Expert: Brad Sears - 1/20/2008

Question
QUESTION: Hi Brad,

I own a 64 Bonneville that is in very good condition, however I recently encountered a problem that I hope you can help me diagnose.  I purchased a new, quality battery , however it immediately began to leak.(I didn't pay too much attention to the old battery as to noticing any leaks).  I exchanged it for another one, but the same problem is occurring.  I tested the charge via my battery tester, and it is showing that the system is putting out 17 volts.  I replaced the regulator, but that did not resolve the problem.  Whhen I disconnet the leads from the regulator, the battery, of course, it stops overcharging.  I checked to ensure that the regulator is properly grounded, and it is. When I turn on the headlights, the volts drop, to an acceptable level. I also believe the ground straps are correctly grounded, as well. Could it be possible that the alternator is bad? I am trying to think of what might cause this issue. The only thing that comes to mind is that I recently had to replace the ignition switch because the old one was smoking. Could that be the culprit?
Any diagnosis or suggestions that you can provide will be greatly appreciated.  Thanks, again!

Vince


ANSWER: Good morning Vince:
Well, I think that we need to check the voltage at terminal 3 on the voltage regulator with the engine running and everything connected. Terminal 3 is the switched terminal from the ignition switch. It is the feed for the rotor in the alternator and the voltage signal used to control the field circuit. It need to be within .5 volts of the battery voltage. If the voltage is lower the problem could indeed be the ignition switch.
Brad

---------- FOLLOW-UP ----------

QUESTION: Brad,

Thank you for your response.  When you mention that there might be an ignition switch proble, you jolted my memory.  Last month, mt ignition was emmitting smoke.  I was able to locate a new switch, replaced it.  Now, with that in mind, does that tell you anything else that will help in diagnosing the problem?  I am going to look at the hrness on the switch to see if there are any obviously odd signs.  Also, you mentioned checking Terminal 3...how do I perform this test?
Thanks.

Vince

Answer
The voltage regulator on your car has 4 terminals. If your car has an idiot charge light on the dash three of the terminals will be used but in the absence of an idiot light only two terminals will be used. This will be terminal #1 or the field terminal. That is the one that sends a signal to the field terminal on the alternator. Terminal #3 will be the signal from the ignition switch. Back probe this terminal with your digital voltmeter, then check the voltage across the battery with the engine running. They must be within .5 volts of each other.
Brad