Hi Guys,

I've just finished stripping the fuse wiring from my car and putting in a new fuse panel from a late model GM car, and after many hassles and hair pulling finally got everything wired up correctly.
During this time the car was not running for a week, and the battery had died.
I've charged the battery up to which it will start the car, but then voltage will start to drop, with radio,amps and headlights on i can sit the DMM and watch the volts drop 0.01v every 4-5 seconds, this is with the 2700mA charger still attached. Once the thermo fans kick in it starts to decrease a bit more, and about 5 minutes later its down to 9.8volts.

The battery is at least 3 years old, and my problem is a chicken/egg scenario where I'm trying to see what's causing the voltage to drop over time.
Is the battery stuffed and this the alternator won't charge it, or is the battery fine but the alternator not charging it?

I've measured the power at the battery, at the terminals, and on the alternator and they're all reading identical.

The alternator has the big bolt which i've checked it at, and a plug going onto the voltage regulator(i think thats it) with 2 prongs, one big red, which is putting out the same voltage, and a brown wire, which I'm assuming is for the gauge cluster.

I guess I'm after 2 things, one is there any wiring in the fuse panel/dash that i might have stuffed up that could cause the alternator not to activate/charge?

Is there any way to identify which item, battery or alternator is at fault?
Logic is pointing to the alternator because even if the battery is stuffed, it should still be pumping out 13+volts while the car is on yes?

 

Yep. Are you getting a steady over 14V at the battery post (ie not the cable terminal, but the batt post itself), that's a must for a working charge system.

If so it's most likely alt or wiring; however can u swap the batt temporarily for another known good one, this would be the next easiest test to perform.

 

the answer: both

Changed the alternator over with new 75amp bosch and we get voltage, however left the car idling and voltage dropped to 10volt.

Revved it and jumped back up to 13volt.

My new question is the plug from the harness with the thick red wire and tan/brown wire. Is it needed? there's no provision on the new alternator for it, so should i hook the red wire up to the battery wire on the alternator, or just leave it there?

 

Is the new alt a 1-wire type? If so, just the batt cable is necessary and forget the 2 wire plug, tuck it out of the way somewhere.

However if the output voltage is fluctuating with rpm, sounds like maybe it's not regulating/exciting properly, and perhaps does indeed need the voltage sense/excite terminals hooked up. You sure there's not a socket for the 2 wire plug, covered up with a rubber/plastic bung for transport?

Also 75A sounds small for TPI, I thought they had to be 100-110 afaik?