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MG MGB Technical - Electrical advice needed

The answer to my questions may well be "poor earth!" but I will ask anyway.
My restoration is nearly done and I connected the battery for the first time. With ignition off -

Park lights (front and rear), number plate lights - zip
Map light - OK
Main headlights - OK
Dash lights - zip

I twiddled the instrument light reostat and the fuel gauge needle hit the F post (the fuel tank is empty BTW). I removed the fuel gauge lamp holder - the needle went back to E.

I haven't dared turn the ignition on until I sort out these problems. Wondering if the restored and painted body, gauges, etc are preventing proper earthing?
I must (?) have power to the park light post on the switch otherwise I wouldn't have a working maplight. So why no lights?
BTW I spent hours checking continuity and labelling every wire in the loom before assembly. I also used electrical contact grease at all the bullet connections with either new of thoroughly cleaned connectors.

Should I install dedicated earths to every light fitting and gauge.

John
John Minchin

John, what age is your car? Have you got a multimeter? You need to start at the switch and see if you have volts there and then follow along the wires to find out where the volts disappear. If the volts get right down to the lamp units, then it must be duff bulbs, bulb holders or earths. To eliminate the gauge rheostat, simply take the two wires off the back of it and connect them together. A lot of owners do that anyway as the gauge lights aren't exactly bright on maximum power. If the rheostat affects the fuel gauge it must be due to there being no earth on the gauge body. The gauge lamp needs to have an earth return as do all the instrument lights.

Work through the circuits logically and carefully using a multimeter and a wiring diagram. There are excellent coloured ones here http://www.advanceautowire.com/mgb.pdf simply find the one that is relevant to your car. If you don't have a multimeter, get one! You only need a simple one and they are really cheap.
Mike Howlett

Have a look down the list to "more electrical gremlins",lots of useful info there!
Allan Reeling

I just happened to have an electrical schematic for a 75 on my desk as I read your post.

first of all go to http://www.advanceautowire.com/ click on stock schematics and scroll to your year.

If the schematics don't help, get back to us with the year.


MGBV81

Always tell us the year. Chrome bumper parking and number plate lights have their own separate earths at each corner where the light unit bolts to the panel, so you either have four bad earths, or one or more bad connections in the red wires from the main lighting switch out to them, via the fuses if you have them. The top two fuses in the four-fuse box (if you have that, one fuse per side, just a single red/green wire on the front of one of them, two reds on the back of each) are for the parking and number plate lights and so are a convenient place to check for voltage. Before that there were two in-line fuses - one for the fronts and one for the rears, another convenient point. Before that there were no fuses but you should be able to find two red wires coming out of the main harness joined to a single red from the rear harness, in the mass of connectors by the fusebox.

I had no problem with the light units at the corners of the car after my restoration but I had to provide wired earths to the number plate lights.

The rheostat and fuel gauge issue shows two circuits are connected together that shouldn't be, there should be no electrical connection between the two spades on the back of the gauge that operate the pointer and the case i.e. where the night-time illumination bulb plugs in. Where something similar happens to the tach that *does* indicate a problem with its earthing. All the gauges hould have a black earth wire - either to a spade on the back of the tach, or under a/the nut that holds the gauges in the dash for the others.
P Hunt

Sorry - Its an early Mk1 (63) with no separate earths on front and rear park lights, number plate lights or gauge lights.

I've sorted the front parks - the spring loaded contacts hade been pushed back too far and hadn't returned to contact the bulbs - earth seems OK through the mounting bolts
Rear parks - earthing is OK - I just hadn't wired everything up properly (the live 12v wire wasn't connected to anything :-(

Numberplate lights - 12v is present - earth probably no good so I will install separate earth wires.

Gauge lights - I checked the one for the Fuel gauge - has 12v but no earth through gauge. I got it working with a separate earth to the bulb holder. And the rheostat works! I've yet to check the other wiring to the gauge.

I'll keep you posted
John
John Minchin

The early gauges may rely on cutting through the paint on the back of the dash to get an earth, then, although the schematic in the Leyland Workshop Manual definitely shows a wired earth going back to a body earthing point with the wiper motor (behind it). However it also shows a local earth on the rev counter, it wouldn't have both, so one or other could be an error. Whilst number plate lights eventually got a wired earth, oddly in 1977 the fuel gauge sender lost the one it had had from the beginning to rely on a series of mechanical mountings and fixings instead.

The early 'swinging needle' fuel gauge does need an earth at the gauge as well as one at the sender.

P Hunt

Update:
Separate earths fitted to number plate lights - problem fixed! Curently doing the same to instrument lights by soldering wires to the bulb holder.
Also discovered that the coil wires were connected for +ve earth whereas the car is converted to -ve earth.
I'll make sure the Fuel gauge housing is separately earthed - the dash is powder coated so the gauge is effectively insulated from earth by the coating and the rubber sealing ring.

John
John Minchin

This thread was discussed between 10/01/2010 and 17/01/2010

MG MGB Technical index

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