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Discussion Starter · #1 ·
Help! where in the UK can I get one of these plugs for my alternator?



Also I don't quite get the wiring.

Painless wiring loom has big cable to go to alty output post from fuseboard - OK
Painless wiring has exciter wire to fuseboard - OK
I need to run another wire to warning light on dash - OK (it's an LED with a resistor - do I need to add anything else in this wire?)

4th pin - switched (power?) from ign switch? do I need to add this one?
 

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Gym Junkie
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As it's a diode you will need to connect the positive side to the ignition switch and the negative side to the L terminal on the alternator.

The original alternator warning light is usually around 2/3W but the LED will be much lower than that and it may not go out even though the alternator is charging, the solution is to add a shunt resistor across the legs of the LED, the value depends on the spec of the LED.

It should be noted that an LED may not dim the same as a bulb does and may just be either on or off, also the LED won't light up if the alternator is over voltage because the current can only flow one way unlike a bulb where it can flow in either direction.
 

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I'm Not Jed Clampett
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How about if I put a std bulb hidden in the wiring as well as the led?
You could wire a small bulb across the LED and hide it somewhere, but why not just put a 50Ω 5W resistor across the LED. this will mimic the 3W bulb that would usually be in the circuit -the LED will still light. It's important that a reasonable current flows in this circuit because it is responsible for "exciting" the field circuit when the alty spins up; just the LED on its isn't likely to provide enough current.
 

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On mine I found it wouldn't charge if you connect all 3 leads Mark. I only use the light and switched live lead ( plus of course the output to the battery). I just used the small female terminals that you can buy rather than the plug. Also found that the cheap alternators are cheap as those plugs don't always fit ;)
 

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Not to disagree Stu but mine is working mine with a 'standard' led warning light fitted , no resistor added by me but is probably built into the light.
 

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Administrator
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Discussion Starter · #10 · (Edited)
You could wire a small bulb across the LED and hide it somewhere, but why not just put a 50Ω 5W resistor across the LED. this will mimic the 3W bulb that would usually be in the circuit -the LED will still light. It's important that a reasonable current flows in this circuit because it is responsible for "exciting" the field circuit when the alty spins up; just the LED on its isn't likely to provide enough current.
Stu,
I have a resistor (not sure the value?) wired inline with the led + lead to make it work with 12v - is this enough or do I need an extra one across the led legs? If so do I keep the inline resistor?

I'm guessing it "should" be wired as the bottom picture?
 

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I'm Not Jed Clampett
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Yes,
Should be wired as the bottom picture.

The series resistor on "12V" LEDs is just fitted to drop the voltage across the diode to the 3V or so which LEDs expect to see. (The exact voltage across LEDs varies according to the type/colour) A conventional 12V charge warning lamp is about 3W, so that means there would be 0.25A (250mA) current in the field excitor circuit (.25A x12V =3W) the LED on its own will only provide 10-20mA of current; the 50Ω resistor across the diode's legs has the effect of boosting the overall current in the circuit so that you can be sure that the field circuit will work- it also has the side effect of making sure that the field excitor circuit will still work if the LED should go Phut -interestingly, or not, depending on your anorak status, Rolls Royce used to fit a resistor across their conventional warning lamp for exactly that reason.
 

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I'm Not Jed Clampett
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On mine I found it wouldn't charge if you connect all 3 leads Mark. I only use the light and switched live lead ( plus of course the output to the battery). I just used the small female terminals that you can buy rather than the plug. Also found that the cheap alternators are cheap as those plugs don't always fit ;)
I have found that the third lead on a couple of Denso altys I've messed with is actually an AC output ("W") , connecting this to +12V would certainly screw up the charging. These altys have to get their voltage sensing via an internal connection to the B terminal (like pretty well all European alternators)

Not to disagree Stu but mine is working mine with a 'standard' led warning light fitted , no resistor added by me but is probably built into the light.
Interesting, I had to fit a current raising resistor when I fitted an LED warning "Lamp" with a Delco alty, so have always done the same with the ones I have worked with since.
 

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I'm Not Jed Clampett
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47 Ohm will be fine, a 7W resistor has to be capable of dissipating 7W of power, so larger power resistors do tend to be bigger.
 
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