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Automotive related question regarding specificity of diode.

hotrodjohn71

Jul 22, 2017
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Jul 22, 2017
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Hi group,

Please help me with a diode question.

In a nutshell, I have a feedback problem, (12v +) from my alternator which is causing my engine to not shut off when the key is switched to off.

This is an occasional problem with some older systems with an updated relay-powered ignition and after changing from the stock 'alternator / regulator' setup to a new style internally regulated alternator.

In order to keep the srock (in dash) charge light functional after the conversion, the 'idiot light as some call it' will have its wire going to the alternator (#1 terminal) so that with the key turned on, but engine not running, positive voltage goes through that indicator bulb allowing the bulb to illuminate (because that number one terminal is grounded at engine stopped). It illustrates to show the bulb is good.

Then after the engine starts, that terminal on the alternator starts to produce positive electricity which goes back to the bulb and nullifies the bulbs lighting to show that there is charge from the alternator.

It is a commonly reported practice to install a diode at the alternator right where that number one terminal wire comes out which is supposed to allow electricity to flow into the alternator from the key switch (making the charge light functional), but not for electricity to flow from the alternator back into the key switch (which can sometimes cause an engine to not shut off with the key switch).

This charge light circuit has virtually no current going through it except for the current to turn on that little 194 bulb. I have read that some say the bulb has enough resistance to eliminate the engine run on problem, but in my case, not so..

I have found that there are about four different styles of diodes which the guys use but I would wonder which would be the best or if it makes any difference at all.

I've seen 1A 100V, 6A 50V, 1A 200V, 1A 50V.

Would these work? Or would there be a better? Thank you
 
Last edited:

ivak245

Jun 11, 2021
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Jun 11, 2021
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107
And also make sure you have a good earth from chassis to engine block. The light is usually around 2w which supplies power to excite the windings.
 

hotrodjohn71

Jul 22, 2017
31
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Jul 22, 2017
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Thank you so much for your advice. Now considering the second part of my posting regarding the diode. Which diode do you think would be best in that question?

""
This charge light circuit has virtually no current going through it except for the current to turn on that little 194 bulb. I have read that some say the bulb has enough resistance to eliminate the engine run on problem, but in my case, not so..

I have found that there are about four different styles of diodes which the guys use but I would wonder which would be the best or if it makes any difference at all.

I've seen 1A 100V, 6A 50V, 1A 200V, 1A 50V.

Would these work? Or would there be a better? Thank you""
 

Kiwi

Jan 28, 2013
471
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Jan 28, 2013
Messages
471
I have seen this problem quite a few times.

The current flowing back through the warning light from the alternator is enough to keep the ignition relay energised if there are no other loads directly off the ignition switch.

Usual fix is to put a diode in series with the alternator warning light.
I normally use a 1N4007, but any 1A diode will work.

The other option is to shift the ignition supply for the light from the relay winding terminal to the output terminal.
 

hotrodjohn71

Jul 22, 2017
31
Joined
Jul 22, 2017
Messages
31
Thank you a million Kiwi.

I have (2) diode styles readily available here in hand. One is a 6A 50V (276-1661), and one is a 1A 50V (1N4001).

Would either of these diodes work? Otherwise I can run down the street and get that 1N4007 you mentioned. Thank you.
 
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