I'm trying to trouble shoot my charging system. I've clean the battery terminals and installed a new serpentine belt. (Goodyear gatorback) The batteries are charged up and read about 12.5 volts, but the indicator windows are dark instead of green. When I turn the key on, bump the starter and leave the ignition on, voltage drops to 11.9V. With the truck running, voltage output at the back of the alternator is 11.9V.

Can bad batteries limit the charging voltage? How can I isolate the problem to avoid throwing parts in until it's fixed?

Thanks

Ron

 

Take your batteries out and have them load tested.
My truck charges at ~13.5 - 14 v.

 

Make sure when you check the alternator output you have a good ground. 11.9v is a little low.

 

I would remove the batteries, charge them, let them sit overnight, and see what they read. They will read hot right after charging, but it's after they sit a bit that matters. Also, you should have close to 14 with the batteries installed and truck running.

 

Well I got them load tested. They're fine so I guess it's alternator time.

Thanks for all the replies....

 

Check your fuses first or test the alternator.

 

The new alternator did the trick. I wonder if the wife will consider it as her Christmas present? Ha! Good luck. I'm glad she never reads these forums.

 

Good to hear the problem's solved.
Now I think I need to load test MY batteries.
The Grid Heaters were dragging them waaaaaaaaaay down this AM.

 

Yep, winter is here!

 

I just bought new batteries and they still go way down when the truck is running, rebuilt alternator, and lights flicker going down the road, no matter what speed I am at. Could use a little help..

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'97 4x4, #100 plate, auto, DTT tranny, 168k, kdp soon do be dead...

 

First thing I would do is check the voltage with a meter - Should come up to around 14 when charging properly as mentioned above. Clean battery posts - clean inside post connectors - ends tight - no corrosion. Note that you can take a voltmeter and test between each part of a connection - post and lug - bolt and connector etc. If there is a reading then the connection may have corrosion, be loose or dirty. When clean there should be no reading between separate parts of the same connection. This problem is most common on battery cable connections. After cleaning, battery terminal protector spray can be used. Losses such as this - bad ends etc. can keep the charging system from operating properly.

 

I just put in new Post connectors and all the ends are tight. Cleaned with a great cleaner, and then used a protector on those ends. When alternator was rebuilt they tested and read the 14 volts. That is why I am at a lose here. Could this be a bad ground? Or something else? I am very confused...

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'97 4x4, #100 plate, auto, DTT tranny, 168k, kdp soon do be dead...

 

I like to use a meter for these things - even a fairly inexpensive one will do. Visually inspect the wires - make sure you don't see something obvious then hook the common to a good ground and positive of the tester to the center of the negative battery post. If you get a reading there it would indicate a bad connection. While you are there check the output from the alternator to ground - if it was 14 on the test stand it should be similar here with the engine running. Stopped it should indicate battery voltage on the output terminal of the alternator - once the engine is started it should come up to 13.5 to 14 or so as the battery charges.

Just checked mine as an example for you - 12.8 at output terminal of alternator sitting -rising to 13.95 about 30 seconds after starting running at idle. Yours should be similar if the alternator is working properly.

Here is a bit more info from another thread - be sure to not disconnect the batteries with the engine running - could blow something. The meter would be the best place to start. Search alternator for more info.

https://www.dieseltruckresource.com/...ator+regulator