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MG MGB Technical - Complete failure

Please help my MGB is driving me crazy.

After being idle for a few months I tried to start my mgb and got no fuel to the carbs and no stark at the plugs. I've replaced the battery cleaned the earth. fuel pump didn't click, so I've replaced that. I've replaced the distributor and coil but still nothing. One clue may be that when I replaced the optronic ignition with the old contact breakers they just burnt out. The train of thought I was on was that there was a fault to earth somewhere before the fuses. So that 12v is going straight to earth and knocking everything out.

Please help before I burn the thing.
PJ Tilyard

Would have thought that if you had a short to earth before the fuses not only would you have a very flat battery but probably a burned out wire at best or a burnt out car at worst! Check the resistance from battery earth to the body. I assume it's turning over on the starter OK? You can easily check for a short by putting an ammeter in series with the battery +ve lead (but be careful - if you DO have a large short it will spark when you connect and blow the fuse in your ammeter). Do you have power at the contact points? Is the condensor fitter correctly - did you put the little insulating washer on? If not it will be shorting to earth and you'll get no spark. Have you power at the fuel pump? Have you got the polarities right (you don't say if it's +ve or -ve earth)?

T Jenner

PJ. You need to stop removing and replacing parts and get a copy of the wiring diagram for your model year. You also need a volt meter or a test light.

Tim has some good ideas about why you may have burned up your points. If the points are connected incorrectly, or if the ignition switch is left on when the points happen to be closed, the points can burn, along with a number of the wires leading to them.

Basic power flow is from the terminal of the battery which is connected to the starter solenoid (power output) and from the chassis to the ground terminal of the battery (power return). Depending on what year your car is you will either have a positive ground set up (standard up until 68) or a negative ground set up (standard from 68 onwards). If you have a pre-68 car, it may have been converted from the factory positive ground system to the later negative ground system, allowing the use of a modern radio.

Hence, we need to know the year of your car and what the grounding is to provide specific assistance.

Power from the main terminal of the starter solenoid is distributed through the brown wire circuit. Brown wires are always live and should always show power when checked with a test light/volt meter regardless of the position of the ignition switch.

The ignition switch, when in the run position, provides power to the fuel pump and ignition system through the white wire circuit. Depending on the year of the vehicle, you may have an ignition relay as part of this circuit. Power flows from the constantly live brown wire circuit to the ignition switch. When the ignition switch is turned to the run position, the power flows through the switch into the white wire system. Both your fuel pump and your distributor system are on the white wire circuit and the failure of the fuel pump can be caused either by a bad pump or by a lack of power to the white wire circuit. Burning up the points is a good sign that there was, at one time, power to the white wire circuit. But, by the time the points have burned up, you can have burned wires all the way back to the ignition switch and beyond.

You need to charge up your battery fully. Clean and attach the battery leads to the battery terminals, then, go through the electrical system, using the wiring diagram, to find out where you have power and when.

Anytime you are testing the white wire circuit, but are not attempting to start the engine (such as checking for power at the fuel pump), disconnect the wire from the distributor to the coil. This will prevent the points from being burned up, along with various wires, if the points are in the closed position with power on.

There is a tech article on ignition system troubleshooting on my website, under the MG section, then articles. Please take the time to read it and see if it makes sense to you while you are obtaining a copy of the wiring diagram and charging the battery. Then, let us know the year of the car and whether it is positive or negative ground.

Fairly easy job of troubleshooting if followed in a logical manner, but a very difficult job is gone at in a piecemeal manner. I have done it both ways and a considered approach works best for me. If it works for you, let us know what we can do to help.

Les Bengtson

Many thanks Tim and Les. Seems I have a lot of checking to do. I was on a bit of a throw and replace spree. My B is a 1974, so its a negative earth. I think you have exactly identified what happened to my contact breakers, the ignition was left on Les, and the insulating washers are missing Tim. I will go through all your tips and your comprehensive ignition system troubleshooting guide. I will let you know the outcome.
I really appreciate the advise, it has already been a huge help. My mg will never know how close it came to being abandoned.
PJ Tilyard

This thread was discussed between 12/07/2007 and 13/07/2007

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