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MG MGB Technical - Fuse blowing - and I've only got two of them!

My Mk1 '64 blowing the lower (bottom) fuse of the two in the fuse box, can anyone help with a solution?

The bottom fuse has one white+blue wire screwed into it. It has three green wires on one side (near the driver) and three white on the other (nearer the front of the car).

As soon as I turn the ignition on the fuse blows.

The fuel gauge flicks a little as it tries to register what's in the tank and then goes back to empty as the fuse blows. Neither the indicators nor the interior fan or the wipers then work. The headlights and side light do work.

When one of the thicker white wires on the bottom fuse is removed (two white wires are thicker than the other) the fuse no longer blows but the car will not start (the engine turns but does not fire up). With the fuse removed and the wire replaced the car starts.

I am about to start looking at a wiring diagram but there are experts out there on this BBS! Can anyone help?

Oh, and I have changed nothing electrical on the car lately, the problem has come out of the blue after around 1000 great motoring miles so far this year.

John.
JW Prewer

John,
do you know where the "extra" blue and white goes? I would disconnect that first and then the other green wires go to the voltage stabilizer, wiper motor and reverse light switch. Disconnect all and put them back one by one or get the meter out and check which leg has the short.

MGmike
M McAndrew

John, after a quick look at the diagram there's a couple of green feeds from the voltage stabilizer to the flasher unit and heater. It's a case of finding the offending connected item but my guess is whatever is on the blue/white or the wiper motor.

MGmike
M McAndrew

Mike,

The extra blue+white is connected using with a screw, so I think it's probably putting power somewhere important, I'm going to check whether it goes to the ignition coil.

A look at a wiring diagram is confusing as hell! How a green wire stops the ignition is beyond me. The greens run 1. the wipers, 2. the brake lights and 3. the indicators, fuel gauge and heater fan!

Above all how did this problem come about in the first place? Looking through the archives it could be a switch gone wrong, so I may be under the dash next.

John.
JW Prewer

Mike,

You are looking at a better wiring diagram than me!

After a bite to eat I'll settle down with the one from my workshop manual.

John.
JW Prewer

Mike,

Does this one look like yours? www.advanceautowire.com/earlymgb.pdf

John.
JW Prewer


PROBLEM SOLVED!

Mike you were right, follow the blue & white wire.

I was a couple of hundred miles from home last weekend when the throttle cable broke. The RAC man fixed it, but he put it back hard up against the wiring for my Kenlowe fan manual switch on the dashboard and the cable sawed its way though the wire and shorted it out.

So no changes to the electrics, but...

John.
JW Prewer

John,
nope much more simple! That one is for making your oldie a modern with loads if fuses and relays! But if you count the four fuses at the top of the power block as the one you you have it connects all the same kit except the the extra blue and white......
The blue and white will be important to something but you'll need to find out what and if its at fault.

MGmike
M McAndrew

crossed in post! But...? no fan!
M McAndrew

sorry got it on second read.... But.... change to the throttle cable...

doh me
M McAndrew

Strangely enough a job for the weekend is to fit the Kenlowe thermostat.

I ran the car with a late MGB cooling fan thermostat in the radiator for a number of years then changed the radiator and never re-fitted it, so I've had cooling using a manual switch on the dashboard for 20 years or so. With the MGB 50th I'm getting to be an originality freak, so back into the bottom radiator hose will go the thermostat from the 60s that was on the car when I bought it!

After all that I was going to re-wire the manual switch to tidy the wiring up, but now that is the first job!

John.
JW Prewer

Learn to read a wiring diagram - your life will improve.
White wires are the power from the key to the fusebox and other stuff, with the White side serving as a connector of the three White wires - only one comes from the switch. These actually have nothing to do with fuses.
The W-G fuse is between that hot White and all of the Greens, which are all the fused accessory feeds = TS,Brake lights, rev lights, OD if have, instruments, heater blower, wipers.
fuel pump and IGN are W not fused (but should be).
Strange wires are always suspect and usually in the wrong place. Fan should be off the browN at the other fuse, or the W, but in either case have its own inline fuse, assuming it is the Kenlowe feed.
And really, the Kenlowe should have a direct fused feed off the solenoid main connection; if not, you should fit a relay in the W IGN line, as the fan is pulling more power than the IGN switch and wiring likes.

FRM

FRM
FR Millmore

FRM, I agree.

I re-wired my MGA after a small wiring fire - during was my second drive after buying it!

The MGB has a lot less miles but yes I am going over it getting it right. OK, so I am doing this a few decades later than I should have, but I am down to just one MG now and it's a lot easier to get the jobs done than when I had five!

Note to self: must buy another MG...
JW Prewer

Put in lots of fuses and some relays!
I try to put pic, later car but you get the idea - if this works.

FRM


FR Millmore

Rest of it - headlamp relays.

I can send you the doc/table which describes all this if you email and ask. Is an Open Office .odt file. Word should be able to read it, or download Open
Office for free and and screw Bill Gates.

FRM


FR Millmore

John,
note to self; must reduce fleet to spend quality time on one old girl at a time!
Current status is similar to your past with 6, yes six.... and I'm not as young as I was ;o)

FRM, I know it's cheating to get someone else to do the work but further up the thread John posted a link to a "master block" which, if still available, looks neat.

MGmike
M McAndrew

Fuses & relays. Power feed is from the "box" at bottom right. This has a cable from the starter to 2 studs-one for power to fuses the other enables underbonnet starting /power- similar to a BMW who "provided" the cable! Second pic are headlight relays under a shroud-there is a baffle to prevent direct rain thru the grill.

Second pic won't play....


Michael Beswick

Seems like to have to wait till the editing opportunity expires...


Michael Beswick

Mike-
I studied the Advanced Wire blocks and many others before arriving at my solution. They have things I don't like and you don't need or want, though I do not now recall the details. My system requires no mods to the harness for the headlamp relays, which are fine where I put them and easy to see, work on,etc. On some cars I have put the fan relay on the back of the same panel. It is only necessary to add two fused power leads to the relays.

The fusebox as shown requires minimal changes to the harness, but gives maximum protection of everything, and eliminates the problems of the 4 browN connector on later MGB, and the stock IGN relay, and the fan which are sometimes relayed and sometimes not, but always should be. Fusebox cost like $8 at local parts store, and the only two holes drilled are for those mounting screws..

I was straighten out consultant on a two year installation of an Advanced block like that in a TF, and hated it, but it was amusing to watch the DIY guys try to put the thing in. It is good quality, but overkill, and the wires are like bending rebar. Really for cars with lots more power and people impressed by oversized wires.

FRM
FR Millmore

John I can see you have. Had a lot of imput on this problem,
The problem appears to be on the green wiring put a new fuse in remove the green wires from the fuse box, switch on the ignition and touch the green wires to the fuse the terminal till one blows the fuse isolate that wire. Connect The remaining good greekn wires to the fuse. Box put new fuse in and check to see what's not working that's the circuit were the short is, it may be wipers trafficactors temp and fuel gauge but at. Least you will now know where to start looking.

Regards Patrick
P.T. Tighe

This thread was discussed between 03/08/2012 and 04/08/2012

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