aftermarket tach. light dims when dash brightens

[GSRCRXsi]

ok so ive had this tach installed for a while. ever since ive had it, the light has been burned out. well i just recently got a new bulb for it and niticed it was REALLY dim. so i started playing with the dimmer. and when i turn the dimmer to to brighten, the dash lights brighten, but the light on the tach gets dimmer, and vice versa, when i dim the lights, the dash dims, and the tach brightens. WTF is going on here?

when i wired the illumination light up on the tach, there were 2 wires for the dash lights. did i hook it up to the wrong wire? is that the cause of this?

 

[reikoshea]

lol...youre asking this question.......

Okay, not like it wouldnt happen to me too, but just kinda unexpected. I have no useful input but, that sucks.

 

[GSRCRXsi]

yea i know. 99% of the time, i know what im doing, but i dunno what happened here. i think i just used the wrong wire, lol.

when i was wiing it up, i was looking at the wiring diagrams for how the dimmer works and basically all it is is a potentiometer (variable resistor) between the bulbs and ground, and as you tun the nob, you change the resistance in the circuit, dimming and brightening the lights. i wanted to retain the dimmer on the tach BUT the tach only has one ground, not a seperate one for the light. so i think i hooked it up to the wong wire, and now its backwards. and i think to retain the dimmer, id have to have the light fully integrated into the circuit, with its own ground.

sound right?

 

[reikoshea]

sounds right to me...but youd think as the circuit changes resistance it dim no matter what...doesnt seem right for it to be opposite.

 

[GSRCRXsi]

yep, well that was the problem lol. now the tach is on full brightness when the lights are on. since its not hooked to the dimming circuit.

but damn the tach is bright. wish i could dim JUST the tach, or set it up seperately. hmm

 

[reikoshea]

i feel a home depo mod commin.

 

[MikeBergy]

You should just be able to wire it in parallel with the rest of the dash. Then you get the same voltage as the dash lights are getting. just make sure you tap into the wiring AFTER the dimmer, not before, lol.

 

[GSRCRXsi]

well i cant hook the tach up to the dimmer cause there is only 1 ground. not a seperate one for the bulb. and if i hooked that ground to the dimming circuit, i fear it would affect the ground to the gauge itself, and i dont want to do that.

also, i like the brightness of the dash at full brightness, but i dont like the brightness of the tach at full. its TOO bright. looks like a freaking freight train comming right for me, lol. for now i just take the bulb out at night when im not at the track. i have another tach on the dash thats good enough for regular driving.

so what im thinking about doing is pulling the tach apart and isolating the bulb ground and hooking it to a seperate dimming switch. so i can independenty dim the tach. it should work.

 

[MikeBergy]

I didn't know you wanted to keep the normal dash at full bright. You are complicating things way too much...just hook a pot between the 12V lsource coming from your dash lights and the 12v lead going to the tach light. Or, if you want to be slick, just wire an inline resistor to the tach light's lead instead of a pot, and then wire it to the light dimmer, so you can limit the tach's full bright setting, and then still be able to control the tach with the normal dimmer rather than have to adjust two different dimmers. The tach light and the pot are in series, so it doesn't matter whether the pot is first or second. Resistance adds independent of order in a series circuit... EE201, remember, .You don't need to isolate the ground, it's just a ground. You just need to make sure that your pot isn't in any way connected to the tach's signal input.

 

[GSRCRXsi]

well thats how the dimmer is set up on these cars. the pot is on the ground wire. if i put a resistor on the lumination wire, it may dim the light (no telling how much) but it wont be adjustable like i want it, cause it still wont be hooked to the dimmer circuit.

that why i have to isolate the ground. cause if i hooked the power wire to the dimmer circuit, im hooking + to ground and the shit is all backwards like it was initially.

i still think isolating the ground and hooking it to another dimmer switch is the best idea. that way i dont really have to redo any of my wiring and the dimmer for the tach will be completely independent.

 

[lil_red_si]

why dont u try sticking the tach light wire in one of the gauge cluster bulb sockets and put the bulb back in when u put the wire in it....maybe it will work

 

[GSRCRXsi]

that wont work. that will just give me power in parallel. like i already have. for that to work id have to wire it in series, which would still require isolating the bulb ground. and thats a real half ass way to do it anyway. like sticking a power wire in the fuse to get power for something. thats just ghetto and ugly wiring, 2 things im against.

 

[MikeBergy]

Well I'll be darned, you are right. I just checked my helms, and they do stick the pot on the ground side of the circuit, my bad. You could still wire the tach into the hot side of the dash light pot, and run it in series...crap, you'd still need an isolated ground... I see your dillema. Just run an independent pot for the tach light. That's your easiest solution.

 

[reikoshea]

word....but do you have anymore room for a clean addition of a pot....i mean you got a zillion switches.

I know youll eventually figure it out. Another pot is your easiest solution though, you can have a half bright tac and a full bright dash...it all works.