testing-a-dynamo-and-checking-outputCar Components Electrical Systems 

how to Test a dynamo and check output

testing-a-dynamo-and-checking-output

The generator is a robust and simple type of generator installed in many early cars. Most modern cars are equipped with an alternator.

If for some reason you suspect a generator failure, be sure to check all connected circuit testers.

Check that the generator is also actually running when the engine is running and the transmission belt is adjusted to the correct tension and does not slip.

Checking output with a voltmeter or tester

checking-dynamo-output

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 does not, and if checks on connections and the drive belt have been satisfactory, switch off the engine and disconnect the two cables from the endplate of the dynamo.

The terminals are usually marked D and F. They are of different sizes, but label them if necessary, to avoid confusion.

Use a short length of fairly heavy cable to clip the D and F terminals of the dynamo together. Start the engine and let it idle at not more than 1,000 rpm.

Connect the positive lead of the voltmeter to the D terminal and the negative lead to earth. The meter should read about 14 volts (or the 12 volt bulb should shine brightly). If so, the dynamo is working.

Testing the cables

testing-dynamo-cables

Reconnect the generator cable to the short connecting cable. Disconnect the cable termi- nals of the control box, also called D and F.

Start the engine and leave it idle for no more than 1000 rpm. Positive from the voltmeter connected to the cable disconnected from the D terminal control box is sound.

Then from the F terminal of the cable to the control box.

If the sound of the cable is checked before the generator is charged, the meter should read 14 volts and any errors must be made in the control box.

Checking a low charge rate or failure to charge

checking-a-low-charge-rate-or-failure-to-charge

If the first output test indicates that the generator is not charging, disconnect the D and F terminals of the generator terminals, but remove the connection between the terminals.

Start the engine, then let your helper run it up to 2000 rpm (medium speed).

If the car is not equipped with a tachometer (speed calculator), 2000 rpm is about 30 miles per hour on the speed of the car when the car is in top gear.

Reconnect voltmeter D between terminal and earth.

If the voltage reading is 2 to 4 volts – enough to light a flashlight bulb instead of a 12 volt car bulb, the circuit tester malfunctions in the field coil or brush.

If there is nothing wrong in the armature voltage or output brush. In both cases, check the brushes and the commutator.