QuestionQUESTION: Hi Barry,
I own a 1968 MGB. Fresh motor, fresh battery. The other day I tried to start the car, and found to have a dead battery. I found no short with a 12V test light between the ground wire and the battery. I successfully jump started the car, and out of curiosity I measured the voltage at the fuse block. It was 9.8 volts. Accelerating the motor did not increase the voltage. Running the car for awhile caused the voltage rreading on the meter to fall a number of volts. When I would disconnect the ground wire from the battery the engine immediately killed; yes I know I probably shouldn't do such a test, but it seemed to tell me something! :-) Suspecting the alternator I took it to the alternator repair shop; they replaced a regulator, which is a part I saw in the exploded diagram in the Moss catalog. I put the alternator back in the car, with a charged battery, and I read 13.6 volts when running, and goes to 13.8 volts with acceleration.
Question 1: Is the problem fixed?
Question 2: What is the purpose of the 'alternator control unit', which is Lucas type 4TR? Is there any troubleshooting I can/should perform on this piece?
Thank you for any and all information you can give me!
Regards,
Dan
ANSWER: Hi Dan,
You have the first MGB fitted with an alternator. Earlier cars had a dynamo controlled by a separate regulator box containing 2 regulators and a cut-out. The very early alternators like yours also had an external regulator box containing some regulators and one or more diodes. The regulators prevent the alternator from over-charging the battery, and the diodes prevent the battery from losing its charge when the engine is switched off. My guess is that a diode failed, so the battery was slowly discharging whenever the car was not being used.
If the battery now holds charge when the car is not used for a while, all is fixed. If not, you still have a problem.
There are other possibilities. Flat batteries can be caused by an alarm or immobiliser. Also, your car has a light in the boot (trunk). If the switch is faulty and the light stays on when you close the boot lid ...
I would disconnect the earth lead from the battery and then insert an ammeter between the battery earth terminal and earth. A test lamp is not sensitive enough. There should be zero current. If the current is micro-amps, that could be just the memory on a radio. Anything more, and something is still draining the battery. Start with a 10 amp range and work down. If the drain current is say 0.3 amps, then whatever is causing the drain is taking (12 x 0.3) = 3.6 Watts, probably a small bulb.
Hope this helps.
---------- FOLLOW-UP ----------
QUESTION: Hi Barrie,
thank you for the fast response! Here is some new data...
I am able to illuminate a small LED (T 1 3/4, 20 milliamps current consumption) across the earth lead and battery earth terminal. With my old fashioned analog meter I measure about 20 milliamps on the 250 milliamp scale, so I think there is a correlation. I do measure battery level voltage across this junction as well; I would imply from your first response this is an expected finding...
I am not sure if my boot has a light with a switch. I looked, and the only electrics I see are for the tail lights and back up lights.
Did you mean microamps (1,000,000 microamps = 1 amp), or milliamps (1,000 milliamps = 1 amp) in your statement above?
Again, thank you for any and all information you can give me!
Regards,
Dan
AnswerHi Dan
The memory on a modern radio takes micro-amps (1/1,000,000). However, I just remembered that you have an analogue clock, and 20 milli-Amps is about right for such a clock.
At 20mA it takes 2 days (50 hours) to consume 1 Ampere-Hour. Your battery is good for at least 60AH, so it would take months for a drain of 20mA to completely flatten the battery.
I conclude that the 4TR regulator was faulty and you have fixed it. If you want to be sure, try fitting the old regulator and see the difference.
