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MG MGB Technical - Voltage Regulator?

Driving along at night in my '77B and all lighting starts dimming. They recover a couple of times, but eventually dim to the point of almost going off. The engine starts sputtering. I move quickly to the side of the road, and at this point, the sputtering engine dies. I switch off the ignition and immediately re-crank, and everything's fine. This happened a couple of times, as I made my way home.

The car died on my wife last week, during daytime, so I didn't make a connection with the electrical system being the culprit.

The alternator's a rebuilt Lucas, about a year old.

It sounds like the voltage regulator, to me. However, I thought the battery would sustain ignition if the alternator failed.
Fred Doyen

Sounds more like a poor connection, if the regulator quit working the battery would not recover instantly. The ground strap from the body to the engine would be a prime candidate for causing this type of problem.
John H

Hi.

I don't see why an engine earth strap problem would affect the lights if the battery still had charge, as it apparently did (or the car wouldn't restart).

I would think it more likely to be a problem in or around the brown wire that feeds all circuits from the junction of the starter motor / battery / alternator output.

Don
Don

Ditto Don. An engine ground strap would have negligible effect, there are usually enough grounds between the engine and the body via accelerator, choke and heater cables to allow the engine to crank (albeit slowly and cooking the cables), let alone supply the relatively small load of engine and headlights. And even if the strap did fail the battery would keep things going, albeit with slightly dimmer lights. My money is on the brown circuit up from the solenoid to the ignition switch, lighting switch, fusebox etc.
Paul Hunt 2

Thanks, Paul and Don. The reference to the battery holding things up answered the question I'd asked, which is the indicator that it's not the VR.
Fred Doyen

Fully charge your battery, then use a meter to test your alternator's ouput. Make sure all connections are clean and secure.
Kimberly

I'm updating to keep this from falling into the archives before I report resolution; however, I DO have some new indicators of the problem, which supports Paul's wager, above.

I pulled all wires from the solenoid, inspected, and repaired a frayed wire, that would not have caused this problem. I took it for a test drive, hoping I had disturbed the wiring in a good way, and it drove beautifully for 10 miles. Coasting to a stop at an intersection, all lights dimmed, and I had problems for the last 5 miles home. Whenever the lights would dim, or when the car died, (about 5 times) I turned the car off, and the lights immediately became bright, and the car was easily re-started.

My car is a '77B, but all the schematics I have, on paper or from internet downloads, do not show an ignition relay. I have to use the "1978 & LATER MGB FROM BENTLY" to show this circuit, which is present on my car.

I'll get down to business and find the problem, within the next several days, and let you know the outcome.
Fred Doyen

Fred - Have you tried the drawings on the Advance Auto Wiring web site? Go to: http://www.advanceautowire.com/ then clikc on Stock Schematics. Good luck - Dave
David DuBois

Clausager doesn't mention it (as far as I can see) but the Parts Catalogue indicates the relay was used for 77 model cars (Sept 76 on) for North America at least. Having said that the lights aren't powered from the relay, of course.

The other possible cause of dimming lights and a spluttering engine (both due to low voltage) is a partial short somewhere which is pulling the voltage down enough to affect the lights and engine, but not enough to smoke the wiring. Without inserting fuses in various circuits to see which blows, it will be difficult to determine whether low voltage is simply due to a bad connection in the brown circuit, or whether a short is causing a 'less than perfect but otherwise reasonable' connection to sag under the very high current. Going back to the bad connection theme for a moment, I believe late model cars have two browns coming up from the solenoid to a special type of 4-way connector (translucent?) the other side of which also has two browns, one to the fusebox and ignition relay and the other to the lighting switch and the ignition switch, could be problems in that connector. A voltmeter semi-permanently connected to various points in the brown circuit at various times should allow you to progressively locate the point at which the low-voltage is occuring, this is something I have done myself in the past to track down intermittent electrical problems.
Paul Hunt 2

The special 4-way Paul speaks of appears to have been the problem. The connection into the 4-way of one of the two brown wires coming from the solenoid was corroded. I dissected the connector, burnished the copper, re-soldered slip-ons onto the wires, reassembled everything, and had no problems during a 20 mile trip, Saturday evening.

How could this have caused the lighting to dim and the car to sputter and die? You might answer the question by asking why there are two wires feeding the supply side of the 4-way, instead of a single wire of larger gauge? (Really, Paul, why is this?)It appears that the intermittent connection would remove half the current capacity of the circuit, and under heavy load, the voltage drop at the connection will rob the ignition circuit. Had I thought of it before the repair, I should simply have turned off the lights when the car started to sputter, to see if the engine would recover.

Is this a plausible explanation?
Fred Doyen

David,

Advance Autowire's schematics are my favorite, but they are the same as the others. I use a combination of schematics for the 77B, and later ones for the ignition circuit. It's no big deal. Knowing where the problem is solves the problem.
Fred Doyen

This thread was discussed between 14/06/2006 and 27/06/2006

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