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MG Car Repair/MGB alternator

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QUESTION: Hi Howard,

1968 MGB, negative ground US car.  I have owned the car 5 years. I have already replaced the battery, and had the alternator "rebuilt" at a reputable shop.  Prolonged periods between driving have sometimes led to difficult/no starting: I jump start, and no problem.  Today the car won't start, and clicking from the solenoid, etc.  Jump starts just fine.  However, I am disgusted because the battery is less than two years old.  I pull the connector from the alternator, and decide to test continuity from the wires, specifically the hot (brown) wire and the ground (black) wire.  The brown wire tests good continuity to the starter.  The ground wire shows no continuity to the negative side of the battery; obviously this is wrong.

What to do?  I traced the black ground wire to the passenger side by the dashboard, but I do not see where it proceeds from there.  I am tempted to cut the black wire leaving the alternator, crimping two new connectors to both wire ends, and creating a new ground.  Please talk me out of that idea, and give me a better one!  :-)

Thank you in advance for any and all information you can give me.

Regards,
Dan

ANSWER: Hi Dan,

Here is the test procedure you need. First charge up the battery and then use a volt meter to check across the battery posts. Should read 12+ volts. Now start the car and put the RPM at about 1500 RPM and check the battery voltage again. Should read 13.8v to 14.5v (not lower nor higher).

If you have an alternator with a 4 wire plug, one wire (black) should be a ground wire but if you have a 3 wire plug, there is no ground wire.

Set your volt meter on a low volt DC scale (if it needs to be set). Start the engine and set it at a fast idle (about 1500 RPM) put the negative test lead on a good body ground and the positive test lead on a clean metal part of the engine (a clean bolt or nut). The meter should read less then .5v DC. If it reads higher then that, you for sure have a ground problem which can be the black wire you found or even the ground strap from the engine to the frame.

Another test that needs to be run is a "voltage drop" test on the ground system and the positive cable system. The method is to get a long piece of wire to use as a test lead, preferably a piece of 12 ga. wire long enough to run from the battery out to the engine.
Bolt one end of the test lead wire to the engine itself and run the wire in to the battery and connect the other end of the long wire to the (+) positive test lead of the voltmeter and put the (-) negative test lead on the (-) post of the battery (not the cable end). Set the meter to a low DC volt range and hit the starter as you watch the volt meter. Note the reading and set the RPM again at 1500 RPM and note the reading. It should read less then .5v.
It is difficult to access the power wire on the starter solenoid so for the next test you can  attach the test wire to the large brown wire on the alternator and run it inside to the battery and with the engine running and set at about 1500 RPM connect the other end of the long test wire to the (+) lead of the volt meter and the (-) negative test lead to the (+) positive post of the battery (not the cable end). Again (if your charging system is working, you should see less then .5v DC.

Now with the engine shut "OFF" remove the large brown wire or wires from the back of the alternator and take a 12v test light and connect the alligator clip of the test light to the brown wires and touch the posts of the alternator that the wires connected to with the point of the test light probe. The test light MUST NOT light up. This is just a test of a shorted diode in the diode pack inside the alternator.

After you run these tests, then you can ground the black wire you found that was not grounded. Providing that the alternator is a Lucas and the wiring has not been altered. (Black wires on Lucas wiring systems are ground wires) If the wiring has been changed, You need to trace that black wire to see if it truly is a ground wire.

let me know what the test results were.

Howard

---------- FOLLOW-UP ----------

QUESTION: Hi Howard, thank you for your prompt response.  Here is more data:

Charged battery  12.1 V
Battery at 1500 rpm:  13.9 V
1500 rpm body ground-engine ground  80 mV (negligible)
1500 rpm negative side voltage drop 90 mV (negligible)
1500 rpm positive side voltage drop 90 mV (negligible)
12 volt test light  Did not do, should ignition be on or off?

I did further investigating into what parts/wiring I have.  Originally the car was supposed to have a 4 wire Lucas alternator (hot, ground, and two wires (NY and NG) to the external voltage regulator). What is in my car is a 3 wire Lucas piece placed by someone else.  The 3 wires coming out of the connector:  Brown (hot) going to starter, black (ground, which was cut, which explains the no continuity), and an orange wire that is butt spliced to the the original NY (brown with yellow tracer) wire that goes to the voltage regulator.  The other, original (fourth) NG (brown with green tracer) wire that also comes from the voltage regulator is disconnected.  This entire mess of wiring was copiously wrapped in black electrical tape, which I unwrapped.  Is there a way to measure current output from the alternator?  Hopefully you can make some sense of this...

Regards,
Dan

Answer
Hi Dan,

The 3 wire alternators usually have two large posts and one small post and a few had one large post and two small posts.

The three wire alternators with two large posts and one small post take two 12 ga brown wires on the two large posts which are actually only one connection. The one small pin gets the "IGN" (charge light wire from the instrument panel)

The three pin alternators with one large pin and two small pins get the large 12 ga brown wire on the large pin and one of the small pins gets a (+) small brown wire and the other is the charge light called the "IGN" light wire. On both versions of the car (4 wire with external regulator) and the (3 wire with internal regulator) use the brown with Yellow (N/Y) which is the charge light wire. A simple test of the "trigger" wire (N/Y) is to have the N/Y wire disconnected and turn on the ignition and note that the "IGN" charge light is "OFF". Then just touch the N/Y wire end to a ground and the "IGN" light should come "ON".

This circuit is very important as it is not just a indicator light but is in-fact the "Exciter" circuit for the alternator to start charging. In your case the reason they spliced the orange wire (Probably the "Exciter" circuit on your alternator) to the brown w/yellow wire is that at the old regulator, it is doubled on one connector at the regulator with a brown /black wire which does go to the charge light. You don't need the regulator any more but you must keep the N/Y connected to the N/B wire at the regulator position and insulate it. The fact that you have 13.9v on a charged battery at 1500 RPM proves that the charging system is working.

The reason the black wire (ground) is no longer needed is because the alternator uses the alternator case as a ground and no longer needs an external ground. The reason the N/G wire is cut is because the alternator has an internal regulator and thus no longer needs the external regulator. The reason the new alternator used the N/Y wire is because it still needed an "exciter" circuit from the charge light and that connection is made at the double spade connector on the old regulator (N/Y to N/B). It would be wise to disconnect that spade connector from the old regulator and tape it up or slip a piece of hose over it to insulate it. Then discard the old regulator.

All of this does not explain the need for a "jump Start" so you need to run another simple test. You got no light on the test light between the large brown wire and the large pin on the alternator so the diodes are good in the new alternator. (just because a part is new does NOT mean it is good thus the test)

Now you must run another test. Run the car normally and shut it off. Then confirm that nothing is on and key is out of the ignition switch. Remove either battery cable off of the battery and put your test light between the battery post and the removed cable end. The test light must NOT light up even dimly. This tests to see if anything is draining the battery while off.

Now, test the voltage of the battery and write it down. Now let it sit for what you termed "prolonged period". What do you call a "Prolonged Period" ?  At the end of this prolonged period don't try to start the car but do check the battery voltage.

If you find it has gone down and the test light did not light up at all, you should return it to where you purchased it and ask for it to be "Load tested" and tell them about it running down after sitting with no load on it. (I think the reading of 12.1v you have is a little low anyway.)

let me know,

Howard

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Howard M. Fitzcharles III

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MG from 1956 (USA versions only) up and Engine theory.

Experience

Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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Associate member SAE EAA member

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Import Car magazine

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ASE Master Auto with L-1 certification up to 2000

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