alternator output
Thread Starter
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
alternator output
i recently installed a new battery in my 72, after having it tested as being bad after 4 years of service. all seemed ok until yesterday when i tried to start it and it was dead. i charged it for about 2 hours and it started right up. the alternator output voltage shows 12.5 (about 1 volt low) with the battery connected, and with the engine off, the battery voltage is 12.25.
two questions, could a battery malfunction cause the alternator voltage to drop to 12.5, and what could cause the alternator to put out only 12.5 volts?
two questions, could a battery malfunction cause the alternator voltage to drop to 12.5, and what could cause the alternator to put out only 12.5 volts?
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
Wouldn't be the first time in my experience, that a new battery was bad right out of the box. But based on the voltage readings you mentioned. I'm thinking alternator, or voltage regulator. Not sure if your charging system is an alternator internally regulated, or an externally regulated set up. If recently jump starting another car the wrong way, may have popped something. If you can pull the battery fill caps off, see what happens with a charger on. If one cell isn't bubbling, then your answer there. If fine, then time for more checking into the charging system. Kinda sounds like the way you'll be going.
Pro
Joined: May 2011
Posts: 581
Likes: 5
From: BC
Rule of thumb is that at start system voltage should be a min .5 volts higher than standing battery voltage. So, it does seem like you have a charging system problem.
However, a shorted battery or a heavy system draw with engine running can cause low voltage like that.
You want to bypass the regulator and check alt output as a next step.
Process differs on int reg and ext reg.
Steve g
However, a shorted battery or a heavy system draw with engine running can cause low voltage like that.
You want to bypass the regulator and check alt output as a next step.
Process differs on int reg and ext reg.
Steve g
Advanced
Joined: Nov 2005
Posts: 51
Likes: 0
From: Perth
Have had a similar problem with my 74. Got to stage was disconnecting battery but still having to charge battery occasionally. Got a 'rebuild kit' off the net for the major components in the alternator. Now alternator reading is 14.5v instead of 12.2v starts every time and haven't charged battery since.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
There should be a way of checking the max. alternator output while in the car. That would tell you if the battery was bad (not accepting a charge) or the alternator was bad (not producing adequate charging voltage). Another alternative is to rig up a substitute battery (from another vehicle) with jumper cables...to test if the alt. charges at a higher voltage with it.
Pro
Joined: May 2011
Posts: 581
Likes: 5
From: BC
There should be a way of checking the max. alternator output while in the car. That would tell you if the battery was bad (not accepting a charge) or the alternator was bad (not producing adequate charging voltage). Another alternative is to rig up a substitute battery (from another vehicle) with jumper cables...to test if the alt. charges at a higher voltage with it.
But in the real world all you would normally do is turn on all the electrical items in the car. You don't care if it's not as much as the alt is rated for, that's as much as the car is ever going to draw anyways.
Bypass the regulater. If it charges 14 volts or more with the reg bupassed but won't without, you need to replace the reg. If bypassing the reg does not bring the voltage up you have alt issues. Pull it apart and go through your checks there.
To bypass the reg on an Delco SI alt there is a small hole in the back of the alt with a metal tab at the bottom of it. This tab is on the reg. You insert a small screwdriver and ground the tab against the side of the case. I'll try and find a pic, there's several small holes back there. Inserting screwdrivers blindly into the back of alternaters is not a good idea.
I'd have to look up bypass procedure for a Delco DN (ext reg) if you need it. It's been a while.
Steve g
Thread Starter
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
while i was checking voltages at the #1 and #2 terminals (as per the manual) i noticed that the terminals looked corroded. i cleaned them and the alternator now shows 13.5 volts output. 
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