• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)
Tips
Tips

Wiring my "driving lights"

Share this page

Lukens

Lukens

Jedi Warrior
Offline
Now that I'm assured my driving/fog lights are proper (see previous post), I'm going to wire them. I assume since my car is positive ground that I can simply reverse the coil contacts on a typical Bosch type relay. Am I right?
Thanks,
Russ
 
Relay is simply a switch - you shouldn't have to reverse anything. But please let at least one person confirm before you act. :D
 
"Polarity" for the relay won't matter much, as long as the indicated relay "ground leg" is actually to the chassis ground.

And wiring the lamps as separately controllable is better. When doing this we usually used a headlamp switch, wired so it is only energised when the parking light circuit is on (so you have tail-lamps on too). First position lit the "fog" lamp only, second position lit both.

HTH!
 
JPSmit said:
Relay is simply a switch - you shouldn't have to reverse anything. But please let at least one person confirm before you act. :D
Click to expand...

I agree, the contacts (30 & 87) are simply make or brake, but the coil that closes them should be current direction sensative. No?
 
Nope. Won't make a bit of difference. Electrons don't know the difference in this case. As long as the indicated (relay) ground lug goes to system ground.

EDIT: The relay coil is an electromagnet, NOT a solenoid. Current flow either "direction" will close the contact.
 
Last edited:
DrEntropy said:
Nope. Won't make a bit of difference. Electrons don't know the difference in this case. As long as the indicated (relay) ground lug goes to system ground.

EDIT: The relay coil is an electromagnet, NOT a solenoid. Current flow either "direction" will close the contact.
Click to expand...

You're the Doctor, and I hate to answer my own question, but isn't a solenoid an electromagnet by any name? And isn't an electromagnet pole sensative to current (electron flow)?
I hate to waste the money on a relay that won't close.
 
Once the coil on the relay energizes, it will pull the contacts down. As doc says, the coil doesn't care about the polarity as long as it's got a neg and pos hooked up to it.
 
A solenoid has a ferrous CORE in the center of the coil and ~IS~ (mostly) polarity sensitive. Changing "flow" changes the direction the mechanical core will go (to do work).

The relay is a simple electromagnet, attracting a contact. No core to mechanically move one way or the other. Thus, not "flow" sensitive. :thumbsup:

EDIT: It'll snap shut like a mousetrap, regardless the "flow direction".
 
Last edited:
Okay guys, thanks for your help. I'm disappointed that I can't see this. But I'll get over it.
Enjoy your turkey.
Russ
 
RUSS!!! Check it out for yourself: Go get a cheezy relay from AutoZone (there's one on Cattleman Road just east of the Interstate, IIRC), skin it and apply 12V to the two coil contacts. Then reverse polarity and see what happens. :D
 
DrEntropy said:
........apply 12V to the two coil contacts. Then reverse polarity and see what happens. :D
Click to expand...

Look out :rolleye:
hinden.jpg


OK, just kidding :D
 
Last edited:
DrEntropy said:
RUSS!!! Check it out for yourself: Go get a cheezy relay from AutoZone (there's one on Cattleman Road just east of the Interstate, IIRC), skin it and apply 12V to the two coil contacts. Then reverse polarity and see what happens. :D
Click to expand...

I must have been sleeping when we wound the wire around the nail back in "the day". For some reason I had it in my mind that the nail attracted on one end... repelled on the other.
And how come you know Cattleman road? Creepy :rolleye:
Russ
 
Last edited:
whats with this relay thing, just used to wire them one wire to ground and one wire to power with an inline fuse (all through a switch of course) and if you use a lighted switch you know when they are on. :yesnod:
 
The coil WILL reverse, meaning the magnetic pole at the end of the core will change depending on the polarity of the applied voltage. But the armature is not magnetized, so it will be attracted to either pole. To get magnetic repulsion you need two magnets, not one magnet and a piece of steel. In your 'nail' experiment, no doubt the other piece was magnetized (either deliberately or accidentally).

I actually have my brake/turn relays wired so they get opposite coil polarity, depending on whether the brakes are on or off. They work just fine either way.

The only exception would be that some automotive relays have integral suppression diodes. In that case, the diode requires that the applied voltage be the 'right' polarity, otherwise you'll blow up the diode. But general purpose relays rarely have the integral diode, and the few relays that I have seen that do have it, have the diode symbol clearly marked on the case.

Some relay sockets also have a diode incorporated. Unless you are using an electronic device to power the relay, just snip out the diode. These sockets had diodes (which I snipped out)
Relaysonheadlightbucket.jpg
 
One other thing, some states have a legal requirement that driving lights be enabled only in conjunction with the high beams. I've never heard of anyone getting a ticket for running driving lights without high beam headlights, but it is at least technically illegal. Might as well wire them that way. You can either tap into the high beam circuit to power the switch, or if it is more convenient, power the relay coil from the high beam circuit and switch the ground side. The latter method is slightly safer, IMO, especially if you use small gauge wire to the switch. That way, if there should be a short in the wires to the switch, it will just enable the driving lights whenever the high beams are on, rather than letting the smoke out.
 
You'd have to hunt the relays with internal diodes on purpose, methinks. Most "generics" are without.

Randall, that one on the left looks to have taken some heat! :eek:

ISTR the PA law was that "auxiliary lighting" needed to follow that scheme when I set up the Lucas lights on the MGB (sometime around 1970). But I wired mine to be able to use them with only the running lights ~or~ with headlamps. Had H-4's in Cibie envelopes in the headlamps, I could run with just the fog lamp, both it and the pencil beam, or headlamps and either combo. Set up the Elan the same way, with Cibie Oscars down in the grill. The +2 came with a set of the Lucas Square-8's, both come on at the same time (switched) only with high beams.
 
DrEntropy said:
Randall, that one on the left looks to have taken some heat! :eek:
Click to expand...

Yeah, I got a little carried away with the heat gun :smile:

Also decided that the sockets were more trouble than they were worth, and used individual quick connects on the other bucket.
 
Back
Top