Hi. I'm new to this forum.

1998 Mustang V6 convertible

After my car would not start, I replaced my battery at Autozone. This is the 3rd battery, replacing an Autozone battery after 3 years. Original Motorcraft battery lasted 7 years.

Thought all was fine, my voltage gauge reads just left of centered. Brought the car back to Autozone to check my alternator. Voltage only showed 12.5 volts on battery terminals with car running = defective alternator or voltage regulator.

Replaced alternator (including voltage regulator) with Autozone model. Checked voltage at battery terminal again, now reading 13.5 volts. Not the 14.5V I expected.

Bought a digital voltmeter to track the problem.

Here's what I get:

Engine off: 12.5V at battery terminal
Engine running:
13.5V at battery terminals
14.8V at (+) lead back of alternator and (-) ground on alternator housing
13.5V at (+) lead back of alternator and (-) battery terminal
14.8V at (+) battery terminal and (-) ground on alternator housing

Note: when I state battery terminal, I'm actually measuring from the wire that is crimped on the batter terminal to eliminate any variance from a "dirty" connection to the battery post.

So it seems that I'm losing 1.3 volts somewhere between the alternator and the (-) battery terminal. The alternator is pumping out 14.8 volts and 14.8 volts is read from the (+) battery terminal and (-) ground of alternator housing.

Is this normal or should I be worried. I don't want to kill another battery. Any chance there is some cause of the voltage drop in the (-) battery terminal wiring? Is there a way for me to measure resistance of the (-) battery terminal and alternator ground using an ohm meter?

EDIT: I did a voltage drop test and found the voltage drop to be 0.02V from (+) alternator lead to (+) battery terminal. So I assume (+) circuit is okay. Voltage drop of 1.3V from (-) battery terminal to (-) alternator housing ground. Here is my problem! Any ideas? I tried to trace the (-) wire bundle from the battery but it gets easily lost under the engine. Maybe goes to starter?

I would appreciate any advice. thanks.

 

voltage drop = resistance.
And by the sounds of it, the resistance is in the ground circuit. Sounds like you may have a corrosion issue. Check the engine block to chassis grounding strap for starters. There is also a pair of fusible links on the wires coming off the alternator that could be at fault, but I am leaning more towards the ground strap being the major culprit.
Good luck. Electrical issues can be a real PITA.

 

thanks for the advice.

Problem solved. I followed the ground wiring from the battery all the way to the starter/bell housing bolt. It appears I lost a retention nut for the lead wire. The wire was simply hanging on to the stud bolt without anything keeping it attached. I really got lucky. Had it fallen off, I doubt I'd even be able to start my car and have no idea of the problem. Went to autozone, to buy some nuts. Had no idea what size I needed other than they fit a 1/2" wrench. Well, the nuts I bought did not fit so I simply removed the stud and attached the ground wire directly to the stud after grinding and cleaning both surfaces.

I then checked my battery terminals with my volt meter. 14.7 volts at the battery and my gauge is now centered again!

 

One for my memory files.