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MG MGA - ignition switch

Hello, The three wires that connect to the ign switch are, on one side two white wires and on the other side one brown with green stripe and one yellow wire. Is this correct? I noticed that when the ign is on that the brown and yellow wire get somewhat warm. Is this normal? Thanks, Neal Turner
neal turner

Two brown/green wires on one side and two white wires on the other. No yellow in my diagram. I believe one of the white wires goes to your yellow wire THROUGH the ignition warning light.
R J Brown

See MGA wiring diagrams here: http://mgaguru.com/mgtech/electric/circ_a.htm
Ignition switch should have two browN/blUe wires on one side, and two White wires on the other side. The NU wires are power in from the control Box A1, and power going to the lighting switch. The W wires go to Fuse A3 and to ignition warning light.

Yellow should be the second wire on the ignition warning lamp. That wire should be permanently affixed to the lamp socket, and there should be no other Yellow wire behind the dash. Your Yellow on the ignition switch is a bodge. It may be a replacement for one of the original NU wires.
Barney Gaylord

Oops
brown blue not brown green. without checking I repeated your mistake.
R J Brown

If the wire is getting warm, you should double check that the connection is clean and tight. Is the whole wire warm or only near the connection?
Jeff Schultz

Hi Jeff. Yes the wires are getting hot close to the connector. The wires are fairly large and are perhaps large to carry the load of the lights and ign. I hope that they are not too hot. Thanks, Neal Turner
neal turner

Some switches carrying a lot of current may get slightly warm to the touch. The dash light dimmer rheostat in particular can run hot because it is a variable resistor passing current. Some wires carrying high current might run slightly warm to the touch, but nothing that could be described as hot.

When you have a hot switch it implies high resistance contacts inside the switch. When you have a hot wire very near to an end terminal, it implies a high resistance connection at the terminal. The aftermarket ignition switch with Lucar (push-on) terminals is notorious for this problem.

Original MGA ignition switch had screw post terminals. Two browN/blUe wires connect to one terminal under a common set screw, so these two wires are intimately connected to each other. Power flows directly from the input wire to the second wire going to the lighting switch, up to about 14 amps with all lights on (including high beams). For that function essentially no power is going "through" the terminal post, as it is conducted directly from one wire end to the other wire end (under the common set screw).

When you turn on the ignition switch power flows through the switch for other functions, ignition coil, fuel pump, brake and turn signal lamps, heater blower, wiper motor, fuel gauge (and possibly a radio), up to 12-14 amps at times. This power (current) is conducted through the switch terminals and internal contacts. While total current draw may be up to 26-28 amps, only about half of it goes through the switch terminals, and those connectors are highly reliable set screws.

The aftermarket ignition switch likely has Lucar push on terminals, two or twins on each side of the switch. This is no big problem on the output side where it splits output current between two connectors. But on the input side the input power wire is connected to one spade terminal, and the lighting supply wire is connected to the other spade terminal. Then you have up to 26-28 amps going through on the first Lucar connector and up to 14 amps for the second one. Running up to 28 amps through a single 1/4 inch Lucar connector is a tough sell. This is more than the maximum output from the dynamo, and the dynamo uses a much larger wire connector.

For a long time I was replacing the Lucar wire end every couple of years after it would overheat, burn a half inch of wire insulation, and lose the electrical connection. My solution to this problem was to install a screw post barrier strip to service all connections that did not have to go through the ignition switch. My ignition switch now has only two wires connected, so input and output terminals carry the same lower current. See picture.



Barney Gaylord

I reduced the current through my ignition switch to a few miliamps by using a relay. A simple mod that required no alteration to the standard wiring harness, so is totally reversable if desired. Reduces wear on the ignition switch contacts and increases current flow to the ignition switch dependant circuits. There, that should start the relay debate again!
Lindsay Sampford

This thread was discussed between 10/04/2010 and 11/04/2010

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