What is a BMW R90S? The BMW R90S, produced in the mid-1970s is a motorbike or “superbike” to those who knew of it then. It can reach sixty miles per hour in less than five seconds from rest and maxes out at 124 miles per hour. The motorbike has a Bosch 12-volt, 280-watt alternator allowing it to produce up to 25 amps. You can use a multimeter (an instrument that specializes in testing voltage and electric current) to test the output of the from this test, you will be able to learn the voltage, amps, and wattage of your BMW R90S.


Test Voltage

1. First, you will need to set the voltage on the multimeter, then find the battery of the BMW R90S and start the engine.

2. Next, you should find the positive battery terminal that is labeled “Pos.” Once you have done this, attach the metal prong found at the end of the red cable that is coming out of the multimeter to this terminal.

3. As in step two, find the negative battery terminal that is labeled “Neg.” Once you have done this, attach the metal prong found at the end of the black cable that is coming out of the multimeter to this terminal.

4. The last step in testing the voltage is to look over the multimeter and copy down the display. It is important to remember the voltage in order to figure out the wattage of the Bosch alternator. The operator is working well if the voltage displayed is between twelve and fourteen volts. It may need to be professionally tested if the voltage reads below twelve volts. This may mean that it needs maintenance e or that the voltage regulator is not working how it should.


Test Amps

5. To test the amps it is important to keep the motorbike running, turning on the lights and other electrical components. Making sure everything is on, choose to test the amps on the multimeter to see how many are being produced by the Bosch alternator.

6. Repeat the instructions from steps two and three (think: red=positive, black=negative).

7. As in step four, it is important to look over the multimeter, this time copying down the amps. If it is working well it should be between twenty and twenty-five amps. If below twenty, make sure all of the electrical components are turned on before continuing. If they are all turned on and the amps still read below 20, then the Bosch alternator needs to be checked out professionally.

Calculate Wattage

8. Retrieve the information copied from the multimeter display on the voltage and amps. Make sure to have a calculator nearby for working out the wattage for the Bosch alternator.

9. All that is left is to multiply the wattage by the amps. In other words, if the voltage read 13 volts and the amps read 20, then the wattage for the Bosch alternator would be 260.

10. Finally, make sure that the final wattage calculation is between 220 and 280 watts. If the wattage is in this range, then the Bosch Alternator should be producing an adequate amount of power, meaning the electrical components that were turned on were using anywhere from 220 to 280 watts of power per hour. If the watts read below 220, then the electrical equipment along with the Bosch alternator should be seen by a professional.

Final notes: If ever in doubt over any of the steps involved in testing the Bosch alternator, do not hesitate to seek help from a professional.

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