I have a 1964, 3.8 litre Mk 2 Auto. In the last couple of years it has been running an alternator instead of the
C 42 Dynamo. Usually, the ammeter charges say 10-15 amps for a short while after starting. The higher level if
the car hasn't been running for say a week or so. Then it drops back to around 3-5 amps for normal running. Just
last week the ammeter jumped up from its usual 3-5 amps to about 14-15. Then when I added the fan in, it jumped
another 2 amps to about 17 and stayed there. At idle it is about zero to one amp, and when headlights are added
in at running say 2000 rpm, amps jump up to about 15 again.
We hooked in another battery isolating the one installed. Same results high amp usage at running speed. The
voltmeter showed about 14.7 volts and the alternator hand book says normal charging is between 13.5 and 15.1
amps.
Any ideas about what this change means?

 

I would say you need a new regulator.
Some are mounted as a separate unit while newer models have them mounted on the end.
Some even include the brush housing.
If you continue using the car, you're probably going to start burning out bulbs if it's constantly running at 14.7 volts.

 

Jeff may well be correct, but it is also possible that something is taking the extra current.

Is the voltage constant at 14.7 no matter what the RPM or what load is on ?

ie Turn the main lights and wipers on when on tickover and see if the voltage drops at all. Post back your findings and we can help more.

Below are a couple of point I have taken from another website:-

A modern automobile has a 12 volt electrical system. A fully charged battery will read about 12.5 volts when the engine is not running. When the engine is running, the charging system takes over so that the voltmeter will read 14 to 14.5 volts and should stay there unless there is a heavy load on the electrical system such as wipers, lights, heater and rear defogger all operating together while the engine is idling at which time the voltage may drop. If the voltage drops below 12.5, it means that the battery is providing some of the current. You may notice that your dash lights dim at this point. If the voltage ever goes above 15 volts, there is a problem with the voltage regulator.

A voltage regulator is designed to limit the voltage output of an alternator to 14.5 volts or less to protect the vehicle's electrical system. If the regulator malfunctions and allows uncontrolled voltage to be released, you will see bulbs and other electrical components begin to fail. This is a dangerous and potentially costly problem. Fortunately, this type of failure is very rare. Most failures cause a reduction of voltage or amperage.

 

Gentlemen,


Thanks for the comments. It looks like the problem is intermittent as the car ran perfectly well this morning. I've been told the regulator electrics (solid state) may be affected by temperature. Today was a cooler low 20's C but last week on the long run it was in the 30 C range.


I've also been advised to start the car then disconnect the battery leaving the car to run on the alternator. Then check the voltage and if this keeps rising above the recommended 13.5 - 15.1 volts the regulator will be at fault. I've also been advised to check the fuses. Trouble is I can't do any more of these until the rain stops here in Canberra when I can get the car out in the open and get some help. I have a regular car shed at my place on Mondays so will probably do the checks then. Will post a reply when the checks completed. I do appreciate your advice. Thank you!!!!

 

I really dislike the idea of disconnecting the battery while running
on the alternator.

A good regulator can seem defective if the ground or power side
do not have a good connection. The regulator sees the voltage
between the two but does not see the voltage drop across the
bad connection. As a consequence, it will try to regulate to a
false voltage reading.

 

Gentlemen,


After some more rigorous testing the voltage even at 1000 rpm was up past 15.2 VOLTS. So we've removed the alternator from the car and dismantled it to find the internal voltage regulator too hard to access. I've seen a replacement model from Amazon at a cost far less than an auto electrician would charge to service the current one so that is what I will do. Cheers and Thanks for your comments

 

What is the make of the old alternator ?
All the ones that I've come across were designed so you could access the regulator easily.
Not that I'm doubting you, I'm just curious to see what kind of design that is where things are not readily accessible.

 

just a wild guess, but check the voltage stabilizer located behind the fuse panel and mounted to the inside of the cowl, sort of between both windshield wipers.

 

No problem, Jeff was spot on.

 

Gentlemen,


I took some pictures of the Quality Built(QB) Brand alternator but due to file size limitations could only include one on this thread. It is the inner side of the rear half of the alternator. The other view from the rear (not on this thread) is to show there is no access to the internal Voltage regulator from that side.


I believe the QB model is a remanufacture of an Hitachi 35 model used on 1978-80 Nissan 4 cylinder models like the 620 and 720.


Cheers