Ammeter Reading/Battery Charging

[ALC 68A]

The needle on my 30-0-30 ammeter hardly moves either side of the 0 mark, where other peoples' seems to swing more widely, indicating a more positive charge. The battery is only about 6 months old, yet I have noticed the starter cranking speed is already slower than when it was new, so I'm wondering whether the battery is being properly charged. The car is S3 with a dynamo, so how can I check whether it is giving a proper charge please? A friend suggested the control box could be at fault, so how do you check it? Sorry if these are basic and obvious questions, but I am hopeless with electrics. Thank you.

Steve

 

[Ken Ellis]

Steve, do you have the Workshop Manual for the car? There's a pretty thorough procedure for analyzing/adjusting the charging system.

What's the difference in voltage between when the car is running and when it's off?

 

[Tom H]

Steve, Ken's pretty much on target. I suggest getting a multimeter and measuring the battery voltage. In general you can measure it right at the battery post on the starter solenoid. Put your red ( plus) lead on that post and the black (neg) lead somewhere on a good clean ground point , like the ground screw (black wires) on the cowling near the solenoid.

With the car not running and key off, you shoud read about 12.6. Maybe a bit higher if you just shut the motor off. Then start the car and rev it up a bit - 3000 RPM or so. You should now read about 14-15 V, with the engine running around 3000 RPM.

If you need to do further troubleshooting , as Ken suggests, the WSM has lots of detail . And if it seems you need to replace anything, i'd recommend changing over to Neg earth and an Alternator .

Tom

 

[Nickodell]

Voltage is a guide, but the only sure way to check amps in and amps out, which was the original question, is with an in-line or clamp-on ammeter. My suggestion would be to purchase a clamp-on DC ammeter (one that only reads AC is no use.) The one I have used for over 30 years was part of a kit given to me by my late employer, who said I could keep all the stuff on retirement, and it's been worth its weight in gold. Back then the clamp-on DC ammeters cost over $450 in today's dollars, but you can get them for 1/10 of that online, and rather more at Sears, Lowes etc.

Clamp it on the hot cable next to the battery and start the engine. You should see a reading of 10 or 15 amps charge, which will drop to around 3 amps after 30 seconds or so if the battery is well charged and healthy. Have a helper bring the engine speed up to about 2,500rpm and switch lights and accessories on. Each time you should see a momentary needle kick toward discharge, followed by returning to 3 or 5 amps charge.

If this doesn't happen, there are several possible problems: faulty generator (may be just needs brushes); faulty control box; internal battery problem, or sulfated plates from standing discharged for a long time; loose or corroded terminals; bad ground (strap broken, or loose/corroded terminals).

You could start by disconnecting all the terminals and clean them, starting at the ground strap, then the generator output lead and continue downstream until the battery hot terminal. Don't forget the terminals behind the dashboard ammeter. DISCONNECT THE BATTERY GROUND CABLE BEFORE YOU START! to prevent a short and possible damage, fire or injury.