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MG MGB Technical - electrical stuff

OK, I buy this 74 MGB and want to bring it back to life. Things are going well, I drop in the batteries today and and decide to turn on the lights before I get down to starting this rebuilt engine. Nothing, I mean nothing, no lights, no power, nota. I trace good power up to the fuse box and then it gets all strange. I doubt we could solve this by internet, so does anyone know of some good reading that might help. I would hate to have to hire someone to fix this unforeseen problem. I went bald scratching my head over this one. Thanks
Rocco
Rocco Spartano

interesting problem you got there...If it was my car I might check what load you are putting on the circuit, with all electrical systems off you are getting 12 Volts at the fuses, what happens when, lets say you turn the ignition on? Are you still getting a reading? My only thoughts might be a bad ground at the batteries, but it has just enough to give you voltage up front to read with a meter but not a strong enough circuit to run anything.
EJW Willis

Roco
I assume form your posting you have only checked whether there is power at the fuse box? Have you checked if power is at the light switch? You see the power comes from the battery to the starter motor then up to the light switch. From the light switch it goes to the fuse box.
Here is a web site that might help you locate any problems you are having without describing all of the ins and outs you are doing on the BBS.
http://www.mgbexperience.com/electrical/

As for books, I would suggest Bentley’s book and/or a Haynes manual. They are shown on this web site.
http://www.mgbexperience.com/

If you have power up to the fuse box and nothing after then I would suggest you ensure that the connections and the fuse holders are clean.
Cecil Kimber

Cecil: I thought the power went from the starter motor to the fuse box then to the light switch, then depending on switch position back through the fuse box. I'll ck the power at the switch tomorrow. I get power to the bottom fuse most of the time, but nothing to any other fuses. Fuses 3&4 should be powered only with the ignition on, and right now that doesn't work either. My grounds seem to be fine. This could prove to be interesting. Without checking the light switch right now I have power to one fuse and that is termittent at best, and nothing downline.
Rocco
Rocco Spartano

The believe headlights should not be on a fused circuit (from the factory anyway.) I'm not sure of their routing in relation to everything else though. In any case a proper wiring diagram will be required if you don't want to drive yourself insane. As mentioned before, the Haynes or official workshop manual from Bentley will provide this information in detail. As to your other problem with the number 3&4 fuses not working, is this an intermitent problem? If it is you might have bad/broken/corroded connections on the back of the fuse block itself, where the tabs are rivited to the fuse clip.

I had this problem on my '70 where some tail lights did not work all the time. If I remember correctly the DPO had jumped one side of the 3 fuse to the other side of the 4 fuse to "fix" the problem. This rendered one fuse innefective in case anything should short out. Turned out the fuse block was bad, once it was replaced with a new one, everything worked properly.
Jared Snider

Rocco. On the 74 there are some plugs that connect the wiring loom to the dash
components. It sounds like these plugs may not be connected. The brown wires on
the fuse block come from the battery terminal on the starter. Are your battery cables
in good condition? In my opinion, if they have the two bolt clamp on battery terminals
you see for sale at most discount auto parts places on them, the cables should be
replaced. I have seen lots of problems with corroded cable strands hidden under the
clamps of replacement battery terminals. Sounds like you may be using two six volt
batteries more connectors for possible problems!! Hope this helps. Clifton

Clifton Gordon

On what looks like all years the brown comes from the solenoid (or RH toe-board stud) to a sealed multi-way connection. On a 74 there are separate branches to the ignition switch, lighting switch, fusebox and hazard in-line fuse. The one to the fusebox goes on to the starter relay and the in-line fuse for the bum detector. Both the wire to the lighting switch and to the ignition switch go through multi-plugs by the steering column. I would check for 12v here (both sides of the plug) as well as at the fusebox, with lights turned on as well as off. A voltmeter could well show 12v on a circuit with no load,but if there is a bad connection back towards the battery it will show much less than that, quite possibly 0v, as soon as you apply a load e.g. the lights.
Paul Hunt

>>bum detector

Paul, where is this located? I have a ’74 but I have never come across one. May be I should have to weight…err.. sorry wait for an answer ;-)

Rocco was quite right when he said >>I doubt we could solve this by internet,
Cecil Kimber

I don't think the 'B ever had one as the Seat Belt warning light on the later variants worked off the driver's belt stalk only.
However, I once had an MG Metro and that had a Seat belt warning light that worked the same way for the driver's side and had a bum detector in the passenger's seat to trigger the activation of the light when that belt was disconnected (vastly superior to the SAAB of the same vintage that I owned which left the annoying light on, unless you drove around with the passenger belt permanently fastened).
Anyway, this was described in the book as a "Seat pad switch". I always assumed that "pad" stood for "Passenger's Ar*e Detector".......
Tim Cuthill

The bum detector was a North American requirement that lasted for about one year only since the NA manufacturers couldn't seem to meet the specs ...

I mentioned it since Rocco probably has an NA spec car.
Paul Hunt

Paul: I have one of those cars, re doing the seats and am torn on putting those little white round devices back in. One good thing is from the help on this post and the outside reading I have on all my lights except for the headlights. The rear lights are on the plate lights, the instrumentr lights and one (right side) yellow running light is on. I figure if I keep plugging away the headlights will go on also. (I hope) Thanks
Rocco
Rocco Spartano

Keep 'plugging' all the connectors in and I'm sure they will ...
Paul Hunt

Well folks:
Rocco Spartano

Guess I hit the wrong key, anyway with your help and some good reading material after about 16 hours of work I have most of the electical working. Still need the brake lights and the widshield wipers, but there is always next week. I found most of the problems nothing to do with Lucas. I never worked with Lucas before so I was a bit shy, but the car was bought from someone trying to restore it and just gave up. I found wires missing, connections not there or broken, grounds gone, some painted over. Wire mixed and some frayed to the point of shorting. Thanks again, if I need help with the brakes or the wipers I'll call. Is winter over, can I drive it soon?
Rocco
Rocco Spartano

Rocco, Good to hear you are making satisfactory progress.
Regards, Clifton
Clifton Gordon

This thread was discussed between 01/04/2002 and 06/04/2002

MG MGB Technical index

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