Hi, I'm trying to figure out the best place to tap power for some lights. -On '05 MC-S


Main power to the lights is already coming off the battery, via a 40 amp relay. I simply screwed a spade terminal right to the place where power feeds the fuse box, next to air cleaner in an S. (The lights have their own in-line fuse)


I just want a good place to tap a small amount power for a toggle switch that will close the relay.


A part of my intention here is to NOT connect to anything that connects to the computer, i.e. the built in fog light circuit. -I know that when the computer is expecting to see the load/resistance/voltage of a light, and it does not (remember, I'm powering a Relay, not a light), then it will give you lower voltage or osculate the power or something weird.... the result is that the relays will not stay closed, they buzz as they open and close very quickly, and the lights flicker. -The way to "fix" this is to put a Capacitor in front of the trigger side of the relay, so that there will be enough juice stored up to keep the relay closed. -However I DO NOT want to plug a capacitor (indirectly) into my computer. My goal is to have an independent stand alone circuit that will not effect the car's computer (then I won't even need the Cap in the circuit either). Do I need to connect right to the battery, or can I just grab power of the cig-lighter or something convenient like that?


Thanks


AlexQS

 

Think you are over thinking it a bit....the car already has/uses a bunch of relays connected to both fuse boxes...
If you are looking for a 5 amp (or so) add a fuse....just pick a fuse (hot all the time or switched...easy yo tell..you pick)..I might suggest the cigarette lighter fuse...then add one of the piggyback add a fuse piggybacks to the fusebox....wire it up to the switch...
Then use the relay (and high amp) from the battery or under hood fusebox for the lights...
Just like adding a hardwired radar detector or GPS......only extra is the run through the firewall to the 40 amp relay....
An add a fuse tap...when done right....is a full new circirut....NOT RUN IN SERIES.....it just shares the hot let up to the 2 fuses that share one hole by way of the adaptor...
And remember....tapping at the fuse box iS PRE COMPUTER....
I think you are seeing the lights "flicker" when the lights are activated cause it takes a second (OK a moment) for the voltage regulator to crank up the amps when a suggen draw happens...not cause your fraction of a amp switch on a 5 amp fuse is drawing "too much" and needs a capacitor "...on some level of electronics perhaps...on the level delt with in cars...NO....THE SYSTEM IS ALREADY SO NOISY.... AND VOLTAGES NOT STEADY LIKE IN A LAB...
Biggest issue folks make is not making a new ground....but splicing into one already used...causing a ground loop....ground loop can cause a flicker or hummm (on audio components).

 

For a toggle switch in the cabin, I'd use an in the fuse box. You can use F32 which is the cigarette lighter fuse (double check with your fusebox diagram).

 

exactly...fast...easy....works great!

 

Sweet! I think that's just what I need. Thank you RKW


You're right Zippy, it does seem like I am overthinking it a bit. I was trying to keep it short, to the point, and avoid admitting my screw up.


Just this past week I had my DCM replaced, because it would not send signal to half of the coil, thus I had no spark in #1 and #4.


The shop said they believed the cause of this DCM failure was that I had a loose ground strap, the computer was having to work so hard to overcome the problem of such a weak/high resistance ground that it got too hot. -Though I know that the ground looked weird to them, because it was not on the passenger motor mount, and was relocated because that little tab broke off, -I know it was tight. Replaced the motor mount and fixed that correctly anyway.


I had asked them if my after market lights could have caused the problem. They said "No", but I'm not sure if they are aware that I added a capacitor to make it work. The relay is actually powering some ballasts. I put in a relay circuit instead of a can-bus adaptor, because I did not want the high-draw during start up to pull through the cars original wiring, but I thought that surely the small amount of current needed to close the relay could go through the cars wiring. I was surprised with the blinking light outcome, but the capacitor (recommended by ballast manufacturer) fixed it.


Today I'm a little embarrassed about my expensive mistake. Though I'm not 100% sure that this light set up caused the computer to fail, the capacitor in the circuit bothers me; and I'll feel better about my new DCM if I can ditch that stupid cap, and not use any of the car's original switches for my after market lights.

 

The reason the Capicitor was needed on the lights with the ballast is...
One a car equipped with a canbus, the lights are run at a LOWER than battery voltage...this is done by the computer "pulsing" the power rapidly on/off..result is on a halogen/regular filament bulb, it is getting run a volt or two less (or much less if DRL are active), and the brightness is ALMOST unaffected, but the life of the bulb is GREATLY enhanced...
As ling as the power you get is UPSTREAM (before) any output from the BCM...like any output directly from the fuse box...you can avoid this issue 100%....
With a xenon..if it gets this "pulsed" power...MOST will flicker and some will fail prematurely....bulb and or ballast....
you can wonder if doing x caused Y...but with so many variables...I would just fix what needs to be fixed (sounds like you did this) and caulk it up to experience...sure maybe a good eye or a Voltmeter might have turned up an iffy ground repair...but that's in the past...enjoy the car and don't second guess everything going forward!!
A relay is JUST a tiny electromagnet and a couple of contacts.....very simple and basic...most need very little voltage....
I would suggest you could just tap into the relay with a jumper to POWER YOUR relay...hence, you no longer need to add a cabin switch.....not sure of your setup...but for xenon fogs...might be ideal...

 

The "computer having to work so hard" explanation makes no sense at all.

Your DCM failed and the loose ground strap was a possible cause. Let's just leave it at that.