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1970 Pontiac Lemans - Short Circuit Troubleshooting
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HighCastle
Novice
Aug 13, 2011, 4:53 PM
Post #1 of 14
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1970 Pontiac Lemans - Short Circuit Troubleshooting
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Year of vehicle 1970
Make of vehicle Pontiac
Model of vehicle Lemans
Engine size 5.7L (350)
Mileage/Kilometers 100,000+
After leaving the key on overnight the battery was completely dead. I charged the battery completely and when I went to hook it up, sparks flew from the terminal to the cable. I did a little research on the Internet and found that you can test for a short circuit or power drain by disconnecting the negative battery cable and connecting a test light between the negative battery terminal and the negative cable. I tried it out and sure enough it lit up brightly.
Since then I have replaced the wiring to the starter, but the short persists. I removed all fuses from the fuse block and then began disconnecting other components one by one, testing as I went. The short circuit went away when I disconnected the voltage regulator. Further testing revealed that I could remove the voltage regulator from the firewall and the test light would go out. Thinking I had found the problem, I began reconnecting the other wiring. After hooking up the alternator, the test light once again lit up (even with the voltage regulator disconnected). The alternator has two connections. One is a two-wire clip and the other is a thicker single wire which hooks to a post. The thick wire should be live all the time, if I understand my wiring diagrams correctly. It is this thick wire that causes the drain/short when hooked to the post.
The voltage regulator and the alternator exhibit this behavior independent of one another, so I am led to believe that both of these are "bad". Can anyone offer a second opinion? Thank you for your time!
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Hammer Time
Ultimate Carjunky
/ Moderator
Aug 13, 2011, 4:58 PM
Post #2 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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Sounds like the alternator has a bad diode and will have to be replaced.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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HighCastle
Novice
Aug 13, 2011, 5:17 PM
Post #3 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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Thank you for your quick response. I will take the alternator in to have it tested. Will check here later if anyone has any further thoughts about the voltage regulator. It causes a short only when grounded.
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Tom Greenleaf
Ultimate Carjunky
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Aug 14, 2011, 5:50 AM
Post #4 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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Hey - HighCastle, you can replace diode trio and clean brushes in this alternator with some know how. If wiring intact you should get it to put out when back on by quickly grounding "F" on regulator on firewall but don't let it go long and have NOTHING on as it could spike over voltage and blow bulbs mostly. PITA with dash bulbs if on at the time.
Good luck. Do it or have it done is up to you. We can help,
T
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HighCastle
Novice
Aug 14, 2011, 3:08 PM
Post #5 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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My brother took the alternator into Schuck's/O'Reilly Auto Parts today to have it tested. He told them the issue we were having but they insisted that there is nothing wrong with the alternator. I am completely at a loss to explain the short if the alternator is good.
With no wiring hooked to anything in the engine compartment (Voltage Regulator disconnected as well), I can touch the one large wire to the terminal post on the alternator and the test light comes on (explained in my original post).
Does anyone have any ideas?
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Hammer Time
Ultimate Carjunky
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Aug 14, 2011, 3:28 PM
Post #6 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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We have already told you what the problem is.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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HighCastle
Novice
Aug 14, 2011, 3:30 PM
Post #7 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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OK Thanks HammerTime. I appreciate the help!
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HighCastle
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Aug 14, 2011, 4:24 PM
Post #8 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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With a brand new alternator I get the same results with the test light. Voltage regulator isn't installed. I'm totally confused. Why would the alternator have a drain in it when the car is off?
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Hammer Time
Ultimate Carjunky
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Aug 14, 2011, 4:28 PM
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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You really need to use an ammeter and not a test light. The test light doesn't give you any idea how much current is flowing. You could have a small key buzzer or something causing that. An alternator will cause a very large draw if the diode is bad.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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HighCastle
Novice
Aug 14, 2011, 4:45 PM
Post #10 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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I connected the positive battery cable directly to the positive post on the back of the alternator. I used jumper cables to ground the body of the alternator. No other wires connected to the circuit and the test light lights up. Brand new alternator, has never had other wires hooked up to it (other clip has two wires). Sorry I do not have a multi-tester. I can only assume that this is normal behavior for the alternator. *EDIT* Thanks again. I know it can be frustrating to help people of lesser knowledge.
(This post was edited by HighCastle on Aug 14, 2011, 4:56 PM)
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Hammer Time
Ultimate Carjunky
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Aug 14, 2011, 5:02 PM
Post #12 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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Yes, but that was with the old alternator. He never fully installed the new one.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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HighCastle
Novice
Aug 14, 2011, 5:38 PM
Post #13 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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Yes it is an external regulator. HammerTime is correct. I have not installed the new alternator. I am going to borrow a multi-tester to see how much it is drawing before I install it. It just seems strange that it would draw anything at all, by itself.
Positive Battery Terminal -----Cable------Positive Post on Alternator
Negative Battery Terminal -----Test Light(lights up)--------Alternator Housing
Nothing else involved in the circuit. I'll post my results with the meter when I can.
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Discretesignals
Ultimate Carjunky
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Aug 14, 2011, 6:28 PM
Post #14 of 14
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Re: 1970 Pontiac Lemans - Short Circuit Troubleshooting
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Leave the test light on the alternator for a few minutes. Does the light slowly fade out? Sometimes they put a capacitor in parallel with the diodes. Once the capacitor charges the light should go out if that is the case.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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