2009 Volkswagen Jetta Electrical

That is absolutely wrong, but don't be too hard on the dealer. You will almost always be talking with a service adviser, not the mechanic. Service advisers aren't mechanics and they know relatively little about cars. Mechanics do a very poor job of communicating with car owners, so part of the service adviser's job is to interpret what they thought they heard from the mechanic into something they think you can understand, and things get lost in translation.

In this case, first of all, you didn't list any symptom or problem you're having. Second, AC generators, (everyone knows what you mean by "alternator, but that term was copyrighted by Chrysler), are physically incapable of generating more current than what they are designed for. What they CAN do is generate more current than the car's electrical system is calling for but that can only be done by raising its voltage. "Voltage" is electrical pressure. "Current" is electrical flow, just like water in a river. It's the job of the voltage regulator to maintain charging system voltage between 13.75 and 14.75 volts, and yours is perfect. In that case the generator will develop exactly the amount of current needed at that time, and no more.

It IS possible for the voltage to be correct but there's not enough current output to meet the demand. AC generators put out three-phase current. When one of the six internal diodes fails, you will lose exactly two thirds of the generator's capacity. 30 amps from the common 90 amp generator is not enough to run the entire electrical system under all conditions and keep the battery charged. If the immediate need is only 25 amps, you very likely will not notice a problem, but as soon as you switch on the heater fan or head lights, if more current is needed than what the generator can deliver, the battery will make up the difference until it slowly runs down over hours or days.

You need a professional load tester to measure that maximum current the generator is capable of, but in addition to low current, due to the missing phase when one diode is defective, "ripple" voltage will be very high. Yours is not. It depends though on the tester. If ripple voltage is measured when the tester draws a heavy load, ripple voltage will show up at its highest value. There are some testers that measure it after that external load is turned off. Part of the battery's job is to smooth out that ripple and that is much easier to do when the generator isn't working very hard. The mechanic has to become familiar with his tester to know how to interpret the numbers.

With a newer battery, it is going to do a better job of smoothing that ripple voltage. As they age they lose their ability to do that. This is a real big problem on '87 and newer GM products. Old batteries that are still working fine are the cause of repeated generator failures. I have not heard of that on VWs or any other car brand.

You still haven't listed any symptoms that would indicate the need to test the charging system.

When a plastic assembly melts it is due to heat build-up on a pair of mating terminals and that is always caused by a poor connection to start with. Current flow through that resistance creates heat. That heat makes the terminals expand causing even more resistance, then more heat. Eventually the terminals turn black and the plastic melts. We see that all the time in high-current circuits like ignition switches, head light switches, and heater fan switches.

No one ever said generators should last the life of the vehicle. Older Chrysler units were really tough. Typically all that would happen was the brushes wore out at about 180,000 miles, and sooner if you drove on dusty roads a lot. The parts to fix that cost $3.00 and took five minutes to install. GM and Ford had very little trouble. Everyone knew how to take the GM generators apart and put new parts in them. With Fords, we just replaced the assembly rather than trying to take them apart. The '87 and newer GM generators are a different story. It is common to go through four to six replacements in the life of the vehicle, and they have shown no interest in improving that terrible design. With import cars we rarely tried to repair the generators because they like to build in the voltage regulator, and once you have the unit apart, there is no way to test it. We don't like to replace parts as a means of diagnosing a problem, and if a new regulator doesn't solve the problem, how do you charge for your time and who pays for the part that wasn't needed? That's why we just put on a whole new assembly and try to avoid thinking about the cost.

And remember, that 140 amp rating of your generator only means that's the maximum it is capable of developing. As long as the system voltage remains within the acceptable range, it will not develop more current than the car requires. Think of a municipal water tower. It might be capable of delivering a million gallons per minute if the main pipe was broken off, but as long as all the pipes remain intact, it will only deliver the amount of water that's needed, and no more. There's only a limited number of faucets and toilets taking water. To increase the flow you would have to raise the water pressure in the tower. To raise the current flow from the generator you would have to raise its voltage. Your voltage is staying within the acceptable range so it is indisputable that the generator is not producing too much current. Anyone who would say otherwise does not understand electrical theory and automotive charging systems.

