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Old 06-27-2010, 07:09 AM
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Alternator run on

i found today that if i switch the motor off it continues to run until it runs out of fuel,, is it possible that the alternator is backfeeding the ignition system.. if i disconnect the alt light wire at the alternator motor stops as soon as i turn it off, also if i remove the bulb it stops when turned off.. when i turn it off the warning light comes on and stays on until motor runs out off fuel..alternator is a powermaster 7294

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Old 06-27-2010, 12:22 PM
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Quote:
Originally Posted by zephyrc
i found today that if i switch the motor off it continues to run until it runs out of fuel,, is it possible that the alternator is backfeeding the ignition system.. if i disconnect the alt light wire at the alternator motor stops as soon as i turn it off, also if i remove the bulb it stops when turned off.. when i turn it off the warning light comes on and stays on until motor runs out off fuel..alternator is a powermaster 7294
A diode in that line will solve your issue. This is common with some aftermarket harnesses that are not 100% correct or poorly designed home wiring jobs. The easiest fix is the diode. If you search here for the word diode you should get the info and part # you need.
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Old 06-27-2010, 01:05 PM
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should the voltage sensing wire be run from the battery or can i join it into the charging post at the alternator
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Old 06-28-2010, 09:15 AM
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Quote:
Originally Posted by zephyrc
i found today that if i switch the motor off it continues to run until it runs out of fuel,, is it possible that the alternator is backfeeding the ignition system.. if i disconnect the alt light wire at the alternator motor stops as soon as i turn it off, also if i remove the bulb it stops when turned off.. when i turn it off the warning light comes on and stays on until motor runs out off fuel..alternator is a powermaster 7294
From your statement you do not have the alternator indicator light wire connected correctly.The wire that runs from the alternator #1 terminal to the alternator (idiot) light, should then go from the bulb to a switched terminal on the ignition switch. I suggest using the ignition terminal.
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Old 06-28-2010, 05:23 PM
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frisco,, the warning light is wired as you have shown,, i have ordered a diode to go in the line
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Old 06-28-2010, 08:38 PM
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Unrealted to your run-on issue but the #2 terminal shouldnt go to the battery (jumpered). Its the sense line that tells the alternator if it needs to output more to charge the battery and ideally should be as far from the battery as you can get (usually the fuse box +12). No harm in just jumpering it other than the fact that it doesnt really sense that you've lost voltage far away from the alternator at the fuse box.
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Old 06-29-2010, 08:42 AM
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Quote:
Originally Posted by zephyrc
frisco,, the warning light is wired as you have shown,, i have ordered a diode to go in the line
To quote myself from the previous post...The wire that runs from the alternator #1 terminal to the alternator (idiot) light, should then go from the bulb to a switched terminal on the ignition switch. I suggest using the ignition terminal.

If you wire it this way you have no need to add a diode.

Here is another quote from a previous post from me on the same problem.

If using an indicator light, you would use a single element bulb with a non-grounded (plastic) socket. It will have two wires coming from it. It does not make any difference which wire connects to the #1 wire coming from the alternator. The other wire from the socket connects to the ignition terminal on the ignition switch. When you turn on the ignition switch, current flows thru the bulb to the alternator (internally grounded) and will light up the bulb. Once the engine has been started the light will go out.

If you prefer to use a diode (not a resistor), you will connect it so that the current will only flow from the ignition terminal of the ignition switch to the alternator. A diode only allows the flow of current in one direction. The diode will prevent current from going back to the ignition switch and this will prevent a "run on" condition when the ignition switch is turned off. Use a Radio Shack #1N4001 diode or an equivalent value.

Speedway Motors offers an alternator wiring harness for the GM internally regulated alternator with the correct diode. Part #910-64100.

