How To Wire A Relay To Power My Dash Lights

New member to this board and fairly new to relay's.  I am into old cars, both of my cars are from the early 70's, I have recently discovered the benifits of adding relays and select other modern electrical devices to my classic cars.  

As I said, my relay experience is quite limited as the only things I have done so far is rewire both of my cars headlight systems using relays and on one of my cars I added a relay to my alarm system so that my park lights flash to confirm alarm activation and deactivation.

There are a few other things I'd like to do soon using relays and solid state electronics to replace the old technology original pieces but at this time I'd like to ask how to correctly wire a relay to supply full 12v power to the light bar that iluminates the instrument cluster.  I've always had a problem with the dash lights going bright and dim depending on the rpm's but since I've replaced the standard row of bulbs with a series of diode's, it seems more sensitive to changes in the rpm's.

 I figure it would be similar to the way I have already used relay's in my car... I figured that 85 should be ground, 86 should come from the switch in the dash, 87 should be output to the dash lights and 30 would be the fused 12v power.  My thinking is to use the stock wire that currently suplies power the row of bulbs and move it over to the relays.

Sorry for the long post.

I want them to go on like they normally would, using the original switch in the dash.  My stock switch is a toggle switch, first stage is the dash and exterior running lights, second stage is the headlights.   I just want to use the factory wire to turn the relay on. 

You have the relay correct.  If you are trying to avoid the dimming I would recommend you take a new source of power for the relay preferably from the ign harness or even from the battery in this case.  The closer to the battery the steadier the voltage will be and get a new good source of ground.

auex, thanks for the help.  When it comes to things like this I like to double check before I go screwing with my cars wiring.  Yes I was planning on a nice strong direct power source for this.  I always use a strong direct power source for my relays.   It would defeat the purpose of using a relay if I didn't.

It is common with old cars for this type of wiring problem as the wiring system is often 30 or more years old, the original wires are literally not heavy enough and the connections are often compromised, lots of current loss results.  I figurred that if I use the factory wire to turn the relay on and off instead of supply power to the light bar it would be better. 

That's how I have my headlight relays, the original harness can not supply ample power to the lights but they are more than capeable to turn the relays on and off.    I'm thinking of using a relay to power my wiper motor in the same fashion.

hey.. i think we all overlooked someting here --- u said u replaced your dash bulbes with a series of diodes....

well diodes are brighter with greater voltage... as your car revs (rpms go from high to low) you're alternator goes from putting out maybe 13.7 to 13.9 or 14 or heck maybe more volts.

i could be wrong -- but a more "solid" alternator might be what you actually need, along with more "solid" wiring.

Thanks for the insight, the alternator is new and the only way to make the stock type wiring more "solid" would be to totally replace the entire engine and under dash harnes' with brand new reproduction harnesses.  I'm afraid that isn't an option at this point as that would cost much more than I can put towards my car right now. 

I figure if I relieve the original harness from some of the more power consuming tasks like the headlights, wipers,  and heater fan, and use the original wires to turn the relays on and off instead of powering the components.  I'll just supply the relays with a nice strong power source using brand new  wires that are heavier than factory guage... it would make the entire cars' wiring system work smarter not harder.

There is a large movement lately of us "old car" guys making such upgrades.  The reality of it is the factory wires are simply not heavy enough to carry the load they they are originally burdened with let alone all the aftermarket things people are adding to their cars these days including multiple amps and monster stereos.  Even the new aftermarked replacement harnesses are made from the same thin guage as the original ones.  I don't see a point in replacing all of the cars harnesses unless they are made with a heavier guage, one that will do the job properly.   Look at all the late madel cars, they ALL use relays throughout the car to run everything, it just makes sense to make similar upgrades to our old cars, just because the cars are old it doesn't mean we have to use the old technology.

One more thing, what size wire would you recomend I use to power the relay and the dash lights?

14-12 gauge to the relay depending on the amount of led's and the amount of current draw each one will require.  And as for the led's depends on how you wire them together ( series or in parrellel).

My original light bar had 3 bulbs in it that stretched across the front of the instrument guages, I replaced each bulb with 4 LED's for a total of 12 LED's.  I have limitimg resistors on the LED's so they don't burn out from spikes and surges.

Why would it matter if they're in series or parallel?  12 in series takes as much power to run as 12 in parallel don't they?   Not to mention LED's take less power that bulbs do.

Hey guys, thanks for your help.  I just got in from wiring my instrument cluster lights to a relay, the factory wire that use to supply power to the bank of lights now turns the relay on and off.  I am very happy, my instrument cluster lights are nice and bright and they don't flicker or go dim as the rpm's drop.

BUT... I have overlooked one thing when planning this upgrade.   I have rendered the factory dimmer switch useless by using that wire to power the relay and not the dash lights directly.  I don't mind, as this isn't a problem and I rarely dim the dash lights anyway.  

If you could help me a little bit further, next question is... Can you recomend a rhiostat or dimmer type switch that I can put in the line between the relay and the dash lights so that I can controll the brightness if I need to?