. So he recharges
the system and makes sure it's pumping out cold
air. It is? Great, and out the door it goes.
The car very quickly is back in the shop and the
complaint, which sure sounds like a repeat, is that
there's no A/C. But nothing is broken, so the technician realizes this is something else. However, he notices a slight hesitation when the A/C is turned on,
and a clicking from the belt tensioner at the same
time. Both symptoms might indicate some tightness
in the compressor. No one waits for a compressor to
"break in," so he installs a new one. Once more, lots
of cold air.
But he's a meticulous guy, so instead of sending
the customer on his way, he carefully ensures a full
range of performance. The compressor does come
on and the A/C operates at idle, but the technician
takes the car for a road test and after about five
minutes, as soon as the driver touches the throttle,
the A/C cuts out, and when it's back to idle, the
compressor engages again.

Aha! A defective throttle position sensor, or a
mishandling of the signal by the engine computer.
No, the scan tool says the TPS is sending a valid
signal and the engine computer is able to process it.
Why would this problem happen when the throttle is touched if the TPS is good? The technician
noticed that engine rpm was almost exactly 1500
each time the compressor cut out, so he put down
his scan tool and went right to the wiring diagram,
convinced he had an electrical problem. (See Fig. 7)
He noticed the directional diodes in the coil circuits for the compressor relay, radiator fan relay,
fan control relay and the condenser fan relay. Could
there be a bad diode? Apparently not, because the
radiator fan operation wasn't affected. What about
the compressor's thermal switch, which we covered earlier in this report? Could it be causing an
intermittent? Maybe, but the loss of compressor
operation was at the engine computer, and thermal
switches are not known for intermittent failures. A
jumper wire would confirm that was not the problem area. What about the pressure sensor? Definitely a possibility and the technician was about to
order one, without even trying a jumper across the
switch contacts, but he noticed the coolant temperature sensor connection to the engine computer. He
went back to the scan tool and there it was: when
the engine rpm goes up, the PID for engine coolant
temperature rises to 230 degrees F. (110 C). That
would be all right for a number of cars, but Hondas
run cooler and when the temperature tops 220° F
(104° C), the engine computer will disable the A/C.
But still, why would the coolant temperature
reading behave like that? The answer was a very
low coolant level, not keeping the sensor bathed in
a solid flow, so when the engine was revved, the
flow of hot coolant over the sensor tip triggered
a hot coolant signal to the engine computer. The
technician topped up the system and coolant temperature returned to normal, along with A/C performance. ■

MACS 2017
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February 15 to 18.

September 2016

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2016