724thomas wrote:My '82 R100RS has been running perfectly for many years with all the standard flaws and repairs. I,m not "electrical" so need some guidance. Over the last month I've noticed by VDO volt meter slowly dropping over a period of about 100 miles. A night on the trickle charger brings it back to life. Yesterday after an average ride the battery dropped to the point that it would not turn the starter. One suggestion was bad brushes or dirty contacts. I've ordered brushes but suspect there might be more. I'm open to suggestions on what to look for while I have the cover off. Thanks for any help..TomJ
From reading your symptoms it is very likely that your charging system isn't working. I know that probably sounds obvious, but the fact that you can charge your battery over night and all is well for a while suggests the charging system. If your battery is very old or suspect now would be a good time to take your battery to a battery store where they can do a load test on it. They will be able to tell you if the battery is in need of replacement.
But, as I said, your charging system is the prime suspect. Your charging system consists of the alternator, the diode board, the voltage regulator, and the wiring connecting these things together and to the battery. Your task is to identify the culprit in a methodical manner.
Do you have a good repair manual? If not you need one. The manual will address the charging system and will be useful in helping you to isolate the problem.
The first and easiest test to do will to see if it is the voltage regulator is bad. For that you will need to lift your tank and find the voltage regulator on the R/H side, mounted to one of the two top frame tubes. The voltage regulator is a plug in. You will need to pull the regulator out of its socket and work with the socket itself. The repair manual should tell you how to use a piece of wire to make a connection between two terminals in the socket. I believe they are the D+ and Df terminals. By connecting the two terminals you are bypassing the voltage regulator and making the charging system charge in full. By making the connection you are applying the full battery voltage to the field windings in the alternator.
You will want to put a voltmeter across the battery terminals as Duane has suggested. Measuring there avoids the voltage drop to the voltmeter on your bike and tells you what you really want to know--what's happening at the battery. If you don't have a digital voltmeter, buy one. They don't cost much money, and they are regularly on sale dirt cheap at Harbor Freight.
With the voltmeter across the battery terminals, start the engine and watch the voltage. You won't need to put the tank back on. You will have plenty of gas in your carb float bowls to do the test. When you start the engine rev it up to about 2,000 RPM. You should see the voltmeter reading well over 13 volts (make sure your battery has been charged before you do the test). If you read a good voltage you know that the charging system can deliver the correct voltage with the voltage regulator operating properly. The one you took out isn't. You will need a new voltage regulator. They are expensive.
If the voltage doesn't rise as it should with the jumper wire, you have a problem elsewhere in the system.
I'm going to stop here because what you discover will guide your next step. If you do the test, post your results. And while you're at it, how about giving us any additional information we might want to know, such as the age and condition of your battery, any prior charging problems you may have had in the past, and what things you've done already in chasing the problem. We're capable of sorting out the information you give us if it isn't pertinent.
One more thing before I go, if your troubleshooting ends up with you needing to check your diode board, Rob Frankham has an excellent procedure on his web site. I use it myself.
Ken