Alternator Harness

NOTE:

Whether you use a diode or an indicator bulb, the connection will go from the #1 terminal on the alternator thru the diode or indicator bulb and then to the ignition terminal on the ignition switch. The ignition terminal is hot only when the switch is in the ON position.
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Old 06-29-2010, 08:59 AM
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Quote:
Originally Posted by bubbahotep
Unrealted to your run-on issue but the #2 terminal shouldnt go to the battery (jumpered). Its the sense line that tells the alternator if it needs to output more to charge the battery and ideally should be as far from the battery as you can get (usually the fuse box +12). No harm in just jumpering it other than the fact that it doesnt really sense that you've lost voltage far away from the alternator at the fuse box.
Below is a photo of the wiring on the alternator on my old '30 Ford Coupe. It is wired the same as I posted above. It worked excellent. I used an idiot light. I have used this wiring method for Chevy alternators at least six times in the past with excellent results.

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Old 06-29-2010, 09:30 AM
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Quote:
Originally Posted by bubbahotep
Unrealted to your run-on issue but the #2 terminal shouldnt go to the battery (jumpered). Its the sense line that tells the alternator if it needs to output more to charge the battery and ideally should be as far from the battery as you can get (usually the fuse box +12). No harm in just jumpering it other than the fact that it doesnt really sense that you've lost voltage far away from the alternator at the fuse box.
Partly true, the sense wire needs to be at the point of 12 volt distribution. That could be the fuse panel or it could be a terminal strip located under the hood or dash. It has nothing to do with being as far away as possible from the battery 12+.

I have set up a number of GM alternators as Frisco has indicated and they also have been working flawlessly.

Vince
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Old 06-29-2010, 11:37 AM
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Quote:
Originally Posted by zephyrc
frisco,, the warning light is wired as you have shown,, i have ordered a diode to go in the line
You will see a band on it (cathode end), put the cathode terminal towards the alternator.
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Old 06-29-2010, 01:31 PM
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frisco,, i understand now what you are describing, mine is wired from the ign live fuse box which also powers the msd..that is my problem, it is feeding the ign directly when i shut the switch off.. it is easier now for me to fit the diode..thanks very much for the info.. next time i wire a car i will do it the correct way
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Old 06-29-2010, 02:54 PM
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Quote:
Originally Posted by zephyrc
i found today that if i switch the motor off it continues to run until it runs out of fuel,, is it possible that the alternator is backfeeding the ignition system.. if i disconnect the alt light wire at the alternator motor stops as soon as i turn it off, also if i remove the bulb it stops when turned off.. when i turn it off the warning light comes on and stays on until motor runs out off fuel..alternator is a powermaster 7294

About 20 year ago,I worked on a Ford Tauris that had the same concern you have.The alternator was at fault, with a bad diode.If you install a in line diode as quoted,you may take care of the problem,but your alternator may have a draw on it and make your battery discharge when turned off.Do the thing right,the less wiring you have the better the better off youl'l be in the long run.Years from now you may already forgotten about the inline repair.
Paisano

Last edited by The Flyin Wop; 06-30-2010 at 07:36 AM.
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Old 06-29-2010, 09:03 PM
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A diode is electricity's check valve, it only allows current to flow in one direction.

Vince
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Old 06-30-2010, 07:34 AM
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Yeah Vince your right.Until it shorts out and lets it flow both ways,causing the voltage to reverse when turned off.Or if a positive diode shorts against the case,it will sometimes let the motor continue to run.Negative diodes usually don't have that problem.
Paisano
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Old 06-30-2010, 08:36 AM
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Quote:
Originally Posted by zephyrc
frisco,, i understand now what you are describing, mine is wired from the ign live fuse box which also powers the msd..that is my problem, it is feeding the ign directly when i shut the switch off.. it is easier now for me to fit the diode..thanks very much for the info.. next time i wire a car i will do it the correct way
Placing the corect size diode and keeping the wiring as you currently have it MAY (should) solve your run on problem.

It would be much safer and correct to run the wire to the ignition terminal on the ignition switch. That is because there will be no current to the alternator unless the ignition switch is turned on.

I still prefer using the idiot light over the diode as it is a quick indicator of alternator failure (such as a thrown fan belt) if it comes on while the engine is running.

Easier is not always the best way to do things. ENJOY!!!
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