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MG MGB Technical - Electricals dead...almost

I thought: Now that the alternator belt problem is resolved, I can trust the car not to strand my wife or son. Mr. Murphy visited this morning.

My son turned the car off to go inside the school office to get a parking pass, and when he returned, the electricals were dead. I left work, got some tools, and was at the car within 40 minutes. I expected to find a loose battery cable.

Symptoms: 12.74VDC at battery terminals and cables. OVDC at the fuse block brown wire. R&Rd the alternator connector, and 12.74VDC appeared at the fuse. Turned the ignition key, and voltage at fuse disappeared. Lightly slammed door, and voltage returned. Each time the voltage returns and I engage the ignition key, the voltage disappears, but is always strong at the battery cables, never wavering.

Is all battery power routed through the starter solenoid switch, even lighting power?

It's a '77B, and is still in the school parking lot.
Fred Doyen

Fred. All of your negative connections are based on the ground cable from the battery negative terminal to ground. The cable needs to be in good condition, the cable clamp should be of the molded on type (rather than the one that bolts to the cable with a strap and two short bolts) and the ground connection should be clean and tight.

All of the positive connections originate at the postive battery connection, the go to the starter solenoid. Make sure the cable and clamps are good at the battery and the solenoid. I have seen this cable as the most common problem with complete loss of power.

There is also a ground strap from the rear, right transmission mount to the transmission cross member, thence to the frame through the cross member. You need to make sure this ground strap is in good condition and that the bolts attaching it are tight.

You should find your problem at one, or more, of these places. The removing and replacing of the alternator plug causing the system voltage to come back argues that the connection at the starter solenoid is the most likely problem. But, check all of the connections and cables for tightness and condition. Otherwise, you will continue to have your problems "piecemeal" fashion. Les
Les Bengtson

My best guess would be to keep looking in the alternator region since thats where you've been working. Check all ground cables and wires to the alt. Some of the wires get corroded and snap inside the insulation. Maybe try doubling up on some of the connections.

Good luck,


No wiring diagrams right now, Les, but just found one similar on the internet, for 75-80 cars. It shows the positive cable going from the battery to the solenoid, which would fit what I suspected and what you said in your last paragraph. When I turn the key, whatever mechanical jolt the solenoid gives probably opens the faulty connection. Sometimes I hear nothing...the clock just stops...which means that if it's a loose connection, then it's very loose.

So, if this were to loosen while driving, what indications would my son have received from the car, if the battery just became disconnected?
Fred Doyen

Fred. If this happened when driving, the engine may, or may not, shut down. If the wires in the "brown wire circuits" were making sufficient contact with the cable end from the positive terminal of the battery, or from the wires coming from the alternator, the car will continue to run. This is especially true during the day and in good weather as there are no lights, wipers or heater motor to draw extra current. These poor connections/cables show up when the system is under load--when the lights, wipers, heater and turn signals are all used together or when trying to start the car.

I have had a similar problem twice over the last five years. Both times it was the cable from the positive terminal of the battery to the starter. Once, it was the cable itself--internal corrosion. The other time it was one of those clamp on battery terminals used by the cheap to replace the molded on terminal. In both cases, a new line from the battery to the starter cured the problem.

If all of the connections are tight, have your son try to crank the engine over while you measure the voltage at the starter solenoid connection. You should see system voltage before he tries to crank the engine. When cranking, you should see at least 11.0-11.5 volts at the terminal. When I had bad cables, the cranking voltage would be 3.0-4.5 volts indicating the cable was not capable of carrying sufficient current under load.

Luigi. If it were the alternator plug, or the alternator itself, the charging indicator light should be glowing when the engine is running. An area worth checking, but not the most likely candidate until the other areas have been checked and found good. Les
Les Bengtson

We went back to the car, still in the parking lot, with tools, jacks and stands. The car cranked right up and we drove it home.

I removed all starter wiring for inspection. A small brown wire came apart in my hand, at a small electrical tape wrap. I soldered the wires together and used heat shrink tubing for insulation. Repaired 3 other pending disasters from a POs legacy. While monitoring voltages at the battery with my son cranking, a nice sized arc came from the positive connector/cable interface (not the battery pole). I had not noticed before that the connector was clamped onto the cable (hidden by a plastic cover). It was very loose, and has my vote as the cause of the "deadness" symptom. Based on your comments, Les, I think you will agree.

The defective brown wire that I fixed probably caused another problem I intermittently experienced a couple of weeks ago.
Fred Doyen

True enough, Les, but I figured that if he hadn't gotten the car started he wouldn't be able to tell if the alt light was functioning. He would have had a previous alt light coming on and off while driving, tho.

Glad to see, Fred got his problem solved.

Fred's experience just reinforces my firm beliefe that the only real problem with Lucas electrics is the halfa**ed repairs that some previous owners do the the electrical system. Cheers - Dave
David DuBois

Fred. I would agree with your analysis. Had exactly the same problem about a year ago--PO had used one of those replacement, clamp on, battery clamps and it had loosened. A through cleaning and a couple drops of Loctite on the two bolts holding the strap tight seem to have cured the problem. Replacing the long cable is something of a problem--dirty and time consuming. So, most people simply replace the clamp when it goes bad. Done properly it will work. But, replacing the entire clamp and cable is a better idea.

Glad you got this fixed. My experience is that, with any newly purchased old car, it takes about six months of "debugging" to find and correct all of the car's problems. After that, they tend to be reliable for a number of years with regular maintenance. Les
Les Bengtson

The alternator plug 'curing' it was just a coincidence. All brown wire power on all cars comes through connections at the solenoid, either spades or bolted lugs. Although the latter are more robust than the former they can still fail, and battery and ground strap connections can also be the cause. Sometimes there will be enough power getting through to operate the courtesy light, which goes out when you turn on the ignition or crank, sometimes not. To find the location you have to do voltage measurements when the fault is evident, often when the load has been applied, as that maybe the only time the voltage is 'missing'.

It is a problem that can stump the professionals, mine chose being up on a ramp for the annual test to fail after giving no previous warning. The garage tried booster batteries (with no effect of course) and in the end I had to go down in another car and diagnose it with a voltmeter. Mine has the bolted brown connections and even though it was tight I could meaure 12v on the battery lug but not the brown lugs. Dismantling and cleaning with emery cloth was a five-minute fix, aided by it being up on the ramp.
Paul Hunt

The fault was only evident when I was unprepared to troubleshoot, but the courtesy light and clock were operating as Paul describes.

I would like to solder the connector onto the end of the battery cable, if I thought the insulation wouldn't burn off the cable for 5 feet. I'll just have to watch it. I also intend to modify the attachment of the battery ground cable to chassis to insure I never have a problem there.

Where is the ground strap from engine to chassis most likely located for the '77 B?
Fred Doyen

Either through the transmission mounts under the car or on one of the engine mont bolts. I decided to do both on my car.


Fred. The factory ground strap on your car is located on the right transmission mount to the transmission cross member as I mentioned in my original post. Les
Les Bengtson

Oops! That's embarrassing. I usually read pretty well.
Fred Doyen

This thread was discussed between 04/01/2005 and 06/01/2005

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