I'm confused. The engine doesn't start unless you let it idle first? That means it is running.
What you're describing is a failed diode inside the generator. The most common generator for your model was a 150-amp unit, which is really huge. A standard GM generator back in the '70s and '80s was a 65-amp unit.
All "AC generators" put out three-phase current which is very stable and efficient. When one of the six main diodes fails, you lose one phase. That cuts the maximum current it can develop to exactly one third, or in this case, near 50 amps. That would be enough for most cars, but given the high number of computers, along with the basics like electric fuel pump, ignition system, lights, and keeping the battery charged, that may not be enough for your car. The battery will have to make up the difference as it slowly runs down over days or weeks.
You'll need to have the charging system tested by a mechanic to identify a failed diode. You can start the testing yourself with a basic digital voltmeter. If you've never used one, check out this article first:
https://www.2carpros.com/articles/how-to-use-a-voltmeter
They're using an "auto-ranging meter" here. That's an expensive feature you don't need. You can find a perfectly fine meter at Harbor Freight Tools for around $7.00. Also look at Walmart, or any hardware or auto parts store. I can help you set it up, if necessary.
First measure the battery's voltage with the engine off. If it's good but fully charged, it will read 12.6 volts. If you find it's close to 12.2 volts, it's good but fully discharged. It should be charged at a slow rate for a few hours with a portable charger. If you find it's near 11.0 volts or less, it has a bad cell and must be replaced.
Next, measure the battery's voltage again with the engine running. Now it must be between 13.75 and 14.75 volts. If that is okay, it only means it is okay to do the rest of the tests, but those require the professional load tester. Besides this "charging voltage test", they will measure maximum output current, and "ripple voltage". A few testers that can make paper printouts will list ripple voltage as a value. Most testers just show it on a relative bar chart as "low" or "high". If ripple voltage is high, it's a good bet all they'll be able to get under the full-load output current test is around 50 amps, not the 150 amps we want to see.
If your charging voltage is lower than 13.75 volts, raise engine speed to roughly 2,000 rpm. All generators are inefficient at lower speeds. If the voltage comes up at higher engine speeds, that is further proof the generator is not able to develop its full rated output current. It should easily keep up with demand at idle speed.
If testing verifies a diode has failed, be aware this has been an extremely common problem with all GM vehicles starting with 1987 models when they redesigned their generators. That new design develops huge, harmful voltage spikes that can damage the diodes, the internal voltage regulator, and can interfere with computer sensor signals. The battery is the key component in damping and absorbing those spikes, but they lose their ability to do that as they age and the lead flakes off the plates. It is real common to go through four to six replacement generators in the life of the vehicle. To reduce that number of repeat failures, always replace the battery at the same time that you need to replace the generator unless it is less than about two years old. That also means a different good, but old battery can cause the same repeat failure. Your old battery can work fine for a few years in a 1986 or older model.
We often rebuild or repair generators for ourselves, but this is one design you don't want to bother with. First, it's just about impossible to get it apart without damaging other parts, and if you do get it apart, there's no way for us to test the voltage regulator or brushes. Rebuild kits include all the parts that commonly fail so you only have to do this miserable job once, but those kits can be very expensive.
One more thing to be aware of is when you do need to purchase a replacement generator, be sure to ask for prices for all the warranty options. Don't assume the best warranty will cost more. My friend recently put his fourth generator in his GMC Suburban, and this last one was no cost to him due the warranty. One with a lifetime warranty a few years ago cost him $150.00. A similar one from a different supplier would have cost $200.00 with a one-year warranty.
Now that I've shared all that, don't overlook a slipping serpentine drive belt. If it is wet or has been contaminated with any type of belt "dressing", it can slip over the pulley. It can take a lot of power to run a generator. If the belt is slipping, you won't always hear a squeal. Also, with the engine not running, check if the belt is loose. You should be able to tug on it with your hand, then is should go back on its own under spring tension when you release it. Those spring-loaded tensioner pulleys can become rusted tight in a position where they aren't holding enough tension on the belt. The clue to this is you won't get the 150 amps we want to see under the full-load output current test, and you will not see the 50 amps we'd expect if a diode has failed. It will be some other value not close to either of those two values. Often the first belt squeal you hear is when this full-load test is being performed. All of these professional tests take just a minute or two, unless further diagnosis is warranted. On many newer models, the Engine Computer has been added to the system to really complicate what used to be a very simple system. Your mechanic will determine what type of further diagnosis is needed.
Also check out this article to get you started:
https://www.2carpros.com/articles/how-to-check-a-car-alternator
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Friday, January 27th, 2023 AT 4:35 PM
