Battery voltage drops when it is connected to the truck?

DRAMER48

MEMBER
2001 GMC SIERRA
5.3L
V8
4WD
AUTOMATIC
210,000 MILES

My truck battery when not connected reads fully charged, but as soon as I hook it up the voltage drops to around 9 volts without anything being turned on. When I go to start the truck and it acts like the battery has drained, but it usually starts but just very slowly. When it is running the voltmeter on the gauges reads between 13 and 14.5 so I figure the alternator is good. But I am lost as to what the battery volts instantly drop when hooked up. Also, there is a clicking/clanking noise that has started coming from under the truck, it is not by the wheels it is close to the middle of the truck. It kind of has the sound of a card in a bike tire. It only makes the noise when the truck is moving.

1 Reply

JACOBANDNICKOLAS

MECHANIC
109,747 POSTS

Hi,

It sounds like we may have two different issues here. First, the noise could be related to a failing universal joint. That is what I would inspect first. Here is a link to help:

https://www.2carpros.com/articles/how-u-joints-work

Now, if the battery drops as you mentioned, chances are you have a short. It does sound like the alternator is doing its job.

Here is what I want you to try. Make sure the battery is fully charged. Connect it to cause the drop in voltage. Then, remove one fuse at a time in the under-hood fuse box to see if the voltage drop stops. If it does, let me know what fuse it is.

Try this and let me know the results.

Take care,

Joe