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MG MGB Technical - Electrical - Harness Install help

69 B Roadster:

Got harness in (British Wiring) and installed except for hooking up one wire and haven't turned the power on.

The one wire is a black with white tracer that plugs into the center of the back of my tach onto a spade connector. It is definitely black/white and not white/black (igntion coil dizzy wire) like I expected. The only thing I can find that is black/white is the brake pressure fail switch wiring and I have no idea why it would hook up to the tach. The brake pressure fail switch on the dash has a black/white, a purple and a black. Looking at the wiring diagrams (on-line, my nice color printed one, Haynes and the correct Bentley one for my car....Diagram #8 in the wiring diagrams section N.....for 68-69 cars in certain jurisdictions) I can't see a black/white wire hooked to the tach. I see the two whites, a green, a black ground on that little mini-harness for the ground wires that hooks under one of the knurled nuts and the red/white that is the light wire when you plug the light in the back of the gauge and is permanently hooked to the light itself. I plan on testing to see if there is any power to that black/white circuit and if not, will hook it up as it was before. I don't seem to have the unit in the engine compartment that the brake pressure failure system hooks to so it doesn't work anyway.

Oh....have circuit tested the two black white wires coming out of the harness in the dash and both wires go to the same place in the engine compartment part of the harness. A little harness extension runs off the main harness with just the two black/whites in it and mounts in a bracket near the heater and have snap connectors on the bullet connectors hanging in the air off the ledge the heater is on. Another small extension in the same bracket is the two wires for the brake lights that hook into the wires in front of the heater that go over to the brake cylinder cover.

Also....on experienced friend's advice....got a low amp charger (2 amps/12 volts) to hook up to the system and test before I hook in the battery to avoid melting something. When I turned it on last night....I got a lot of clicking from the solenoid area. Assume I must have the white/red hooked to the wrong terminal down there. When I hooked it all up...not seeing the spade terminal on the post I converted the spade terminal on one of the 3 browns to a ring connector and hooked all three to that post as it was before. Any thoughts?
J.T. Bamford

J.T.
Both the black/white wires go on the brake pressure light/switch.
The diagram nr. 8 in the factory manual shows the switch (the one you are missing) with both black/whites, which go to the warning light/switch. One of the wires is connected to one side of the light, the other is connected to the one side of the switch.
There should also be a purple wire, which goes on the remaining connection on the light. A black with spade connector goes on the remaining switch connection.

If you connect everything there will be 12 Volts on one of the black/white wires in the enige compartment, maybe not a good thing if you're missing the pressure switch.
You could make the test switch work and NOT have 12V on the black/whites if you leave them off the light/switch and make up a short piece of wire with spade connectors to connect the posts where the black/whites would have gone.
This way there will be a continuous 12V supply to the light and pressing the switch will earth it so the bulb will light.
HTH,
Rufus
R Pool

"If you connect everything there will be 12 Volts on one of the black/white wires in the enige compartment, maybe not a good thing if you're missing the pressure switch."

Rufus,

Yo're right, there will be 12 volts on one of these wires, but it will be coming through the test lamp, so there is no potential for harm if it should come into contact with ground. All that will happen is that the light will come on - this is exactly what happens when the fail switch actuates, it grounds the wire.

"You could make the test switch work and NOT have 12V on the black/whites if you leave them off the light/switch and make up a short piece of wire with spade connectors to connect the posts where the black/whites would have gone."

This will work, but then you are only testing the BULB, and not the circuit, as it would be if everything was connected properly. You might get a flase sense of security by watching the light come on when you press the test button, but the wire could be disconnected from the fail switch, and you'd never know it. Of course, if you don't have the switch, knowing that the light will come on is not at all important. In that case, all you have is a light with an on-off switch.
Dan Masters

Thanks guys. Looks like my black/white connected to the tach was a red herring....doing exactly nothing and leading me astray. I don't care if the test light works until I put the whole thing back together right, later.

Thanks for the "de-mystification".

JTB
J.T. Bamford

Dan,
an unprotected wire with 12V can never be a good thing, i think.
I suggested J.T. make the test light work as it was, in my opinion, the way to make the most of the parts he has.
J.T. wrote that he doesn't seem to have the warning switch and wasn't interested in making it work.

Rufus
R Pool

Rufus,

I didn't say you were wrong, I just wanted to add some clarification.
Dan Masters

Dan
and I wasn't upset/angry/offended, also just wanted to explain my sugestion.
Do you own a 68/69 B aswell?
Rufus
R Pool

Rufus,

No, I have a '74 BGT, getting a Ford 302 V8 soon
Dan Masters

Into the "poor" GT??
I replaced one of those once on a friends Mustang once...remamber it as being quite heavy. HP is not bad, but I don't think a supercharged one will fit under the bonnet :)
Rufus
R Pool

"...remamber it as being quite heavy. HP is not bad,..."

Rufus,

With T5 tranny and in the proper trim, less than 25 pounds heavier than the Stock 4-banger/tranny combo, and well over 300HP!

Lots and lots of them over here, and more being done every day!

As the old American TV commercial used to say - "try it, you'll like it!"

I tried it, and I liked it! I have driven more than one V8 MGB, and the Ford 302 powered version was the most fun of all. At the V8 meet last month, this car was dyno'd with 323HP at the rear wheels!

Awesome!
Dan Masters

"there is no potential for harm if it should come into contact with ground"

I like it.
Paul Hunt

This thread was discussed between 30/08/2004 and 03/09/2004

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