Not sure which solenoid you're referring to, but I have issues with your generator testing. Don't waste your time removing it to be tested at a parts store. That's like pulling your heart out and mailing it in to be tested. Well; sort of!
The first concern is it takes about five horsepower to run a generator wide open. Test benches at auto parts stores use at most, a one horsepower motor to spin the generator. It is not possible to run them at full load, and that must be done for the most critical test. The next problem is you're observing an intermittent problem, meaning everything works fine at times. If the testing is done during one of those times, of course everything will test okay. The third problem, just as with your heart, the generator is just one part of the entire charging system. You left all the rest of it at home when you hauled the generator to the store to be tested.
The only time off-the-engine testing can be valid is when it proves the generator can't develop any output current. Other than that, it can only show if it is capable of developing SOME output current.
To address your dandy observation that the amount of current being produced was changing, there are only three possible outcomes you can find when it's tested on the engine. During the "full-load output current" test, which only takes a few seconds, you will get 0 amps, the amount of current that generator is designed to produce, or exactly one third of that amount. There is no such thing as a weak generator or one that produces a wildly-varying output.
All AC generators, ("alternator" is a term copyrighted by Chrysler, but that's not important now), develop three-phase output current that is "rectified" with six diodes. Diodes are one-way valves for electrical current flow. Only DC or direct current can be stored in a battery. Those diodes change the AC, alternating current, into DC so it can be stored and to run the electrical system. It is not uncommon for one of those diodes to short. When that happens, the most you will be able to get is exactly one third of the generator's rated current. The standard generator for your application is rated at 100 amps, which is pretty impressive. By the way, AC generators are self-regulating when it comes to the current they can produce. It is physically impossible for one to develop much more than what it was designed to produce.
If one diode fails, the most current you'll get during the full-load output current test is around 30 to 35 amps. That may not be enough to run the entire electrical system under all conditions. When more is needed, it has to be supplied by the battery. That is when you might see the lights become dim and the heater fan slows down. The battery supplies 12.6 volts when it's fully-charged. The generator supplies between 13.75 and 14.75 volts while the engine is running, so the lights will be brighter.
The next issue is the voltage regulator, which controls the generator, is built into the generator on your vehicle. It can fail too, but because they can be hard to replace, most people just replace the entire generator when necessary. Voltage regulators fail more often than diodes do, and unlike the typical diode failure, since voltage regulators contain a pile of electronic circuitry, they can fail intermittently. Plus, they can fail in multiple ways. They can stop the generator from producing any output current, they can work properly to maintain the output voltage between 13.75 and 14.75 volts, or they can short and cause the generator to run wide open, the same as that full-load output current test does.
My reason for sharing this wondrous story is only normal operation, obviously, is correct. The surging you're seeing could be the result of the regulator shorting and voltage goes too high, or the regulator could stop working, you see the lights are lower when they're running on just the battery, then when the system goes back to working properly, you see the lights going back to normal brightness as an increase that is incorrectly interpreted as surging. Either defective condition will typically act up more and more over time. The first condition will cause the battery to run down while driving. When that failure becomes permanent, you'll only e able to drive about a half hour before the engine stalls, then you'll be walking. The second condition will over-charge the battery, which can damage it, and you can expect numerous light bulbs to burn out too early. Neither condition can be allowed to continue unless you own the company that builds car batteries.
The proper way to test the entire system is with the generator on the engine, then with a professional load tester. You can start the testing with an inexpensive digital voltmeter. With the engine running, measure the battery voltage. You must find between 13.75 and 14.75 volts. If you do, that means it's okay to do the rest of the tests. The full-load output current should be close to 100 amps. The last test is for "ripple voltage". Most professional testers just display that on a relative bar chart between "low" and "high". We're happy when it shows "low". High ripple voltage is the direct result of what happens when one of the diodes has failed. The high ripple, and output current one third of what it should be, conclusively show a diode has failed and the generator needs to be replaced. Diodes are hard to replace, so we don't get involved with that.
The final concern is there can also be a wiring problem related to the intermittent problem. As I mentioned already, we need to see the results of the tests done while the problem is occurring. If it's hard to catch right now, watch what happens to the "Volts" gauge on the instrument cluster. Those are notoriously inaccurate for measuring voltage, but they are perfect for observing when something abnormal is occurring. You should be able to see if the gauge drops to the low side when the problem occurs, indicating the charging system stopped working, or if the gauge jumps up too high, indicating the system is over-charging and could cause damage.
Thursday, May 18th, 2017 AT 9:30 PM