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Redland1

03’ 9900i with 04 cm870.Voltage steadily dropped and around 8.5 the stop engine light came on so I pulled over.Let it idle since it’s 17 degrees here and it died when it got to 7.5 volts.Don’t know if this has anything to do with it but a couple of days ago I replace the radiator and water pump so I had to take off the alternator.Once I put everything back together and started the truck I could hear the belt squealing and tried belt conditioner spray but it got worse and finally went away half an hour or so before the truck died.I checked the connections on the alternator while it was still running and also the battery connections and everything looked good.What could it be

Redland1

Just turned the key to on position and it’s showing 10.3 volts and on the off position it’s showing 11 volts

snailexpress

Dead alternator?

Redland1

(03-25-2023 )snailexpress Wrote: Dead alternator?

Headed now to go get one.I hope that’s the issue

arazdolgin

Had the same recently, check batteries and alternator if you have voltmeter. My alternator was barely doing over 8-10, so replaced it and way to go!

Redland1

Looks like it was the alternator.Was a bit concerned at first when I started reading up on these symptoms and there was the possibility of ecm going bad or ecm power supply cables from the battery chafing somewhere.

ktblogistics

had a situation where positive stud on alternator was loose and voltage changed when moved it, easy to damage stud when removing old alternator

Redland1

(03-25-2023 )ktblogistics Wrote: had a situation where positive stud on alternator was loose and voltage changed when moved it, easy to damage stud when removing old alternator

The first things I checked were the batteries cables I disconnected to remove the alternator in order to put the new water pump in and also the alternator cables thinking those were the culprits but didn’t find any loose.

DVT873

Would be a good idea to replace the belt and the tensioner. Serpentine belts do wear and will slip. Most of the time they don't make noise. The tensioners get weak with age. If the belt is bouncing with engine running it is definately bad. IF it is slipping it can heat damage the bearing at the front of the alternator.
Fun fact. On ProStars slippage will heat up the coil on the air conditioning clutch. That changes the coil resistance which changes the amperage going to it. The body computer watches that closely and it has to be inside a narrow range. If the amps get outside the range the body computer shuts off the clutch. It will leave a code which reads something like "AC clutch over/under current". As soon as it cools down it will work again. For a while. Not knowing this or what the code means can make you lose your mind trying to figure out whats wrong.I've seen a lot of parts, including body computers thrown at that problem.
I do not know for a fact but suspect that most trucks with a body computer work in a similiar fashion.