You fix an air conditioning leak, recharge the system, and shortly thereafter the motorist is back, complaining that he turned on the A/C, the engine soon started to run rough and
the Check Engine light came on and is still on. And on most cars, the A/C probably stopped working.
Sure enough, that’s what happened. However, unless you did something on the order of installing a new powertrain computer, throttle position sensor or other parts in that area,
your repair of the A/C just brought the symptom to the fore.
All late models (OBD II) have “misfire monitors,” power-train computer algorithms (software strategies) that not only log misfire, but (if applicable) can narrow it to individual cylinders. A/C puts a huge load on the engine, so a borderline spark plug or its wire, or fuel injector, misbehavior in the idle air valve, or a borderline sensor, can cause the engine to misfire. When the A/C wasn’t working, there was no
compressor pumping load to produce the symptom, and in your shop the cooling load was low or the misfire monitor algorithm did not reach the trigger stage. When the monitor
does set, it not only turns on the Check Engine light, but also
may turn off the A/C compressor. In some cases the misfire is so barely noticeable that the motorist doesn’t even associate it with the Check Engine light, but when he turned on
the A/C, he notices it stopped cooling (on those cars where misfire codes disable A/C).
How do you convince the motorist it wasn’t your fault?
Unless he or she is reasonably savvy in this area, the sad answer is “not easily.” It helps if the Check Engine light is accompanied by a misfire code, which is PO300. Although a cracked or improperly tightened A/C compressor bracket may cause idle roughness when the A/C is turned on, it won’t illuminate the Check Engine light or log a trouble
code. However, if the A/C repair did involve a powertrain
component, or if you may have loosened a wire while working under-hood, you are going to have to find the problem.
If you don’t have a scan tool that works on late models with CAN (Controller Area Network) data bus, so you can get the misfire or other code that absolves you of blame,
you’ll surely have to pay a dealer or aftermarket drive-ability specialist for a diagnosis. You may not get the answer from a scan tool every time, but if you aren’t able to at least do that, you can find yourself in a nasty dispute.
The inter-relationship of all systems on the car means you
have to be able to at least defend your turf.
If you’re a service professional and not a MACS member yet, you should be! Become a member and receive a technical newsletter with information like what you’ve just read in this blog post visit http://bit.ly/10zvMYg for more information.
You can E-mail us at firstname.lastname@example.org . To locate a Mobile Air Conditioning Society member repair shop in your area. Click here to find out more about your car’s mobile A/C and engine cooling system.
Mobile A/C professionals should plan to attend 35th annual Mobile Air Conditioning Society (MACS) Worldwide Training Conference and Trade Show, Meet me at MACS Make Connections that Matter, February 5-7, 2015 at the Caribe Royale, Orlando, FL.