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Old 11-20-2004, 06:45 PM
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reversed polarity..what have I toasted?

Today was supposed to be the great day firiing my new 383 engine after a very long period of working at it but it turned out bad!

I had installed a cut-off switch and adversely inversed the ground for the positive wich as a result ended-up having a positive grounded chassis an negative lead flowing through the different circuits.....

I've put the switch (ignition) on and saw some smoke from the alternator..thinking it was my work lamp touching something I went out and saw the whole wire strand running from the alternator just frying and smoking...ran back to cut out everything.

Now I just don't know where to start! I'm close to total ignorant into electrical matters.

The car is an english two seaters sports that was modified with V-8 power..It has an MSD ignition system with retarding (timing) system,electronic (smiths) tach,electric fuel pump,these are components that I wonder if they got affected. What about the starter...I haven't engaged the switch further into the starting position..?

The engine is carbed without any modern electronic devices else the MSD.

Could it be the damages limited to a toasted alternator since it appears only the wires from it were roasting?

All advices are appreciated,thanks,

Ron.

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Old 11-21-2004, 07:58 AM
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Arrow

The wires and alternator are the most likely victims. Reverse polarity doesn't have any detrimental effects on most items. The Alternator, however has diodes (one way electrical circuits) that will get smoked if hooked up in reverse. I'd replace the alt. and repair the wires and try it again, you should be fine. To be truthful, I'd probably repair the wires and try it before getting a new alt., they are pretty tough and with the wires melting may have saved the diodes. Just check charging output after it's repaired to see if the alt. survived.
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Old 11-22-2004, 04:33 AM
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Reverse polarity

Doc here

Anything Digital, or semiconductor that WAS on most likely took a hit...Radio/CD player, Digital Clock, Your MSD brain box may have received Damage if it has a micro chip in it...

Analog Stuff will fare much better...starter etc...

The Diode bridge in the alternator blocks the flow of current from ground to power, if you reverse bias the diodes (reverse the leads) current is allowed to pass and you have a dead short. Hence the melted harness...

Put in a new diode bridge and internal regulator(if so equipped) or replace the alternator, replace all the burned wires, check and replace if needed the fusible link (you do have one, right?)

Check the fuse on the MSD..hook up the battery correctly this time, and fire it up...

If it won't run you may have toasted the brain box on the MSD..however these usually are reverse polarity protected by a zenier diode strapped across power and ground on the circuit board usually D1 on the board, If this is the case, you can replace the diode (or cut and lift one side of it) and it should run.

Test the radio clock, tach and any other Non analog device you have ..If any of these are dead or keep blowing fuses, the zenier diode thing applies here too..

It just depends on if you are inclined to tear into the circuit boards to save the units.

The zenier will usually be a little glass diode that goes from power to ground on the circuit board.

if it has one, usually is the first component from the power wires and should be marked D1 or Z1.

It's job is when it sees reverse polarity, it Crowbars shut, making a dead short, blowing the fuse time after time when you apply power and a new fuse.

You can cut and lift one side of this diode, replace the fuse and it will run normally, but you will no longer have reverse protection..so it's advisable to replace it, which requires minor soldering on the circuit board.

But if your lucky, you'll only have to fix the melted wires, and alternator and it will be all good.

BTW, it sounds like your not running a fusible link, (else it would have burned up) If not you should install one.

Doc
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Old 11-22-2004, 07:18 AM
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Docvette,

Thanks for a very complete look into my problem...surely hoping for the least dammage theory!

The car is a 1977 TVR that saw its original guts (english) replaced with all american things...383 chevy stroker and all.

the electric is a mixture of both original and improved! I haven't done the work and looking behind the dash makes me doubtfull the least I can say.

The alternator is a 70 amps.Denso. Yesterday I spent many hours just to track the melted wires..from what I see it is the big 10 gauge or so,from the alternator that melted but doing so it affected some other ones that were in its nearby basically the two smaller gauges wires that also go to the alternator.

I've just ordered a book on cars electrical (all new stuff to me) and since winter is here anyway I might just make it a winter project to go through all the system and make it working and safe...

thanks for the interest,

Ron.
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Old 11-22-2004, 02:59 PM
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Wiring

Doc here

That's the best way to go!

Take your time, do it right!

IMO, Robert Bosh invented the dead short, compounded By Lucus! LOL

Seriously,I have had a lot of malady's with English cars wiring in the past..But the older ones, the ones with Cotton wire and screw terminals instead of harness plugs! I think a 77 is out of that window.

In addition to your schematics, You might want consider purchasing the following,:
  • A fair DVM (digital Voltmeter) That can measure :
  • Volts,
  • Amps,
  • Ua (micro~amps) ,
  • Resistance, and:
  • diode check/continuity check.(With a "Beeper")
You should be able to find one for about $39.00 locally,like at "Rat Shack" or less on the Internet, if your willing to wait for shipping.
  • Also a good 12 volt Test light, with a sharp probe on one end and alligator Clip on the other end.
  • Solder Iron (about 40 watts ) is good to add along with 50 feet of 22 Gauge Acid Core Solder.
  • Heat shrink tubing, is very handy and:
  • butt Splice kit (assorted Blue, Red, And Yellow) and Crimp pliers .