Also be aware the charging system voltage will change each time you measure it at the battery. Almost all voltage regulators have some type of temperature compensation built in. Charging and discharging a battery is a chemical reaction, and any chemical reaction slows down when those chemicals are colder. We need to get the battery recharged right away after cranking the engine. We raise the voltage a little in colder weather to make that happen as quickly as possible, and we lower the voltage in warmer weather to slow the process down and prevent boiling the water out of the battery. As long as you find the voltage between 13.75 and 14.75 volts, there is no reason to be concerned.

I have never actually measured ripple voltage. Every load tester I've ever used had a bar graph to show relative ripple. Only a few segments lit up if ripple was low. If there was a bad diode, almost all the segments would light up.

In this application, diodes are either good or bad. There's no in between and there's no such thing as "about to fail". There are always six diodes, and some generators have additional diodes, but if you can get the rated current during a load test, all the diodes are good. If one of the six diodes fails you will lose two thirds of the rated capacity, but it is possible for system voltage to still be correct. The reason for YOU to measure ripple voltage is you don't have a load tester to measure output current. When we do have a load tester, if the generator will deliver the current it's designed for, within five or ten amps, the diodes are good, period. When you can get the rated current, ripple voltage is irrelevant except to diagnose an aging battery, and we don't do that very often on anything except GM products. They have a huge problem with voltage spikes when the battery gets to be more than about two years old, but that real common problem is not a problem on other car brands.

To look at this another way, all AC generators will produce ripple voltage just because they are producing alternating current that has to be rectified. It's the job of the battery to smooth out that ripple. If your mechanic says ripple is too high after the battery has been replaced, ask him what's wrong with that new battery.

I think the mechanic is confused. While he probably has your best interest at heart, he doesn't understand how AC generators work and how they fail. A diode is either good or it's shorted. Once it shorts, it never becomes good again, and not long after it shorts, it overheats and burns open. There's two sets of diodes with three diodes in each set. If any one diode shorts in each set, you have a dead short to ground and a fuse or fuse link wire burns open. That happens so rarely because the first one to short burns open long before the second diode shorts, if it ever does.

If a diode is not shorted now, you can't say one is failing, and if a diode shorts in the future, that does not cause a failure of the starter to crank the engine. That failure to crank is due to the battery being run down, and that is due to not enough output from the generator. That's the result of a bad diode. If testing shows output current is near the generator's design rating, you don't have a bad diode and you don't have a failing generator.

Even with a bad diode, output voltage will usually still be okay so we need to go one step further. It's the output current under full load that's important. You said the generator has a capacity of 140 amps. That's really high but there are some that high. If testing shows it can deliver 125 - 140 amps, you can sail off into the sunset without worrying. A diode or another part in the generator can fail tomorrow but that would be totally unrelated to the previous service. I don't think you have anything to be worried about.

Yes, most independent mechanics don't want to risk working on VWs. They are one of the world's three most customer-unfriendly manufacturers when it comes to business practices. German engineering isn't better. It's just different, and many mechanics who can transfer knowledge and experience to other brands find they can't do that to VWs without specialized training, and they are reluctant to get that training for cars that they won't be working on many of. Also, test equipment is very expensive and it's not cost-effective when it can only be used on one brand. It's less expensive to leave those cars to the experts who understand them and are already equipped with the training and tools. We ran into that with tvs too. If you understood how one tv worked, you could troubleshoot any tv, except a Zenith.

I really think you're over-thinking this issue. If you overlook one mechanic's comment about needing a new generator, what other symptoms or problems are you observing? If the answer is "nothing", you can drive off into the sunset with no worries.

My replies to you are going to be sporadic for quite a while. I had a major house fire and I'm staying with relatives who don't have internet access. I have to stop at public libraries to get online.