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MG MGB Technical - loom lunacy

Got an MGC with some sort of late B loom in it. (UK spec) Has extra wiring for various bits. Lots of white-with-brown wires arround the ignition area of the loom.

Basically I want to find the wiring diagram that relates to this specific loom so I can eliminate the extra junk added on without regard to the original scheme of colours.

Better still would be to find a kind soul with a spare MGC loom (overdrive and UK spec for a 69-ish) even if it is burnt out and only good as a template.

Also, when the coil CB/negative side is unclipped there is still continuity to earth when the ignition is switched off. Is this correct? My midgets and pre-war stuff has different setups.

TIA!

Rob
rob

Sounds like a loom that caters for the ignition relay which arrived in 1977 for UK cars. Haynes has a diagram for what it calls 'later UK models' on page 199 that includes this.

Do you mean the coil CB/-ve spade still shows continuity to ground when the wiring is removed from that terminal? Or that the wiring removed from that terminal shows continuity to ground? If the former then this is normal (if the coil +ve/SW wiring is still connected) because there are several loads connected to ground on the white and green like fuel pump and voltage stabiliser. If the latter then after 1972 the coil negative had a white/black which goes to the voltage-operated tach. I would not be suprised to get some continuity to ground on this wire, possibly in one direction only, but it would mean that your original tach (mechanical or current-loop operated) would also have been replaced. This may have been necessary because the later loom does not have the extended wiring in the white to go from the ignition switch via the tach to the coil +ve. Which leads me on to the fact that the later loom had a ballast resistance embedded in it to reduce the 12v on the white to 6v on the white/light-green for a 6v coil. If you still have the 12v coil you will be getting a lower spark than you should unless the ballast has been bypassed in some way. If you have the 6v coil do you also have the later starter that shorts-out the ballast during cranking? If not you will be getting a lower spark than you should during cranking.

PaulH.
Paul Hunt

Sounds like it IS a post-77 car since it does have relays for the starter circuit. Didn't think to isolate the voltage stab! Using a Jag revcounter to try to remove the impulse faults inherrent when using a Lumenition "black" kit on an RVI type revcounter (loop of wire or two whites instead of just one white connection) that lumenition forgot to mention on their sales leaflets!
RVI-type will cause confusion to the system that calculates the correct dwell. Didn't want to buy a new electronic revcounter so I modded a Jag RVC one.
Anyhow, back to voltages. negative (distributor end) off and still some eart continuity. 12.5-ish volts at fuse box but only 11.5 at the other coil terminal conection. Losing voltage across the wires. White/brown, plain white and white/black have been spliced into each other. There is even a white/black (coil negative, usually) powering the overdrive solenoid.
Also (going off topic again) what is the perceived wisdom on coil-capacitor values? Do you have to have a different capacitor on a 6-cylinder car to store a charge more quickly for an engine that effectively runs 50% more sparks than a 4-cylinder? Coil resistance the same on a 6 as a 4?

Anyhow, paul, thanks for the help. Don't suppose you could mail me page 199?

PS Carb backfire on dellortos started a nice engine bay fire. Bought an FIA plumbed-in extinguisher system. Works great! Snagged my sleave on the handle and accidently fired 2.25 litres of AFFF foam into the engine bay!

{note to self...sell MGC...buy volkswagen}
rob

Starter relay came after 1969, it was the ignition relay that came in 77 or so.

12.5 volts at the fuse-box and 11.5 at the coil implies two things - you have no ballast in the coil feed and there is still a decent ground on the coil -ve from somewhere.

Ye gods! White/black spliced into the white and powering the overdrive? Sounds like a bit of a butchers job. Bit difficult to fault that on email.

The coil for the ballasted V8 is the same as for the ballasted 4-cyl so I would not expect the condenser to be different in value even though it is different physically. Any one spark event on a 4, 6, 8 etc cylinder is the same, the only thing that varies is how many time a second it happens for a given rpm. Since all those engines have a wide range of overlapping rpms there must be many occasions when the spark rate on a 6 or 8 cylinder is the same as on a 4.

I'll mail you that page tomorrow, stand by for a big file!

Cheers,
PaulH.
Paul Hunt

Got the file. Many thanks. Any chance you could send the decode for the picture? I can't work out what some of the parts are.

I seem to be getting some sort of short through the White/black wiring that is intermittent and causes a reduction in voltage at the coil when the revcounter is attatched. Might be a fault in the volage stabiliser circuit as a handheld tacho works OK and doesn't give a missfire.

Getting closer!

TIA

Rob
rob

MGC coil has different values from Midget and Mini. Can't work out why. Anyone got the part number for the Midget and MGB condenser from an original parts book. I know some places sell them as identical parts for B/C/Midget, but there might be some sort of difference between them(?)

Rob
rob

For the MGB it is
GSC101 for the 25D with HSs
47H 5250 for the 25D with HIFs
GSC110 for the 45D
It's quoted as .18 to .24 for all years.

PaulH.
Paul Hunt

Hmmmm 0.32 for the "C".

Wiring sorted. Thanks for the diagrams, Paul.

Problem was that the revcounter earth was actually only connected to the voltage regulator and not to any earth itself (butchered loom from a B in a C) and it seems the path of electrons somehow got reversed whenevr the car got above a certain voltage from the alternator. Late MGB loom fails to meet the British Standard (BSau 37) wiring colour code standard.!!!
As for the sticking reevcounter needle, it wasn't simply an electrical glitch, a piece of swarf had dropped into the casing and was fouling the windings against the magnetic ring that forms the worrkings of the system. Just got jammed up as it passed between thew moving parts.

Now it is time to burn some serious rubber!!!

TTFN
rob

This thread was discussed between 20/11/2001 and 26/11/2001

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