Alternators don't get replaced unless they have a problem. They aren't a maintenance item. If the charging system does develop a problem, the first symptom is the battery is not strong enough to crank the engine for starting. That is overcome with a jump-start or by placing a portable battery charger on it for a few hours at a slow charging rate. If the charging system fails completely while you're driving, you'll see the dash "Volts" gauge drop slowly and / or a warning light or message. At that point you'll be able to drive up to roughly a half hour before the engine stalls; less if a lot of lights and the heater fan are turned on.
The alternator can partially fail in another way that limits how much current it can develop. If that isn't enough to meet the demands of the entire electrical system, the battery will have to make up the difference as it slowly runs down over days or weeks. You may or may not get a warning message on the dash.
You can start the testing of the charging system yourself with an inexpensive digital voltmeter. If you aren't familiar with them, start by looking at this article:
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 for around $7.00 at Harbor Freight Tools. Also look at Walmart or any hardware store. If you need help setting it up, I can help with that.
Start by measuring the battery's voltage right at the two posts, with the engine off. You should find close to 12.6 volts if it is good and fully-charged. If it's near 12.2 volts, it is good but fully-discharged. That's about the time to expect it to crank the engine too slowly to start. It will need to be charged with a portable charger. If you should find the voltage is down to around 11.0 volts or less, it has a shorted cell and must be replaced.
Next, measure the battery voltage again with the engine running. Now it must be between 13.75 and 14.75 volts. If it is too low, it will usually stay around 12.6 volts or less. That tells us the charging system is dead and the truck is running on just the battery. Unless the system starts working intermittently, you'll only get about a half hour of driving before the engine stalls. A common failure at high mileages is worn internal brushes. Those almost always cause intermittent operation, often for months, with the periods of failure becoming longer and longer.
If the charging voltage is between 13.75 and 14.75 volts, that only means it is okay to do the rest of the tests, but that requires a professional load tester, so you'll need to visit your mechanic for that. He will repeat the charging voltage test, then perform a "full-load output current" test, and a test for "ripple" voltage. A few testers that can make paper printouts will read ripple voltage as an actual value, but most testers just show it as "low" or "high" on a relative bar chart. I mentioned earlier that an alternator can partially fail. They all have six "diodes" that are one-way valves for electrical current flow. When one of them fails, you will lose exactly two thirds of the alternator's maximum current capacity, and ripple voltage will be "high". The most common alternator for your truck was a 136-amp unit. That's close to what should show up for the full-load output current test. With one failed diode, all you'll be able to get will be very close to 45 amps. That might not be enough to meet the demands of the entire electrical system under all conditions. On the other hand, if you rarely use the head lights, and the heater fan is always on a lower speed, it can be possible to not notice any symptoms except for an unusually loud whine on AM radio
This professional charging system test just takes a minute or two as long as there is not an intermittent failure involved.
It sounds like you have to hold the accelerator pedal down a little to get the engine to start and for it to stay running. You didn't say if you disconnected the battery during the recent service. If you did, the Engine Computer lost its memory. Once the battery is reconnected and the engine is started, most of the sensor personalities and fuel trim numbers will start to be relearned right away, without you even noticing. The one big exception is "minimum throttle". Until you do that procedure, the engine may not start unless you hold the accelerator pedal down 1/4". It will not develop the nice "idle flare-up" to 1500 rpm at start-up, and it will tend to stall at stop signs.
Friday, February 24th, 2023 AT 3:40 PM
