fastivab6tg25mr

not a miata but i trust the feed back on here more than the other forums im on.

i need constant battery power to my passenger side floor/firewall area and was thinking of tapping into the alternator charge wire that runs directly from the battery to the back of the alternator. the wire is going to go to a fuse block that will be powering my MS1, innovative LC-1, and 1 or 2 other little things. im trying not to run a wire all the way to the battery.

Braineack

iTrader: (62)

you want to wire your megasquirt to constant battery?

18psi

iTrader: (76)

he will likely have a switch so its not powered on 24/7

fastivab6tg25mr

the power will be controlled by a relay and activated by a hidden button. but my lc1 and my knocklight/ shift light need a 12v constant. im trying to keep everything in its own "standalone" harness... just in case i pull this car in half too i can swap it to another cheap craigslist find lol

Joe Perez

iTrader: (8)

I'm wondering if I've misinterpreted this. Do you really intend to supply the LC1 with a constant 12v supply all the time, whether the car is being operated or not?

There seems to be a lot of superstition surrounding the LC1, particularly with regard to its apparent distaste for marginal power supplies, such as the ~9-10v supply typically available while the starter is operating. The most reliable workaround I've seen for this is simply to supply it via a circuit which is on the ACC side of the keyswitch, or via a relay (from the battery or alternator) whose coil is supplied by the ACC circuit.

fastivab6tg25mr

The power to all devices that require switched 12v will be controled by a relay. There will also be a constant 12v at the junction block for the devices that require a 12v to retain memory

Braineack

iTrader: (62)

that's limited to your radio--pretty much.

Joe Perez

iTrader: (8)

My thought as well. A typical (stock) vehicle has unswitched +12 going to the radio (to store the presets) and the ECU (to store fault codes), and that's it.

A car with a non-stock ECU shouldn't need unswitched +12 for anything other than the radio. Definitely not for accessories like a wideband O2 sensor, shift light, etc.

It makes me wonder if I'm not vastly misinterpreting the OP here.



EDIT: I am obviously ignoring things like power locks, lights, horn, etc., as the OP was specifically referring to things that need to retain settings in memory.

18psi

iTrader: (76)

JT you should link them to your build thread.

I think it would clear up quite a bit

Braineack

iTrader: (62)

wont help us understand why he wants to constantly drain batteries.

fastivab6tg25mr

The power I'm talking about getting from the back of the alt will be going through a relay. My concern was voltage fluctuation at the rear of the alt that would mess with the megasquirt.

Edit:
i was thinking there was a mem constant power to the lc1 i guess there is not.

Joe Perez

iTrader: (8)

The current flow across the battery-starter-alternator cable is sufficiently small during normal operation that the voltage will be essentially identical at all three points. Thus, the voltage should actually be more stable at the back of the alternator than at most other points in the harness.


It's really not a bug deal, and I suspect that you may be over-thinking this based on having heard some horror stories about the LC1. The thing that the LC1 doesn't like is being powered on while the starter is operating and dragging the system voltage down into the 8-10v region, thus the recommendation that ACC be used to supply or switch it.

fastivab6tg25mr

My lc1 has always been powered by the same circuit as my MS and have had no problems with over 2 years in my daily. I was more worried about voltage fluctuations messing with my sensor signals.