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Alternator Charging / Digital Dash (no charge light)

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Evening Guys,

So I am looking for a more elegant solution to something i already have in place and 'working' per se.

Car: VL Commodore / RB30 Engine

I have replaced my standard alternator with an LS1 Alternator (Mitsubishi 12v 140amp unit / A003TA7991). I run a haltech IC7 via my Adaptronic M2000 o CAN. So i no longer have a factory dash as its all been replaced with a 3d printed item.

Initially - i couldn't get the alternator to charge; found out after a bit of back and forth that i need to provide a positive feed to the "L" pin on the alternator (it has a LS pin diagram on the unit) So my Sense pin was connected to my 12v Buss bar, and after testing and mucking around - my first step was to use a voltage regulator (7805) with a 100uf capacitor. I'm not classically trained in electronics so i had youtube and confirmed with the systems teacher at the high school i work at. This was to step down the 12v ingress to 5v so this could be used to excite the charging circuit of the alternator (forgive the incorrect terminology here...)

This worked and i finally got the alternator to charge and this got me off the dyno and rolling. In the end i then went to ebay and got a dc dc step down charger unit that did the same thing in a much nicer package - offered med 13.6 - 14v off the alternator. Happy Days.

Now that we are back to the present; I'm looking to see if there is a more elegant solution to my problem of the light wire and giving holistic view on what has been done and where it is at present. Seems to be a common question but i never get a clear answer.

What id like to see is a nice high alternator output (14v +) unless this is unrealistic. But I am asking now as I have recently splurged on a PMU16 over Christmas and have just finished redoing the EFI harness for the car and will be looking to move the rest of the cars power supply over to the PMU in the coming months once i have it running again. But i'd also like to know what options i have to do this 'light wire' better sans PMU.

If anyone here can provide what they have done in the situation - I just want to see if I have the most efficient and elegant solution with my wiring.

Attached is one of the sides of the EFI harness - its not milspec or instagram worthy but im very proud of it given im just a Sys admin who moonlights trying to build cool stuff

Attached Files

This has been asked several times before.

You need a resistor to approximate the value, and power dissipation, of the original bulb, between the field terminal and a ignition switched 12V supply. A common bulb is 3W, which would be [edit - brain fart] 48 Ohm, so a possible resistor would be 47 or 51 Ohms with a 5W rating. Try and place it where it can dissipate the heat, as if it's in an insulated location the heat will build up, which could cause problems, like melting insulation and even fire.

In practice, though, many people will just wire the bulb into the wiring for it - if you go that route, remember, it's less reliable than a resistor and you should place it where it's easily replaced, if required.

Hi Gord,

Thanks for the feedback I appreciate that this has been asked a few times before - but i just got only part of the equation or what I needed.

I'm back at work tomorrow so ill head over to the systems building and see what i can find there. Thanks again for the advice and taking the time to spell it out for me

OK, not sure where you have a problem, but in summary.

The field windings need a small initial current to give them a magnetic field, this is provided via the warning light which has two main functions a/ to indicate when the alternator isn't charging b/ as a voltage and current limiting device to prevent damage to the field windings.

Regarding a/, I assume you're using the display/ECU to monitor charge voltage and don't need it.

Regarding b/, 3W @ 12V is a 0.25A current (I=P/V). This means R=48 Ohms (R=I/V) - see edit above!

48 Ohm resistors aren't a standard value, 47 or 51 (or a little higher, especially for a 2.2W bulb)) should be around the value required. 5W because the resistance is converting the electrical energy into heat and it should give plenty of reserve for the ignition being left on and/or being in a restrictive area for cooling.

Doh, just thought, even easier would have just been to check the bulb's resistance and used a similar resistor value.

I use a solid state battery isolator. It sends the amperage over canbus. Positive amperage means the battery is powering the car, and negative amperage means the car is charging the battery.

On the digital dash, I added a rule:

If (amperage across the isolator is > 0A for greater than 0.5seconds), then it activates the battery warning light.

It's simple and elegant and it works fantastic.

I was having some alternator issues too, and added a secondary rule:

If (battery warning light = true AND rpm > 500) Then throw an big red alert on the screen: Charging System Failure!

Thanks again guys for all the input and feedback. I'm just in the process of dropping in my new EFI harness and ill take some side by side shots of my solution vs a simple addition of the resistor in the L feed to the alternator and come back with my results.

We usually reply within 12hrs (often sooner)

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