You Can get a heat shrink gun (about $40), but if you don't want to spend the money, you can use a hair dryer (it takes a lot longer to shrink tubing) or an open flame (mini torch, or butane Cigarette lighter)

These are ALL good tools to Add to your tool box if your getting into the hobby for under $150! And learning and doing your own Electrical Work successfully, will save you a TON of bucks!

If the Damage to the harness is TOO server, you may want to consider a Kit like Painless and put it all back new.

You might want to Check Your local library, Reference room Desk, for for your schematic 's .. They often have the factory Manuals you can copy there, or may be able to outsource them for you. A lot cheaper than buying one from the local auto parts store. Here, our's does, and I use it all the time for obscure Schematics.Also a good place for "How TO Books" on Auto Electrical.

The Most important Items To Consider:
  • Fusible links on the Starter to the wire going to the rest of the car's electrical, Should be located at the solenoid main terminal, and about a 10 gauge wire usually red.
  • A Fusible link off the car's Alternator Main wire...for obvious reasons!
  • RELAYS to operate high current devices, like lighting, heaters, and electric cooling fans...

A Relay will provide a lower amperage Control circuit (the switch to Coil) to control a higher current device (the relay contacts) You'll deliver more current to the device simply because of no current loss through an under~rated switch alone (headlights will be brighter, Fans run faster) ..and you'll eliminate the danger of wiring/device burn outs from over current at the switch.

Good luck!
Keep us informed of the progress!
And post a pic of that beast!
A 383 in a TR must be sweet..

Doc
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Old 11-22-2004, 08:35 PM
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Doc,


I've added few pictures on my gallery....these are two years old picts. Basically it looks similar except the engine got rebuilt since then and valves covers are changed and headers got ceramic coated and engine got painted same color as the car...I'll post new picts. when I'll be through...humm thinking about my past experience gone through (totally) could be very long from now!

regards, Ron.
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Old 11-22-2004, 09:38 PM
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Hey Ron,

Just looked at your gallery and that is a beautifull car.

John
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Old 11-22-2004, 09:44 PM
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Yeah!

Doc here:

I totally agree!

A sleeper on the street...And probably a monster in the quarter...

Doc
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Old 11-26-2004, 06:02 PM
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Re: Reverse polarity

Quote:
Originally posted by docvette
Doc here

Anything Digital, or semiconductor that WAS on most likely took a hit...Radio/CD player, Digital Clock, Your MSD brain box may have received Damage if it has a micro chip in it...

Analog Stuff will fare much better...starter etc...

The Diode bridge in the alternator blocks the flow of current from ground to power, if you reverse bias the diodes (reverse the leads) current is allowed to pass and you have a dead short. Hence the melted harness...

Put in a new diode bridge and internal regulator(if so equipped) or replace the alternator, replace all the burned wires, check and replace if needed the fusible link (you do have one, right?)

Check the fuse on the MSD..hook up the battery correctly this time, and fire it up...

If it won't run you may have toasted the brain box on the MSD..however these usually are reverse polarity protected by a zenier diode strapped across power and ground on the circuit board usually D1 on the board, If this is the case, you can replace the diode (or cut and lift one side of it) and it should run.

Test the radio clock, tach and any other Non analog device you have ..If any of these are dead or keep blowing fuses, the zenier diode thing applies here too..

It just depends on if you are inclined to tear into the circuit boards to save the units.

The zenier will usually be a little glass diode that goes from power to ground on the circuit board.

if it has one, usually is the first component from the power wires and should be marked D1 or Z1.

It's job is when it sees reverse polarity, it Crowbars shut, making a dead short, blowing the fuse time after time when you apply power and a new fuse.

You can cut and lift one side of this diode, replace the fuse and it will run normally, but you will no longer have reverse protection..so it's advisable to replace it, which requires minor soldering on the circuit board.

But if your lucky, you'll only have to fix the melted wires, and alternator and it will be all good.

BTW, it sounds like your not running a fusible link, (else it would have burned up) If not you should install one.

Doc
Docvette,


I had looked for a fuse on the feed line to the MSD system but cannot see anything that look like a fuse to me....altough the 12 v. 10 guage feeding wire to the 6400 MSD box doesn't go directly into the box ....intercepting te said wire there is a device that I ignore the function (like about for everything in that system but at some deeper extent!) it is a cylinder of about inch and a half in diam. two inches long with two terminal posts on its top...two black 10 guage wires (GRD?) gets on one post and two same size but posit. gets on the other.I can read on it: Mepco Electra
26000 uf,
15V DC 20 surge,
85c max ambient...Could this be the protective device you are referring to in your answer to my initial post?

My MSD include a rev. limiter and a variable (at distance) timing..there was no chip into the rev. limiter at the time of my mistake..

thanks again.

Ron.
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