battery connection screw up Options

[dt4]

No radio fitted at present so don't have that to worry about (IMG:style_emoticons/default/idea.gif)

[Superhawk996]

Electricity is somewhat of a mystery to me so bear with me.

I don't understand how switching the + and - leads can cause a dead short and burn up wiring. I can understand how polarity sensitive devices (like a diode) might be harmed but why wiring?

Also, there is so very little that gets power with the key in the off position. How would the radio (for example) be damaged?

Most basic answers:

1) Since chassis is assumed to be ground, it can basically source infinete electrons (in theory) and those electrons moving = current. The chassis ground strap between the body and the transmission for example is about 1" wide and is pretty thinck braided wire that can easily handle several hundred amps drawn though it upon cranking and to run the coil and alternator. When the connections are switched (+ for negative) now all the positive wires become ground. Current goes looking for a ground path where ever it can find it - electricity always tries to follow the path of least resistance. If that path happens to be a small gauge + wire that was only sized for 1A but is now trying to support 10A of current draw being drawn through it, poof it melts.

1a) IF, every circuit had an equally sized positive and negative (ground) wires, this situation wouldn't occur. However, that isn't the way cars are wired. Often there is only a positive wire that feeds the circuit and the ground path is provided by "chassis ground". Now you have an asymetric sizing between positive and negative current paths. Positive is limited by wire size. Chassis ground is very large.

2) Electronic componenets like diodes. A diode is only designed to flow current in 1 direction. Diodes have a characteristic called Peak Inverse Voltage (PIV). A diode will resist reverse current flow up to the PIV but beyond that it will "leak". The rate of leakage depends on how much higher the PIV is and whether or not the diode overheats and fails open. As it continues to "leak", allowing current to flow backward, that situation becomes a basis for situation #1. Small gauge + wires can melt since they weren't sized for that current. Really big and well heat sinked diodes (i.e. the alternator) can have some substantial leakage before the overheat and eventually fail open. What circuits flow current in reverse up until the diode opens is up to the electrons trying to find their way home is somewhat of a guess (IMG:style_emoticons/default/sad.gif)

You are correct that if a circuit was completely turned off and has no circuit path, it would be unafffected, unless there are diodes in that circuit that fails, and, creates a path where otherwise it would not have existed when connected properly.

[Superhawk996]

Since OP was from England and now that some of the sting of the situation has worn off, I can now cut him some "extra" slack.

Some old British cars were actually reverse ground. Early Jag's were like this and I'm sure they weren't alone. In this case positive terminal does go to the chassis. Perhaps he has a collection of Jag's and had a momentary lapse when connecting the battery. (IMG:style_emoticons/default/beerchug.gif) Cheers Mate!

(IMG:style_emoticons/default/happy11.gif) The Lucas - Prince of Darkness moniker didn't come without justification.

[falcor75]

Since OP was from England and now that some of the sting of the situation has worn off, I can now cut him some "extra" slack.

Some old British cars were actually reverse ground. Early Jag's were like this and I'm sure they weren't alone. In this case positive terminal does go to the chassis. Perhaps he has a collection of Jag's and had a momentary lapse when connecting the battery. (IMG:style_emoticons/default/beerchug.gif) Cheers Mate!

(IMG:style_emoticons/default/happy11.gif) The Lucas - Prince of Darkness moniker didn't come without justification.

I wrote this for a friend a few years back when he got his first Jaguar... (IMG:style_emoticons/default/biggrin.gif)

Our Father Lucas who arth in Hell,
condemned by thy name.
Your darkness will come,
your shortcurcuit unfound,
positive earth as standard.
Give us today our burning harness,
and forgive us our use of heatshrink,
as we forgive those that use fuses.
Lead us not into electronics
but deliver us from logic.
For the darkness,
the nonexistant spark and the mysterious smoke are yours.
Now and intermittently.

Ohm.

[ctc911ctc]

Cannot resist..........

Years ago I met an older gentleman who's last name was "Lucas". He said that he had just retired (sold) a few auto dealerships in the northern SF area specialized in English cars.

I said that he must be tired of hearing the Lucas Joke. He stated that he had never heard it. So I told him....

Why do the English drink warm beer?

Because Lucas makes refrigerators too!

He was not amused, a friendship was not made.....

CTC911CTC

Since OP was from England and now that some of the sting of the situation has worn off, I can now cut him some "extra" slack.

Some old British cars were actually reverse ground. Early Jag's were like this and I'm sure they weren't alone. In this case positive terminal does go to the chassis. Perhaps he has a collection of Jag's and had a momentary lapse when connecting the battery. (IMG:style_emoticons/default/beerchug.gif) Cheers Mate!

