battery volts

Stang67401 · Oct 8, 2012

hey guys ill keep and quick and simple... what would cause my battery guage to jump all over the place? when i drive and come to a stop is when i notice it the most. but whenever i hit the gas just a little the battery volts go back up. also, when i floor the stang the battery drops low which is causing me to have almost no power.. what would cause something like this? the battery and alternator tested out just fine. 1994 mustang automatic

thanks guys you have no idea how much it means!!

MustangChris · Oct 8, 2012

how much "jump" occurs?

If it just wiggles a little, this is fairly common.

I'd suggest checking your battery's water level (as applicable) and fill it if needed (with distilled water.)

Testing the battery and alt can be inaccurate... How old is your battery? Are the terminals clean? It may need replaced due to the winter. Is it leaking? Are their fluid stains on the battery? Is the ground clean, or rusted over? Are the coils in the alternator clean and red/brown? or have the faded to black/gray? How old is the alternator? have you personally tested the battery? is the belt slipping? do you have under drive pulleys?

CC'S95GT · Oct 8, 2012

Test the Alt. in the car with a digital volt meter. It should be putting out about 14.5 vdc at any rpm.
It might also be the connector on the back of the alt.
Also as Chris said, make sure all connections on the alt. and batt. are clean and tight. including grounds.

ttocs · Oct 8, 2012

start simple and check all the connections at the battery. Give them a good pull/tug and there should be NO movement from them. IF there is corrosion can get in there and cause problems.

IF that is all good check your grounds. There should be one to the body next to the battery and as well as from the block to the chassis on the passenger side bottom of the motor.

Stang67401 · Oct 10, 2012

could this be caused by a bad fuel filter?

MustangChris · Oct 11, 2012

nothing you mentioned could be caused by a bad fuel filter...

if you feel inclined to replace the fuel filter, go for it... its like, $10.00 for parts and it only takes 20 minutes to do.

slowroll96 · Oct 11, 2012

i had a somewhat simillar problem with my 96 3.8l 5spd, not sure if the setup is the same, but the ground from my batt. to chasis was loose, and there was corosion where the wire went into the termonal end at the batt. tightened ground and cleaned terminal and no more problems. hope this helps. good luck.

ttocs · Oct 11, 2012

also, under drive pullies can cause this to happen as well.

slowroll96 · Oct 11, 2012

ttocs said:
also, under drive pullies can cause this to happen as well.

ok, noob question here...what's the differance between regular pullies and "under drive" pullies?
and how can they cause this problem?
thanks

CC'S95GT · Oct 11, 2012

slowroll96 said:
ok, noob question here...what's the differance between regular pullies and "under drive" pullies?
and how can they cause this problem?
thanks

UDP's slow down the accessories and in theory free up some HP. The issue is when the alt. is slowed down to the point where the alt. can't charge the battery anymore.