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I went to start my '68 Gt yesterday and found that the battery was dead. I hooked up the charger and after 30mins tried it again. It worked, the car started. I checked the battery voltage and found a steady discharge. Over a period of 3-4 mins it went slowly from 13.5 to 12.5 I did a quick test of the alternator by disconnecting the connection to the +post. The engine immediately started to die. Obviously no power from the charging system, the car was running off the battery. I held a small screwdriver to the plug in the center of the back of the alternator, it was magnetized. So, now to my question: what's wrong, the alternator or the VR? If I remember correctly the VR is an old style points type. If it's the VR that's bad I'll change it for a new electronic type. Which leads to another question: what electronic VR interchanges with a '68?
 

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What was the voltage while running? Or was that it? First, i'd bring it up to a fast idle and see if the voltage rises, this would indicate that the alternator is working. The alternator should put enough power out at an idle to at least keep the engine running. But alternators don't put that much out at idle anyway. Compounded by a low idle speed or incorrect pulley size from a rebuilt unit by skew things.

How old is the battery? Don't rule the battery out either. BTW, when running the engine with the battery disconnected, shut off all accessories and lights. This will only compound the problem. Remeber, lights, ect. use a lot of power!
 

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Based on your description it is the alternator. Take it to an auto parts store that has the ability to test it (NAPA, POOP BOYS, Autozone). If it is fine have them select an electronic replacement for the VR.

Frank
 

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Remove the battery first, take to Autozone or other place having a battery load tester. Seems like its bad to me based on your description. Car should run for several hours on battery charge alone. If only running three or four minutes then the battery must be very nearly shot.
 

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You can do a simple alternator test to determine if the alternator or regulator is not functioning;

Start the engine. Unplug the harness at the regulator, with a jumper wire jump the yellow and white wire. If the alternator is OK, then it will fully charge, you will be able to hear a load on the engine and full amperage will go to the battery. This indicates a bad regulator.

If still no function, more than likely the alternator.
 

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Typically one of four problems:

(1) Alternator
(2) Regulator
(3) Battery
(4) Wiring

They can all be tested individually. Diagnosis is difficult and confusing when more than one item is bad, which happens. Bad wiring kills the Alt, bad VR kills the battery, etc.

Below is the VR test done by jumpering the A and F terminals on the regulator. If it can produce 14-16 volts at 1000 rpm with the test, you know the VR is bad. But remember, if it fails the test, all you know is that at least one item other than the regulator is bad, and the regulator could still also be bad. The best test for the alternator is to remove it from the car and take it to Pep Boys for a bench test. Wiring is a bear to test and usuall has to be done with an ohm meter and the other three components disconnected.
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From a previous post about 6-months ago:

Anyone know the correct way to wire to bypass the volt reg to test the alt and/or give me a good volt meter proceedure to find my problem?
-Nick
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Nick,
Pull the plug on the regulator, noting that the regulator is marked I A S F, from left to right. Between I and A is a gap -- the distance between A S F is the same. Construct a 6" wire (14 ga. is ok) with male spade terminals on each end. Start the car with the regulator unplugged. Plug one end into the F terminal, the other into the A terminal. Note that A is battery voltage. As soon as the A plug is engaged the engine should slow a bit as the alternator goes 'full field' or full output. Inside Ford alternators (of the era) the field coils are grounded. Applying full 12V to the field terminal puts the alternator into full output. A voltmeter connected to battery will show about 14V at just above idle, and around 16V at 2500.

If you show battery voltage only, then there is something wrong in either the wiring or the alternator. If you show voltage readings similar to above, then the regulator is at fault. Common sense says don't leave the connection hooked up too long and don't run any accessories during the test. In case you're wondering, there is a tool sold by the SnapOn/Mac guys that is a plastic plug that emulates the connections of the regulator. Plugging this tool in does exactly what the 50 cent spade plug we built does...it's just goof-proof.
-BossBill
 
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