Made a headlamp harness for the Miata

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I made a headlight harness for the Miata tonight. Well, I have a few wires left to patch in, but it's 90% done. The battery puts out 14.9 V at idle, and I was only getting 14.0 V at the headlamp
shocked.gif
So I should see nearly a whole volt of increase when it's all said and done. Overall, it wasn't too hard. The battery is in the trunk, so that made things a little interesting, but running the wire was still very easy. I'm using 12-gauge stranded wire for everything, except for the trigger wires I'm using the OEM harness (which is quite thick surprisingly). The only odd thing, which I discovered upon metering things, is that this car uses a ground-switched electrical system. I work with low-voltage for a living, and I cannot understand the purpose of ground-switched systems. Anyway, I'll report back in a few days once things are all done.
 
Originally Posted By: Rand
14.9v from the alternator? ouch.


It is higher than normal, but anything under 15.5ish is okay. IIRC, my Sedona puts out like 14.7 V.
 
Originally Posted By: 901Memphis
14.9v would be acceptable for a fully charged battery on equalization if the temperature is low. Alternator voltage goes down as temperatures rise.


Battery and alternator are new also. And it was cold out.
 
most vehicles use undersized wiring to the headlights that loose some power + give weaker lighting. using relays to carry full power + using switchgear for just on-off gives better headlight performance
 
Ground switching reduces the load on the switch. Most electronic stuff on cars is ground switched,
 
Originally Posted By: CT8
Ground switching reduces the load on the switch. Most electronic stuff on cars is ground switched,


Sorry, but this is incorrect.
 
Originally Posted By: 901Memphis
14.9v would be acceptable for a fully charged battery on equalization if the temperature is low. Alternator voltage goes down as temperatures rise.


While true it seems on the high end of high.

Have you ever checked your meter?

I have a 100$ meter that is less accurate than my free harbor freight unit.
 
Last edited:
Originally Posted By: Rand
Originally Posted By: 901Memphis
14.9v would be acceptable for a fully charged battery on equalization if the temperature is low. Alternator voltage goes down as temperatures rise.


While true it seems on the high end of high.

Have you ever checked your meter?

I have a 100$ meter that is less accurate than my free harbor freight unit.


This is the good meter I use for work everyday, verified by other meters as well.
 
All my meters are about the same readings. I have an Extech multimeter and a cheap HF one too. They are both right on what my Innova 12v cigarette meter is.

My system doesn't get that high, but i know that some do. My system never goes over 14.7v last i remember.
 
Originally Posted By: Klutch9
Originally Posted By: CT8
Ground switching reduces the load on the switch. Most electronic stuff on cars is ground switched,


Sorry, but this is incorrect.


My Mitsubishi Eclipse was ground switched for the headlights too.
 
Originally Posted By: Klutch9
Originally Posted By: CT8
Ground switching reduces the load on the switch. Most electronic stuff on cars is ground switched,


Sorry, but this is incorrect.
I would say this is a Kirchoff's Laws exercise.
 
UPDATE: So I used the headlights this evening with the completed harness. It wasn't completely dark out, so I don't have any impressions yet. But I did notice something odd... As with any dual-filament bulb, the low beam filament is supposed to turn off once the high beam filament ignites (with the exception of the flash function, the low beam is retained). But now, the low beam filament DOES NOT turn off with the high beams on. This will certainly kill the bulbs in short order if high beams are used. I wonder why this is happening?

Going back a bit further... I did notice, when I was metering things before the project, that if I metered the socket with the bulb removed, I still got a good ground on the low beam circuit with the high beam circuit on (ground switched). My only theory is that no bulb in the socket, or the relay by itself, is not enough to fool the car into thinking a bulb is there. If necessary, I could fix this issue easily with an SPST normally closed relay that will automatically kill power to the low beams when the high beams switch on, but that would add more wires and more mess. Ideas?
 
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