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MG MGA - Fog Lamp Wiring & 6RA Relay

I finally got around to wiring my fog lamps this summer (about two months ago). In that time I have purchased a new battery which I have had to replace twice. Both times the battery was dead and showed bad when tested. Last night upon connecting the third battery, I heard a noise that I previously missed. Clunk, remove cable, replace cable, clunk, and so on. I reasoned that I was activating the relay and to verify this, I removed the inline fuse and reattached the cable with no further clunk sound. So I have a problem.
I did not have anything to go on when wiring the 6RA relay, otherwise I used Barney's very helpful website and followed the instructions left there by Mark Lambert. I then pulled some pages out of a Lucas fault manual to determine how to wire the relay.
I came up with the following, which other than being a current drain (details, details!) functions as I want it to.

Terminal W1 Brown and White wire to inline fuse to Starter terminal (side with protective cover on it)

Terminal W2 Black wire- Ground to fender bolt

Terminal C1 Red/Yellow wire to lights

Terminal C2 Red/Yellow wire to existing red/yellow wire in harness coming from the fog lamp switch.

The relay is not insulated from the radiator duct panel on which it sits (I don't suppose this matters, but thought it might be important)

I cannot see the forest for the trees here and would greatly appreciate any help.
Thanks in advance,
James
James Hurm

Bad wiring. If lucky nothing happens. A little less lucky and you blow the fuse by shorting power to ground when the relay clicks (if it clicks).

C1 and C2 are the coil connections, used for the signal to trigger the relay, and are interchangeable. W1 and W2 are the load connections (power contact) and are interchangeable. A 5-terminal relay would have two terminals on W2 where you might connect two lamps or two horns. See relay installation and wiring diagram here: http://mgaguru.com/mgtech/electric/et205.htm

When in doubt about which relay terminals are which, apply power and ground to various terminals until it clicks. The two terminals that make it click are the coil terminals. The other terminals are for the load.

Connect your fuse to live battery power (as you already have). Connect output of fuse to relay W1 (as you already have). Connect relay W2 to lamps. Connect lamps to ground on bumper or through a black ground wire to chassis ground. Connect original Red/Yellow switch wire to relay Ci. Connect C2 to ground. Switch power through the relay coil C1-C2 to ground triggers the relay. Relay power contact W1-W2 connects auxiliary power (your fuse) to the lamps.






Barney Gaylord

Barney-
Sorry, you have it entirely backwards.
W stands for Winding, ie, the operating coil. So, W1 is control power, W2 is ground.
C stands for Contact, the supply-to-load switch. So, C1 is power in, and C2 is load out.

For the above:
W1 = RY from harness (switch)
W2 = B to earth.
C1 = NW FROM hot feed through fuse
C2 = load out to lamp(s)

In modern terms:
30/51 = power in
87 = load out
85 & 86 control - one is power, one ground, but 86 is always negative for diode equipped relays.

Some older Lucas relays only have a single W; in that case, power is supplied via C2 for both control and load, C1 is load out, and W is earthed to operate. (from Magnette manual)

It is possible or likely that C1 & C2, as well as W1 & W2, get reversed for positive or negative earthed cars, but I've never really sorted it out. The difference would be in the material of the actual contacts in the relay; practically it doesn't matter until you get to modern relays with diodes.

As James had it wired, the W1-W2 is always draining power and keeping the contacts closed, but the lights only work whenever the switch is on, with power for the lights coming from the light switch. The relay is doing nothing but killing the battery.

FRM
FR Millmore

Duh, sorry. It might help if I actually had one in my possession for reference. The relay picture I have is too fuzzy to read terminal markings. Go with FRM's notes on terminal markings. My description to test and determine which terminals are which is still valid.
Barney Gaylord

Barney-
S'all right, every time I start on old relays I take "C" to mean coil. That leaves "W" as "What????" Then I get to recalibrate my noggin again!

Tip: Virtually all Lucas diagrams have at least one relay shown, and the terminal markings are consistent, so you can figure it out from most British wiring diagrams. Even the late MGB diagrams show the old Lucas designations, though the cars actually came with relays with the Bosch markings.

FRM
FR Millmore

And here I thought we were MGA people. ;^) The only relays in a stock MGA are for the control box (A1 A F D E) and 1500 turn signals (1 2 3 4 5 6 7 8 E), none with W or C terminal markings. I had to dig through a Haynes MGB manual all the way up to 1970 before I found one use for starter solenoid relay.
Barney Gaylord

I'm just promiscuous!
Too many Healeys and Jags I guess. Very first B shows OD relay with markings, some later ones don't show the markings. Magnette has a horn relay. "turn signal relay" is correctly referred to as "the eight legged box"! Annoyed to find that all my MGA books mysteriously grew legs, so I don't have one - maybe that's when the 1500 ELB walked off and left us with the 1600 turn indicators!

FRM
FR Millmore

Barney and FR Millmore,
I certainly appreciate your help. I rewired the relay today and am up and running again. No more clunk on reconnecting the battery, and everything works well.
Thanks again,
James
James Hurm

This thread was discussed between 07/09/2007 and 09/09/2007

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