Calling all electrical experts
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
Calling all electrical experts
The problem is after driving my 77 for half an hour, the battery runs down and the car won't restart. I have a new battery and starter. I charged the battery then checked the battery with a voltmeter, 12.5 v with engine off, 14.5 v engine idling, 13.5 v idling with all electrical stuff turned on. I took the alternator to local parts store for a test. They said its OK. I reinstalled it, then disconnected the NEGATIVE cable at the battery and put the voltmeter across that. Its reading 12.4 v there. Is that normal? Also, my battery will discharge in a week if I leave it connected. (My clock works.) Any ideas?
Burning Brakes
Joined: Apr 2004
Posts: 847
Likes: 0
From: Des Allemands LA
First I would check the battery-even if it's new.
Then this may or may not work but I would start by checking fuses, by this I mean:
1. Put voltmeter across battery
2. Pull fuses one at a time while watching meter (may need some help for this)
3. If voltage "fluctuates" when pulling a fuse, investigate that circuit to determine what's using the power and if it is supposed to be using power.
Also have you installed aftermarket radio, alarm or anything like that?
After checking the above items, I would start taking wires off components (alternator, starter) 1 at a time and try to determine where the drain is coming from.
While not a cure, installing a cut-off switch on the battery may help you while you try and figure this out.
Then this may or may not work but I would start by checking fuses, by this I mean:
1. Put voltmeter across battery
2. Pull fuses one at a time while watching meter (may need some help for this)
3. If voltage "fluctuates" when pulling a fuse, investigate that circuit to determine what's using the power and if it is supposed to be using power.
Also have you installed aftermarket radio, alarm or anything like that?
After checking the above items, I would start taking wires off components (alternator, starter) 1 at a time and try to determine where the drain is coming from.
While not a cure, installing a cut-off switch on the battery may help you while you try and figure this out.
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
ltlevil - Thanks. I was afraid I was going to have to start pulling fuses. BTW, battery checks good according to my charger's diagnostic system. Also I have a new radio but I am wondering about that original alarm system which doesn't work but has never been disconnected.
Burning Brakes
Joined: Apr 2004
Posts: 847
Likes: 0
From: Des Allemands LA
Bruce,
You didn't mention and I should have asked, but did this problem just start? If so think of any work that was done prior to this, I'm sure you have thought of this, but someone else reading here may benefit from the idea.
You didn't mention and I should have asked, but did this problem just start? If so think of any work that was done prior to this, I'm sure you have thought of this, but someone else reading here may benefit from the idea.
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
Steve - Problem started last November. As a "quick-fix" I put in a new mini starter. That didn't solve it, but it did take 10 lbs. off the car. Didn't drive the car Dec thru mid March. But before all this I didn't work on anything electrical.
As I pull fuses, should I be watching voltage across the battery terminals or voltage across the negative connection only?
As I pull fuses, should I be watching voltage across the battery terminals or voltage across the negative connection only?
Burning Brakes
Joined: Apr 2004
Posts: 847
Likes: 0
From: Des Allemands LA
Originally Posted by BKbroiler
Steve - Problem started last November. As a "quick-fix" I put in a new mini starter. That didn't solve it, but it did take 10 lbs. off the car. Didn't drive the car Dec thru mid March. But before all this I didn't work on anything electrical.
As I pull fuses, should I be watching voltage across the battery terminals or voltage across the negative connection only?
As I pull fuses, should I be watching voltage across the battery terminals or voltage across the negative connection only?
Le Mans Master
Joined: Jan 2000
Posts: 8,918
Likes: 27
From: New Hill NC
You describe two problems. First if the alternator is delivering 13+ volts while running there is no reason for the battery to discharge while its running. If that occurs, one of two things is happening. Either the alternator is not really charging or the battery has failed internally and is killing itself. If it dies over time while sitting there is some kind of draw on the battery at rest. This can be caused by a lot of things, usually in a circuit (courtesy clock) that is live all of the time. Or maybe an alternator diode is shorted or the the battery itself is. The common thread in both scenerios is the battery and the alternator. I would look there first, then at the lesser draws that are common, like the courtesy delay or the alarm, etc.
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
wombvette - Is it normal to have a voltmeter reading over 12 across the negative cable on the battery with the engine off? Only the clock should be drawing current at that point. Thanks for the help.
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Le Mans Master
Joined: Jan 2000
Posts: 8,918
Likes: 27
From: New Hill NC
Hmmm, I dont know that I have ever checked one that way before. I dont know why you would want to. You are basically floating the ground and I guess if you have a 12V battery in there it would show 12V. I think you are confusing voltage with amperage. Any circuit with 12V applied will continue to read 12V until and unless the resistance of the circuit approaches zero (short) and the supply cant deliver the amperage to keep it up. To check for current draw, you must place an amp meter in line with the circuit. At very low current draws of less than an amp, you are not likely to see any voltage changes on a volt meter, so it is not the weapon of choice.
Melting Slicks
Joined: Aug 2004
Posts: 2,165
Likes: 950
From: Keizer Oregon
Originally Posted by BKbroiler
wombvette - Is it normal to have a voltmeter reading over 12 across the negative cable on the battery with the engine off? Only the clock should be drawing current at that point. Thanks for the help.
