Battery installed backwards

D

Dick So. Cal.

Senior Member
A new member to our club was having problems with his car cutting out. He had to be towed home last week. He was given the car from his dad and has little knowledge of it's history. He called me and asked if I could help him get it going. I got a crew together (three of us) and started checking things out.
One of the first things we noticed was the 6 volt battery was in backwards (negative to ground). Kinda confusing to me because it had run. Several other things were noted and fixed, ie. timing, point gap, corroded terminals, misadjusted carb, etc. The car has an alternator.
We decided to correct the battery installation, and start the car. The car had a battery cutoff switch installed also.
When we turned on the ignition switch, smoke came out from the dash. We quickly turned everything off and tried to figure out what was wrong. Not knowing the problem, we disconnected the alternator and tried it again. No smoke this time and it started. After a few adjustments, it was running pretty well, even without the alternator. It was running off the battery.
I attempted to reconnect the alternator wires while it was running, but it sparked and shorted the ignition so I quickly detached the wire. The car still kept running

Question: What are we doing wrong and how do we get this thing set up properly, (Pos ground) and charging? Is there a way to polarize or check the alternator and get it charging properly?

Also, can a battery be "charged" backwards by improperly attaching the cables?

Any assistance would be appreciated.

Dick 'sparky' Wyckoff
 
Frank is exactly correct! Look on the exterior of the alternator and you will see the required---"NEG GROUND"!
 
IMHO, you just blew up the alternator. They are polarity sensitive and if it was charging with the negative ground 6V battery, it won't work with a positive ground and by hooking the battery up to positive ground, my bet is the alternator is trashed.
 
What a great 'we'll help you' to a new club member! Once wiring smokes, it is trash. If you have any conscience you will pay for the repair or replacement of the alternator and all damaged wiring. Only an oscilloscope on a load test bench can determine if internal diodes have been damaged. Replacing the alternator would be cheaper.

This still does not address diagnosis of the original 'cutting out' complaint. Removing parts and making multiple changes increases the difficulty of finding the original fault.
 
I also have done this once- tried to wire up the alternator for a positive earth when it should have been negative.- Fortunately as it kept sparking when I connected it I went no further. No permanent damage was done and the alternator worked Ok once I realised and got the polarity correct for it. You tooo may be lucky - try it before you trash it.
 
A battery can be charged backwards, but only if it's completely discharged first. Look for the largest post and + symbol, then connect a voltmeter to determine if it is charged backwards.

In all my years as a mechanic I've only seen this done 3 times. Once was in the Army with those large batteriies, and the guy that didn't know what he was doing installed vaporized the battery post.:eek:
 
This may be cheaper than a new alternator.:D

Bob
 

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Be careful here. There are modern alternators that have been modified from negative to positive for the A. I have one on my bench right now waiting to be installed. So the alt may start out life as a 6 or 12v neg and actually set up to be a 6v pos grd.
The new markings are a very faint 6 v p.
 
if it smoked then the jury is in. it was deliberately wired to neg ground because it had a neg ground alt.

As far as a car itself, (the motor, not the gen or alt), it doesn't really care much whether pos or neg ground; lights don't care, horn doesn't care, starter doesn't care. While it is true the spark is better one way than the other, the engines will still work. The spoiler was the neg ground alt. ya gotta look at the whole picture....
 
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My '28 has a negative ground and I am not going to mess with it. I was surprised to see it though.
 
My 29 is negative ground 6v too. Confused the hell out of me trying to get the rear high-mount LED going!
 
Dick, what a great nickname. The alternator may have survived. When you get it all done use a fuse.
All that smoke was some load the alternator did not see. Worth checkin it and if you blew the rect you can put one in yourself. When guys come in after having all the fun you've had I typically rob a core rectifier and donate it. I'm a bit of an odd duck however.
 
Just for kicks and giggles, I'm going to show why I believe an oscilloscope load check is in order after the ordeal described.

BOTH of the following waveforms represent an alternator that does work, but the second one has a damaged diode and will crash at about 1/2 the rated capacity. A simple voltage output test, internal no-load diode testing with a meter, or an in-car ammeter will not reveal the defect. Only an oscilloscope:

Good, fairly well matched diodes:
07f860cc7c0b062b543d5ce4878ef82c0c4df1cf_small.jpg

Compromised diode:
46366e1a2ab1d2c0548f35991f1263665fe8087c_small.jpg

Here's another one. The alternator checks OK for voltage out, but . . .
BlownRectifier.gif
 
Now you see why generators are so nice to have!:D

As Mike shows, on modern cars with alternators you might not know you have a bad diode until you drive at night with the air conditioner on and find the battery going dead.:eek: An alternator with an open diode will still charge the battery, but not keep up with the load.
 
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