MG Car Repair: MGB Charge Indicator

Question
Howard, you've helped in the past so I hope you can help now.  I'm stumped on how to diagnose this problem.  I have a 1972 MGB roadster (US Spec).  The car starts and runs fine.  The charge indicator light on the dash is bright and solid when I turn on the key and dims significantly after starting but continues to flicker while the engine is running.  The headlights and electrical accessories seem to surge with the light.

My first suspect was the alternator and I swapped it out with other known unit and the problem remained. I verified good connections on the battery terminals and to the chassis from the battery and all connections on the starter are tight and in good shape.  I removed the spade connectors from the alternator and cleaned them.  I tried other known good distributors and ignition coils and nothing seems to affect the flickering charge light.

I have two other ideas yet to check, swap out the ignition switch in case it is faulty and swap out the battery with a known good one in case of an internal short?  Other than that I'm at a loss.  The ground strap from the engine to the chassis is in place and in good shape.  The variation in voltage is real as I can see it in the headlights and courtesy light when the engine is running.

I have owned and maintained multiple MGB's and only seen this when there was a problem with the internal regulator or diode bridge.  That doesn't seem to be the problem here.

I'm big on troubleshooting in a methodical way but this has me stumped.  Any ideas you have would be most welcome.

Thanks

Dave

Answer
Hi Dave,

Here are a few things to do in an attempt to correct the problem.

Even though you did a physical check of the cables and wires this is what we had to do in odd problems that the normal checkes didn't correct.

First is the voltage test. Check battery voltage before starting. (12.+) at the posts (not the cable ends) Then start the engine and set the idle at about 1500 RPM or a little more. Then read battery post voltage (not cable ends). Should be 13.8v to 14.5v (not less nor more)

Next with the engine at the fast idle do a "Voltage Drop" test on all cables. (negative test lead of the voltmeter set on a low volt scale DC on the negative POST of the battery and Positive test lead on a good bare metal on the cylinder head. Voltage should read well below .5v DC. Now put the Negative test lead on the Positive post of the battery and the Positive test lead on the large flat spad terminal on the Alternator (12 ga Brown wire) Again you should not see more then .5v DC. (Engine running)

Now shut the engine down and remove the 12 ga Brown wire (+) from the alternator and put a 12v test light between the wire and the the spad terminal on the alternator. The test light should not light up at all. (This is a test of the Diodes for a short but not for an open diode)

Now take the small trigger wire from the back of the alternator and turn on the Ignition to "ON" (not Start) and look at the charge light. It should be out completely. Then have someone touch the end of the wire to a good ground on the Alternator case. The light should burn brightly. If your alternator has a small Brown wire on the back of the alternator remove the wire from the Alternator and test the wire for voltage with a volt meter. It should read 12v +.

If all that is correct remove the wires from the back of the Alternator and remove the black plastic cover from the back of the Alternator and then plug the wires back on the the alternator. Now take a jumper wire and ground one end and start the engine and put the volt meter across the battery POST (not cable ends) and read battery voltage (12v +) Now set the RPM on about 1500 or above and the voltage should be 13.8v to 14.5v and while you see that reading touch the grounded test lead to the metal strap on the regulator case or if it don't have a metal strap then touch the grounded test lead to the GREEN wire end. Dont't leave the grounded test lead on long, only enough to read battery voltage. It should read 15v +.

This test will require two people or long test leads with good clips on the ends.

Other things to look at, Be sure that the charge light bulb is tight in it's socket. The charge light bulb is the "Exciter" circuit for the alternator and if that small amount of current does not reach the regulator the Alternator will not charge.

I did run into one charge circuit that was a problem and several mechanics in the dealership tried to find the problem and I had a difficult time too but I had the shop scope and I put the scope on the charging system and I spotted a trace of AC current in the DC cable (large brown wire) and that was a leaking Diode. The charge rate was good on a volt meter

Don't consider a change of any part as a sure test. Even though it is rare I have replaced a bad part with a new bad part from the factory.

Let me know what you find.

Howard