(IMG:style_emoticons/default/happy11.gif) The Lucas - Prince of Darkness moniker didn't come without justification.

I wrote this for a friend a few years back when he got his first Jaguar... (IMG:style_emoticons/default/biggrin.gif)

Our Father Lucas who arth in Hell,
condemned by thy name.
Your darkness will come,
your shortcurcuit unfound,
positive earth as standard.
Give us today our burning harness,
and forgive us our use of heatshrink,
as we forgive those that use fuses.
Lead us not into electronics
but deliver us from logic.
For the darkness,
the nonexistant spark and the mysterious smoke are yours.
Now and intermittently.

Ohm.

[dt4]

Since OP was from England and now that some of the sting of the situation has worn off, I can now cut him some "extra" slack.

Some old British cars were actually reverse ground. Early Jag's were like this and I'm sure they weren't alone. In this case positive terminal does go to the chassis. Perhaps he has a collection of Jag's and had a momentary lapse when connecting the battery. (IMG:style_emoticons/default/beerchug.gif) Cheers Mate!

(IMG:style_emoticons/default/happy11.gif) The Lucas - Prince of Darkness moniker didn't come without justification.

thanks Superhawk but no slack is deserved in this instance
It was a monumental moment of fuckupery
I have had the battery off and on a dozen times, have 20 years of restoration experience and 35 years of diy mechanics so have no excuse.
I bet I dont do it again

[KELTY360]

The English language evolves through creative combinations of words in common usage. You sir, have skillfully woven such an example that should become a permanent part of the lexicon: fuckupery!

I can see the scene at a spelling bee clearly: Fuckupery - F-U-C-K-U-P-E-R-Y - Fuckupery.

Or better yet, the contestant asks, “Can you use it in a sentence?”

Classic! (IMG:style_emoticons/default/beer.gif)

[dt4]

The English language evolves through creative combinations of words in common usage. You sir, have skillfully woven such an example that should become a permanent part of the lexicon: fuckupery!

I can see the scene at a spelling bee clearly: Fuckupery - F-U-C-K-U-P-E-R-Y - Fuckupery.

Or better yet, the contestant asks, “Can you use it in a sentence?”

Classic! (IMG:style_emoticons/default/beer.gif)

thanks Kelty
it looks like you reside in Townsend?
that's my surname (IMG:style_emoticons/default/smile.gif)
how about that !!

[dt4]

so the latest...

I tracked an alternator down over here and fitted it at the weekend

With the ignition on I can hear the fuel pump running at the front, when I turn the key to start it is very slow to crank and is not firing, its as though the battery is virtually flat
I swapped out the battery for a spare that I believe to be good but the same result.

Looking for suggestions as to what else to check please

I am an electrical numbskull and whilst I have a circuit tester I have no clue how to use it.

[BeatNavy]

Assuming one or both batteries are actually charged it sounds like a grounding issue. Check all your electrical connections to make sure they are snug, but particularly check connections to starter and all your grounds. Specifically, make sure you've got the ground strap snugly attached from rear of gearbox to a clean ground point that's on the underside of the trunk. Also check to make sure your battery ground is clean and snug, and that the actual negative terminal is snug to battery terminal -- but do not overtighten (else it can stretch and ruin the clamp).

Good luck.

[ctc911ctc]

Think of the electrics as "Plumbing for Electrons" all of the connections need to be tight and clean or you will have 'leaks'.

I would start with all of the POWER connections. Start with the battery ground. How solid is the metal around the connection on the side of the engine compartment? If the lug on the compartment wall is connected to rusty metal, in essence, this is one big resistor and the NEGATIVE needs to be SOLID and consistent at every point it is used.

On this, a VOM might show zero ohms between two chassis points, however, when 90+ amps are being drawn, the current flow characteristics can change. RUST or loose connections.

Clean the Negative connection! Super clean and tight.

On the Positive (+) side, the cable from the battery to the starter must be in good shape. As a question of maintenance, many people replace this cable for a variety of reasons that have to do with age of a plumbing part and it getting leaky.

Make certain that the connection from the battery to the starter is solid, clean and tight.

The starter, of course, has two components - solenoid and the starter motor. if you are certain the cabling is solid - replace the starter.

Let us know how things turn out!

so the latest...

I tracked an alternator down over here and fitted it at the weekend

With the ignition on I can hear the fuel pump running at the front, when I turn the key to start it is very slow to crank and is not firing, its as though the battery is virtually flat
I swapped out the battery for a spare that I believe to be good but the same result.

Looking for suggestions as to what else to check, please

I am an electrical numbskull and whilst I have a circuit tester I have no clue how to use it.

