low voltage

[KJThaDon 10+ year member]

I've just changed my alternator twice in the last few months. The first one was a 250 amp one and the one I put in last night is a 300. Both of them seem to drop volts when shifting, idle, and sometimes at startup. This all happens without the head unit on too.

The battery / alt test good

What could be the possible causes?

Thank you!

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[Flex68 10+ year member]

3 possibilities come to mind:

1) Voltage dropping across bad connection / bad ground,

2) Your battery is shiznit, or

3) Your regulator is shot.

Easiest to check is battery voltage to see if it is holding charge with engine off, then to see what it is with motor running, and you note that both the alt and batt check okay.

If you have an external voltage regulator, that may be the culprit....

 

[KJThaDon 10+ year member]

3 possibilities come to mind:1) Voltage dropping across bad connection / bad ground,

2) Your battery is shiznit, or

3) Your regulator is shot.

Easiest to check is battery voltage to see if it is holding charge with engine off, then to see what it is with motor running, and you note that both the alt and batt check okay.

If you have an external voltage regulator, that may be the culprit....

1. Thank you I will have to check grounds everywhere

2. Battery is only like 6 months old, here's a pic of it

http://i9.photobucket.com/albums/a89/kjthadon/IMAG0359-1.jpg

3. Voltage regulator is built into my alternator

 

[KJThaDon 10+ year member]

Ok battery with car off reads 12.53..

 

[Flex68 10+ year member]

What does it read with motor running....should see at least a +1V rise

 

[KJThaDon 10+ year member]

What does it read with motor running....should see at least a +1V rise

Sometimes it will start at 13.8 and go down to anywhere around 12

 

[mlstrass 5,000+ posts]

what brand alt? Have seen that issue with a few companies out there...

 

[MANTI5 10+ year member]

What car?

 

[Flex68 10+ year member]

You have only a single battery?

Seeing a reasonable sitting/unloaded voltage reading, then a falling idling/running voltage under load, really makes me question if ur battery is any good.

IF the alternator+regulator is sending it out, the batt sure doesn't seem to be accepting it.

If u were running 2+ batteries of different types, I'd say the weak one was draining it, but if you only have the one....

Suggestion: get a friend or family member to let you swap out ur battery with theirs....see if the issues follow ur battery

 

[mlstrass 5,000+ posts]

or start the car and disconnect the battery to test the voltage the alt is putting out and see if it's stable...

 

[All_Logix 10+ year member]

Has anyone considered the voltage drop due to idle and shift being caused by the electromagnetic mechanics of the alt. There are alot of HO alts on the market that sport BIG amperage like these you have that have absolutely terrible idle amperage.