MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Fuses and guages

75 -MGB
Problem -
No Fuel Gauge, Temperature Gauge, Windscreen Wipers, Directional Indicators, Tachometer.

Green fuse burned. Replaced. Burned again when I turn on ignititon.

Where should I start?
HC Wallace

HC. You should start by getting a good copy of the factory wiring diagram. If you have a copy of the Bentley manual, or the Haynes, go to a copy shop and have them make a reproduction of it. Then, have them expand the reproduction by 300%. Then, have the enlarged reproduction laminated. (Dan McMasters has produced a color coded wiring diagram which may be of significant value to you. Do an archive search under his name, go to his website, and see if he has the proper diagram to fit your car.)

Then, disconnect the wires from the green fuse. Start the car. Connect one of the connectors. (There are, as I remember it, two wires connected to that fuse--the third fuse from the top with a white wire input on the front and the two green connectors on the back.) See if the fuse blows.

If so, disconnect that wire, install a new fuse and connect the other wire. If, the fuse blows again, you have bigger problems than I can help you with. Go to Paul Hunt's website, "The Pages of Bee and Vee" (someone was kind enough to list the website address on the thread about brake disc replacement--Thanks). Contact Paul. He is a very highly qualified individual, holding a degree as an electro-mechanical engineer. A Gentleman, a Scholar, and a Teacher--he has taught me much of what I know about electrical systems over the years. A friend and honored companion in things MG.

If only one circuit tends to blow the fuse, you have a short somewhere in that circuit. Use your wiring diagram and an ohm meter to see what is on that circuit, then, disconnect all of the wires at their terminals. Turn on the ignition, removing the wire from the coil to the distributor to prevent burning up the points and all of the wires back to the ignition switch, including the brown wire running to the switch. (Yes, I did make this mistake. Myself. I am an "MG Guru". I am also human and make my fair share of mistakes.)

So remove the wire from the coil to the distributor before turning on the ignition switch. Hook up the various circuits until the fuse blows. Replace the fuse. Disconnect the circuit that blew the fuse and hook up the other circuits on that system. Disconnect any that tend to blow the fuse and leave attached any that do not tend to blow the fuse.

By the time you are finished, you will have isolated the problem down to a single circuit or two. At that point, it is simply a case of running jumper wires to the bad circuits. If the fuse blows, it is a problem with the item that is being hooked up to. If the jumper wire does not cause the fuse to blow, there is a problem with the wire running to the instrument. If the fuse blows with the jumper wire, there is an internal problem within the instrument which needs to be addressed.

Les
Les Bengtson

http://www.advanceautowire.com click on stock schematics. Make yourself a couple of enlarged copies of the appropriate diagram, one for reference and one to make notes on. I made one for myself that is approximately 14"x20" by printing on four sheets of paper and taping them together. Some people will go to the expense of having the diagram laminated, I didn't bother. If I need a new copy, I'll just print one. There are a couple of different diagrams for '75. Look for the one that best matches your car (look at the wiring schematic for the seat belts to see which one best matches your car).

Unplug the white/lightgreen wires to the coil and insulate them from shorting to protect the ignition circuit. Then turn the car on, but don't start.

One of the easiest ways to find a short is to unplug all the green wires to the fuse box. Then plug the green wires in one at a time until the fuse blows (you will probably need a few fuses). Unplug the wire that caused the fuse to blow and plug in any remaining wires. This will help to narrow down what part of the green circuit is giving you trouble. If possible note what you are turning on when the fuse blows (does it blow when you signal to go left?). On my car it turned out to be a pinched wire going to the brake light switch. You then have to physically trace the green wire(s) that are affected. Unplug various components on the section of the circuit to narrow down the problem area even further. Look for worn/cracked insulation, faulty/loose connections, pinched wires, and test the components. A possible example of a bad component would be the wiper motor. If the wiper motor is not working properly and is causing a high draw of power it will blow the fuse.

As you check each connection, make sure it is clean and secure, and use dielectric grease to prevent future problems. Dielectric grease will also make it easier to plug and unplug connectors. If you have a bad connector, replace it. Connectors are availble from Moss Motors, Victoria British, britishwiring.com , and a few other suppliers of british car parts. I usually avoid Moss and VB if I can due to their high shipping charges. When I do have to order from Moss, I go through a local distributer, this saves me on shipping charges and I usually get better service because the person answering the phone is not just an operator, he/she is someone whom is actually familiar with the cars and knows what the parts look like so the correct parts get packaged and sent to me.

After you have found the short/problem, fix or disconnect it until it can be repaired. Don't forget to turn off the car and connect the wires to the coil.

Be patient and methodical in your approach.

http://www.britishwiring.com/CAT14_15.PDF
http://www.britishwiring.com/CAT20_21.PDF
The above links are a price list for connectors and fuses. If you need to order connectors, order an assortment of the type your car uses so you will have spares for the future.
Kimberly

A good tip when looking for shorts is to make yourself a test lamp, from a headlight bulb (You need a high current bulb) with flying leads and crocodile clips, (or you could solder to the ends of your dead fuse)

Connect the lamp in place of your fuse.

Whilst you have a short, the lamp will be on*, and the wiring wont be damaged, because the lamp takes 4Amps or so.


Disconnect things until the bulb goes out, and you've found it, by following the advice above.


*At full brigntness. If it is dim, then you have something turned on, this is normal.
Martin Layton

Lots of good troubleshooting tips above. I usually use a 10 amp battery charger to test circuits that are blowing fuses. You can disconnect the car battery when using a battery charger as a troubleshooting tool. Attach the battery charger to the suspect circuit or item, if it has a short the circuit breaker in the charger will trip. It works for me and doesn't use up my spare fuses.

Clifton

Clifton Gordon

The green circuit is one of the hardest to detect shorts on as there are so many branches. If your fuses are blowing straight away then be thankful, it is much worse when it is intermittent!

I favour the headlight replacing the fuse method. You can start by disconnecting one of the (usually) two connectors on the green side of the fuse and see which one causes the light to glow it full brightness. Then replace it with the other one and see which circuits work and which don't which narrows it down. Then do the same at the various 4-way bullet connectors in the faulty branch. You will need the exact diagram as the green circuit varies greatly across the years.

Check the brake light switch wiring first, this can get trapped or chafed more than pretty-well anything else. Also check the inlet manifold heater wire.

Thanks for the kind comments Les, although my 'degree' is from attending the 'school of LBC ownership and maintenance' for 40 years rather than any formal qualification.
Paul Hunt 2

This thread was discussed on 28/08/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.