You can't measure current with a multimeter unless you set up a known load and measure the voltage drop across it.........To me it sounds like you might have a small drain on the battery somewhere in the wiring....this can be a real pain in the a$$ to test...the steps that ltlevil mentioned are a good way to start.
Also, I agree on the battery cutoff switch...It's a good thing to have anyway.
Last edited by oregonsharkman; Apr 8, 2006 at 10:13 PM.
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
I do have the battery cutoff. I'd rather not use it when the car is driven fairly regularly since I loose the radio settings etc. I guess I will try the fuses. If I find anything, I will advise. Thanks.
Le Mans Master
Joined: Jul 2005
Posts: 5,752
Likes: 4
From: Austin Texas
The most common things that run a battery down are those that are unswitced. These are the horn relay, interior lights including the timer, (if applicable), the alternator, a battery that has an internal short, and a poor install of an aftermarket alarm or audio add-ons. We had one a couple of days ago, the diy-er had a similar problem, he had reversed the alternator plug-in wires when he replaced the connector...
Burning Brakes
Joined: Aug 2005
Posts: 1,126
Likes: 0
From: Hudson Falls, N.Y. 76 Vette Modified L-48
Originally Posted by wombvette
You describe two problems. First if the alternator is delivering 13+ volts while running there is no reason for the battery to discharge while its running. If that occurs, one of two things is happening. Either the alternator is not really charging or the battery has failed internally and is killing itself. If it dies over time while sitting there is some kind of draw on the battery at rest. This can be caused by a lot of things, usually in a circuit (courtesy clock) that is live all of the time. Or maybe an alternator diode is shorted or the the battery itself is. The common thread in both scenerios is the battery and the alternator. I would look there first, then at the lesser draws that are common, like the courtesy delay or the alarm, etc.
There should be no reason for the battery to die while driving unless the alternator charge is not reaching the battery, OR the battery will not accept the charge....as you've indicated wombvette...if the voltage between battery terminals is showing 13+ volts while running, I'd say the battery has an internal dead short in a cell or 2 and is not taking a full charge, I'd swap the battery first....just my 2 cents...
Wait a minute...womb just said that....better make some coffee...
Last edited by kb2fzq; Apr 9, 2006 at 06:21 AM.
Instructor
Joined: Nov 2004
Posts: 189
Likes: 0
From: Somerville NJ
My 76 started fine drove about 5 mi. went in to a store came out would not start ( dead bat.) got a jump got home was checking volts and found the wire inside the plug that goes into the alt. was broke. Changed the plug and its been fine but I now always look at the ammeter. It may be worth a look.
Racer
Joined: Jul 2005
Posts: 499
Likes: 1
From: Winnipeg Manitoba
Originally Posted by kb2fzq
There should be no reason for the battery to die while driving unless the alternator charge is not reaching the battery, OR the battery will not accept the charge....
Sounds like battery.
BTW:
If your trying to measure the clock current,
you probably won't find any.
The clock is mechanical, with an electric winding mechanism.
Two contacts close when the clock winds down,
The contacts power a little solenoid which winds it back up.
It only draws current for a fraction of a second.
You can hear it rewinding if you sit real quite in the garage for a few minutes.
Instructor
Joined: Jul 2000
Posts: 173
Likes: 0
From: Pumpkin Center CA
Originally Posted by 427V8
Sounds to me like your battery is not charged,
Or most likely you have a bad cell.
A good battery should be 14+ volts
A bad cell can also cause the bat to self discharge
Or most likely you have a bad cell.
A good battery should be 14+ volts
A bad cell can also cause the bat to self discharge
Aggree with others, use a good multimeter to check AMP draws.
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
First - thanks for all the advice.
Now the latest - Was a nice day today, so I started the car, no problem. Drove around for 45 min, speeds up to 80 at times so the alternator was spinning. Volt gage in car stayed over 13. Got home, shut it off, wouldn't restart. Cranked VERY slowly. I decided to retard the timing and try again. Made no difference. Let the car cool for 2 hours. No change. Battery voltage was reading 12.5. Checked voltage at the starter - 12.5 also. Changed starters - put my old 20 pound unit back in - no change - still cranked slowly. Decided to charge the battery. The charger indicated battery voltage as 13.0 when I started. Checked back in an hour, charger said FULL. Rechecked battery voltage, now 13.5 with my voltmeter. Guess what - now the car starts normally. I think, as a lot of you suggested, the nine month old battery is bad.
Now the latest - Was a nice day today, so I started the car, no problem. Drove around for 45 min, speeds up to 80 at times so the alternator was spinning. Volt gage in car stayed over 13. Got home, shut it off, wouldn't restart. Cranked VERY slowly. I decided to retard the timing and try again. Made no difference. Let the car cool for 2 hours. No change. Battery voltage was reading 12.5. Checked voltage at the starter - 12.5 also. Changed starters - put my old 20 pound unit back in - no change - still cranked slowly. Decided to charge the battery. The charger indicated battery voltage as 13.0 when I started. Checked back in an hour, charger said FULL. Rechecked battery voltage, now 13.5 with my voltmeter. Guess what - now the car starts normally. I think, as a lot of you suggested, the nine month old battery is bad.