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MG MGB Technical - Wiring Driving Lamps to dip with high beams

I just got a set of driving lights and a switch for them to fit the fog light hole on the dash in my 1973 B. I would like to wire them to cut off when I dip the high beams, and I want to wire in a relay. Please help.
Greg Bowman

Two relays woiuld do the trick. Wire the driving lights to a relay only on the feed to the load side place another relay between the driving light relay and the source of the load that uses the high beam lead as a switch. That way no power would be to the driving light relay unless the high beams are on. Also, the driving lights would not work without the headlights being on. There may be other combinations.
Jim Lema

Jim, Greg

Wouldn't a single relay, with the power to the switch controlling the relay coming off of the high beam "on" lead, do the same thing?

Larry
Larry Hallanger

You may be right. I'm certainly no electrical guru, but get by with help from you guys.
Greg Bowman

Greg,
My aux driving lights are wired so that they are operate any time the lights are on except when the low beam is on, as long as the aux light switch is on. i used 2 relays, 1 normally open and 1 normally closed. i took power from the main light switch that supplies the parking light circuit and fed that to the aux light switch. switched power from the aux light switch then goes to the normally open relay. When the lights are turned on and the aux light switch is on, this relay gets power and switches the main supply to the aux lights. Pretty simple, the driving lights come on any time the switches are on (parking lights, or low beams, or high beams). in order to make the aux lights go off when the low beams are on, I placed the normally closed relay in the supply between the aux switch and the main switching relay. I then powered the normally closed relay from the low beam circuit. Thus, when the low beams are off, there is no power to the relay, it remains closed (since its a normally closed relay), and power from the aux light switch goes to the main relay. When the low beams are on, the normally closed relay is powered which then opens interrupting the circuit to the main relay from the aux light switch. Said another way, switching the lows on switches the main relay off, and therefore the aux lights. When the lows are switched off, the normally closed relay loses power and closes the circuit from the aux switch to the main relay and the aux lights come back on. Does this make sense? I told you it was complicated.
good luck,
Ben Pender
Ben Pender

Larry
you should have your headlights on relays already. I would go with a double relay and leave the main feed lines on the headlights alone.
Jim Lema

A different approach uses a relay for each "beam" coming off of the column mounted lighting switch (High beam, {Blue/White wires}/Low beam {Blue/Red wires}). When each beam's relay is energized by power passed to it by the wires from the switch, that relay then passes current, straight from the battery (solenoid battery cable post, actually) to the headlamp filaments without having to run all that current through the switch - saving wear and tear on flimsy switch contacts. That would take care of both your basic high and low beam circuits, providing a bit more power to the lamps, and a longer life to the switch.

Now, as for your driving/fog lamps, all you would need to do would be to EITHER branch off to your aux lamps from the output of the High beam relay with a heavy gauge wire (say a # 14 ga.) or to have the current from the high beam relay energize a third, Aux lamp, relay that would control another feed from the battery to those lamps. Either way, your Aux. lamps would only be on when your high beams were on. In the second case, they might get a little more juice from the battery than if they shared the feed for the high beams. FYI. Good luck!
Bob Muenchausen

Bob's way is easiest. To be really safe, use two relays: one for the RH headlight main beam + LH driving light, and one for the LH headlight main beam + RH driving light. That way if a relay calls it quits on a dark country road at high velocity, your pulse rate needn't go up more than a beat or two. Because of the way lamps work, maxing out the voltage using fat wires and relays makes a huge difference as far as brightness. Definitely don't try running four lights through your poor old Lucas switch, unless you like the scent of melted plastic and are similarly amused at sending dough to Moss! And using cheapo Pep Boys relays for this is the falsest economy ever ... get serious Hella, Delco, etc. ones ... after all, we're talkin' about your headlights here!
Ted

This thread was discussed between 08/04/2003 and 10/04/2003

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