That "BS" is a misconception. If they feed you a line, then you tell them to go ahead with the repair, they know how they're going to look when their recommendation doesn't solve your complaint. They might be incorrect or mistaken in their diagnosis, but no one is going to intentionally give you bad advice unless they're trying to make a competitor look bad. Every industry has its insecure people like that who think they look better by making competitors look worse.
Testing the charging system only take a few seconds. It takes longer to connect the equipment and start the engine that it takes to get the readings. When I worked for a mass merchandiser in the '80s we charged $5.00 for that test, but it was included free with the installation of a new battery or when you had one of our batteries already but were having a problem. Most full repair shops that I'm aware of do not charge for the test specifically. It is done as part of the mechanic's hourly labor charge you're already paying.
Electrical is so confusing to people in an industry where they learn and diagnose best by seeing and feeling things. A lot of mechanics understand just enough to cover the basics, and that typically means testing the charging voltage in this case. You can do that yourself with an inexpensive digital voltmeter, but if the generator has a bad diode, the voltage will still be very close to perfect, (between 13.75 and 14.75 volts with the engine running). It's the current that is insufficient. That alone can be made up by the battery for those few times when a real lot of current is needed. The bigger issue is "ripple" voltage. Imagine a three-cylinder water pump feeding a hose on a fire truck. You'll get a nice strong stream of water until one cylinder fails; then you'll see regular momentary dropouts in the flow once per revolution of the pump. That's what happens to the three-phase output of AC generators, (alternator). With one bad diode one phase is missing. During that time the output voltage drops real low causing the output current to drop too. The battery is going to smooth out the voltage that you measure with the voltmeter but it's the current pulses that also affect computer sensor signals.
If the battery is more than about two years old so much lead has flaked off the plates that it loses it's ability to dampen and absorb voltage spikes and those are what destroy the diodes and voltage regulator inside the generator. When current flows from the generator to the battery it always creates a magnetic field around the wire and that can "induce" a voltage into the adjacent wires in the harness. THAT is how it interferes with computer sensor signals. For most of them a change in voltage of one or two tenths of a volt is quite significant. The induced voltages can be much higher, and the computers will think those are the proper voltages and will make incorrect decisions based on them. That's how you can have unsteady idle speed, misfires, reduced power when the computer thinks it's getting a signal from a knock sensor so it retards ignition timing, and things like that.
The reason I always point to having the charging system tested first is because generator failures on all cars are very common but the characteristic way they almost always fail on GM cars is a failure of a diode or the voltage regulator. As I mentioned, a simple voltage test at the battery will appear to show it's working fine, but it's that ripple voltage you have to test for along with the ability for it to only be able to develop one third of its current rating. You must have a professional load tester to measure those two things.
Anytime you have multiple problems we always start by suspecting the cause is something common to all of them, and that's often the charging system. If I'm wrong, that's fine but it's silly to overlook such a simple and quick test. If you DO have a bad diode no matter what else you do or what parts you replace you will never solve those problems. If testing shows ripple is low and the generator can easily produce its design current, typically around 90 amps, you can eliminate that as a possible cause, then troubleshoot each malfunctioning circuit individually.
One more comment on mechanics giving you incorrect information. Most mechanics are very poor at communicating with car owners. Just like doctors use different terminology with other doctors than they do with their patients, one mechanic can say three words to another mechanic who will instantly know volumes about that car, but it would take volumes to explain the same thing to the car owner. On top of that, in most shops the mechanics don't have time to speak with every customer so there are service advisers or service writers who do that. They very often know nothing about diagnosing a car problem, but their job is to listen to what the mechanic said, try to understand it, then interpret that to you in a way you will understand. You can be sure things are going to get lost in translation. I've had conversations with car owners where I didn't recognize my own story the way they repeated it. There was no intention to defraud or mislead anyone, just poor communication. I've also been the customer when listening to people in car repair and tv repair trying to explain something. I knew what they were trying to say but if I were not an expert in both fields I would have heard something entirely different. That doesn't mean they were intentionally feeding me a line of BS. You can look at it another way right on this site. When someone posts a car problem, then adds a real lot of detailed information and / or they say they are a mechanic, they will get a totally different answer than when we assume we're dealing with someone whose expertise is in something other than cars. The saying we had in school was "what you heard is not what I said". It causes a lot of unwarranted mistrust in any profession but service people get it the worst. That's because owners don't know much about the product except how to turn it on, it's broken, and it's going to cost money to have someone else fix it. Even here you already have two different recommendations so you know at least one of us is going to be wr-wr-wrong. You have at least three things to look at. To a single mechanic they might approach the problem the same way but they aren't going to say "I'm going to test this thing first, then that thing, then if I'm still wrong I'll test the third thing. That's what they do but it's not how they describe it.
Thursday, April 11th, 2013 AT 12:35 AM
