Here you can get how to Testing a dynamo and checking output. The dynamo may be a robust and simple sort of generator which was fitted to many earlier cars. latest cars are fitted with an alternator. If you think a fault within the dynamo, check all the connections thereto with a circuit tester. Check also that the dynamo actually turns when the engine is running, which the drive belt is adjusted to its correct tension, and isn’t slipping (See Checking, adjusting and refitting drive belts ).
Checking output with a voltmeter or tester
Make these checks with a voltmeter if possible. If not, use a circuit tester or test lamp. The instructions are for a car with a negative (-) earth system. For a positive (+) earth system, read negative for positive, and positive for negative. Connect a voltmeter across the battery terminals while the engine is running. Have a helper rev the engine up from idling speed.
The battery voltage should rise, or the tester lamp (or headlamps) should brighten. If it doesn’t , and if checks on connections and therefore the drive belt are satisfactory, cut the engine and disconnect the 2 cables from the endplate of the dynamo. The terminals are usually marked D and F. they’re of various sizes, but label them if necessary, to avoid confusion.
Use a brief length of fairly heavy cable to clip the D and F terminals of the dynamo together. Start the engine and let it idle at less than 1,000 rpm. Connect the positive lead of the volt meter to the D terminal and therefore the negative cause earth. The meter should examine 14 volts (or the 12 volt bulb should shine brightly). If so, the dynamo is functioning.
Testing the cables
Reconnect the dynamo cables, leaving the short bridging cable in place. Disconnect the cables at the control-box end, where they’re also labelled D and F. Start the engine and permit it to idle at less than 1,000 rpm. Connect the positive lead from the voltmeter to the cable disconnected from the D terminal at the control box to ascertain if it’s sound.
Then do an equivalent with the cable from the F terminal at the control box. If the cables are sound, and if the dynamo is charging as previously checked, the meter should read about 14 volts and any fault must be within the control box.
Checking a low charge rate or failure to charge
If the first output test (see left) showed that the dynamo wasn’t charging, disconnect the D and F terminals at the dynamo endplate again, but remove the link between the terminals. Start the engine and have your helper run it up to 2,000 rpm (medium speed). If the car isn’t fitted with a tachometer (rev counter), 2,000 rpm is about the speed of the engine when the car is travelling at 30 mph in top gear
Reconnect the voltmeter between the D terminal and earth. If the voltage reading is 2 to 4 volts – enough to light a torch bulb but not a 12 volt car bulb during a circuit tester the fault is within the field coil or the brushes. If there’s no voltage the fault is within the armature or the output brushes.