Auto Service World
Feature   July 1, 2008   by Chuck Carman Curriculum Developer CARS

A/C Inoperative? Don’t forget the Middleman

It's that time of year again where the A/C technician is back in full swing. After a long winter and a cool spring, bring on the heat. Bring on the inoperative air conditioning systems.

It’s that time of year again where the A/C technician is back in full swing. After a long winter and a cool spring, bring on the heat. Bring on the inoperative air conditioning systems.

Air conditioning systems in today’s vehicles-as with the other systemsnow incorporate multiple computer and communication systems. These are becoming so complicated that one hardly even needs an old set of manifold gauges.

Ok, maybe we can’t go quite that far, but, with the amount of data available with some of the automatic systems, that’s not too far of a stretch. But, with all that data floating around, information is not always found in just one or even two modules “What’s a tech to do”? Read. Read all the information you can get on the system you are working on. The information is almost always there, it’s just not always staring you in the face. Technicians must be resourceful and think outside the box.

So, where am I going with this you ask? As a technical consultant I always advise following the diagnostic procedure for any code or symptom and review the associated wiring diagrams.

One talented technician I know calls these “The road maps to success”.

But, don’t forget the boring part of the Service Manual; the “Description and Operation” section. I know you know how the system works but do you know how this system works? It’s amazing how many different ways the same general system can be designed to operate.

Take for instance, GM’s beautifully designed Pontiac Aztec and Buick Rendezvous; the diagnosis for the compressor not engaging on a manual A/C system will hardly ever lead you astray. But, with the automatic system, the waters become a little murkier.

The diagnosis appears on the surface to be very straight forward. You follow the procedure and it comes to a point where it asks “Is the HVAC

control module inoperative?” Nope, all the buttons light up and you can see the switch inputs in the HVAC control module data stream. So, now verify the A/C request in the PCM data.

“There’s the problem, the A/C request is missing.” Well then, simply throw a new HVAC controller in… Oh, still nothing, ok, it’s got to be the PCM. Replace… This is where panic sets in.

Ok, now stop here, step back and think, better yet, read. Don’t keep replacing modules, at least not those two. After reviewing the system’s “Description and Operation”, a little tidbit of information becomes exposed. The BCM receives the A/C request from the HVAC controller prior to the PCM receiving it. The BCM must resend this information to the PCM before it will allow the clutch to engage. This is where the “light bulb” turns on. The BCM is the “middleman” and it did not complete its job.

Oddly enough, the service information makes this very clear for the manual A/C systems but leaves it out

on the automatic versions. This happens on more than just these models so be careful with this and on systems other than A/C.

Diagnose an inoperative speedometer, coolant gauge or fuel gauge on a Chevrolet Cavalier or Pontiac Sunfire without reading and understanding the system’s Description and Operationforgetting about the “middleman”-and you too may find yourself replacing modules unnecessarily.

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