[dt4]

thanks guys, I will follow the pointers given and let you know how it goes (IMG:style_emoticons/default/thumb3d.gif)

[Spoke]

About what could happen if the battery was reversed; check out the 914 charging circuit diagram below. With the battery connected correctly as shown, all the diodes in the alternator block current from the battery.

When the battery connection is reversed, the diodes are forward biased and shorted across the battery and will likely be damaged.

[dt4]

About what could happen if the battery was reversed; check out the 914 charging circuit diagram below. With the battery connected correctly as shown, all the diodes in the alternator block current from the battery.

When the battery connection is reversed, the diodes are forward biased and shorted across the battery and will likely be damaged.

so it should just be the alternator that is kaput?

I have had an hour checking the negative battery lead, the earth strap, live feed to the starter motor and any other wires to the body for good connections and earths. I didnt see anything untoward but cleaned them up in any case. No improvement, the starter barely responds.
I did figure how to get a reading from the multi meter and it showed the battery to putting out 12.5v
Is there a way of testing the power getting to the starter and the alternator?

[ctc911ctc]

About what could happen if the battery was reversed; check out the 914 charging circuit diagram below. With the battery connected correctly as shown, all the diodes in the alternator block current from the battery.

When the battery connection is reversed, the diodes are forward biased and shorted across the battery and will likely be damaged.

(IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif)
Agreed, many of those diodes are either welded shut (unlikely) or toasted open

[ctc911ctc]

About what could happen if the battery was reversed; check out the 914 charging circuit diagram below. With the battery connected correctly as shown, all the diodes in the alternator block current from the battery.

When the battery connection is reversed, the diodes are forward biased and shorted across the battery and will likely be damaged.

so it should just be the alternator that is kaput?

I have had an hour checking the negative battery lead, the earth strap, live feed to the starter motor and any other wires to the body for good connections and earths. I didnt see anything untoward but cleaned them up in any case. No improvement, the starter barely responds.
I did figure how to get a reading from the multimeter and it showed the battery to putting out 12.5v
Is there a way of testing the power getting to the starter and the alternator?

I would get another battery - i know you have tried another - however, this sounds like the battery is not juicy enough.

Try this test, this may take two people: put the Volt meter on the battery and then when you grind the starter what is the voltage? If it falls close to zero - try another battery. If it stays above 8-9 volts - there is a wiring or starter problem.

GOOD LUCK!

[porschetub]

About what could happen if the battery was reversed; check out the 914 charging circuit diagram below. With the battery connected correctly as shown, all the diodes in the alternator block current from the battery.

When the battery connection is reversed, the diodes are forward biased and shorted across the battery and will likely be damaged.

so it should just be the alternator that is kaput?

I have had an hour checking the negative battery lead, the earth strap, live feed to the starter motor and any other wires to the body for good connections and earths. I didnt see anything untoward but cleaned them up in any case. No improvement, the starter barely responds.
I did figure how to get a reading from the multimeter and it showed the battery to putting out 12.5v
Is there a way of testing the power getting to the starter and the alternator?

Try this test, this may take two people: put the Volt meter on the battery and then when you grind the starter what is the voltage? If it falls close to zero - try another battery. If it stays above 8-9 volts - there is a wiring or starter problem.

GOOD LUCK!

(IMG:style_emoticons/default/agree.gif) best way to test provided you have a strong battery,the starter pulls huge current,an old battery will tip over really fast with this load and a fully charged one will indicate a bad starter as appears you have checked all connections.
914 starters are pretty strong ,strip down and service it ,pretty easy.

[RickS]

Considering how often I have seen this happen on 3 bbs, I am surprised the SAE doesn’t develop a standard where batteries have round and square or star shaped pegs and the connectors have similar so there is never a screwup unless really drunk.

Good luck to the OP and yes your alternator is toast.

[dt4]

I got the Mrs to turn the key whilst I checked battery volts, one battery dropped from 12.5 to 9.5, the other dropped from 12.5 to 10.6
does that suggest healthy battery and a dodgy starter motor?
can a starter motor be tested on the bench with a battery?

[ctc911ctc]

If the wiring is solid, all connectors tight and clean - I would move to the starter mother/solenoid as the culprit Also, is the starter fully engaged? Does the motor turn? Makes that Errr,Errr, Errr noise? Or, do you hear clicking? Errrr,,, Click,Click,Errr,Errr,Click,Click???

DOUBLE CHECK THE WIRING -please,,,,,,,, (IMG:style_emoticons/default/biggrin.gif)

I got the Mrs to turn the key whilst I checked battery volts, one battery dropped from 12.5 to 9.5, the other dropped from 12.5 to 10.6
does that suggest healthy battery and a dodgy starter motor?
can a starter motor be tested on the bench with a battery?