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Bad alternator?

just put a used snowplow on my 2000 silverado 2500. put brand new motor on plow pump. when i lift the blade the volt meter drops to about 9 volts. i think i have a standard 105amp alt. would a 140amp alt help?

Is this voltage drop noticed at idle,or above 2000 rpm?

When you lift the blade, what is the engine rpm? An alternator has higher output at 1,000+ rpm. You need the spec sheet for the different alternators to find out if the 140 amp alternator has more output at the rpm the engine operates when the blade is raised.

Drops to 9 volts. Then what does it do?

The voltage is dropping because the motor for the pump is drawing a lot of current and that is most likely normal. It would help to have larger alternator and you may need to replace the battery if it doesn’t have the cranking amp capacity it should have. Have the charging system load checked to see how well it is working.

I would go for a second battery (to increase reserve capacity), not a bigger alternator.

this voltage drop is at idle. after lifting the blade the gauge goes back to 14volts.is there a kit to add a second battery setup?

You need some device that increases engine rpm when the load of the pump is put on the alternator,why won’t you try this before adding a second battery?

There are kits available at most parts stores that includes an isolator to keep the second battery from draining the first. An engine RPM kit is not the best way, IMHO, because the drag on the entire electrical system is still going to be high due to the power demand of the pump. Adding a second battery, and isolating it from the rest of the electrical system, except for the alternator, would also save the very expensive electronics, like the ECM.

You are certainly not making best use of the alternator when you restrict it to only idle rpm.

The OP needs to take advantage of the increased alternator output that 2000rpm would offer.