Question about 1983 BMW R 100 RT

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I have a 1983 BMW R100RT. Potential charging problem

I have a 1983 BMW R100RT. Recently, the voltmeter started hovering just above 12 volts. Normally, it reads just shy of 13 volts. Five weeks ago, I replaced the turn signal solenoid. About two years ago, I replaced the diode board with an aftermarket one from Motorrad Elektrik. Questions: 1) Does a voltmeter reading just above 12 volts indicate a charging problem?
2) Is there anything specific to look for?
3) How should I go about trouble shooting the problem?

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Hi Tom,

Typically, the charging voltage should be somewhere around 13.5VDC-14.5VDC, so you may be experiencing signs of a problem.

Before doing much else, I'd recommend you do two checks:
1) Assuming you are referring to a voltmeter mounted to the bike in your message, double check the voltage using a multimeter at the battery terminals to confirm there isn't a fault with your bike's voltmeter.

2) Because I'm unfamiliar with your particular model, I'd recommend checking the spec for your bike -- if you have a service manual, it should list the proper voltage range. Otherwise, you can probably contact a dealer or BMW tech to get the spec.

If you are below the recommended voltage range, there are a variety of possible faults.

The first that comes to mind is your regulator/rectifier circuit. The purpose of this circuit is to 1) rectify (turn the AC current generated by the engine into DC current) and 2) regulate (make the current stable at a given voltage, say 13.5 VDC, and dissipate the excess voltage as heat). Because this part turns excess electricity into heat, it's not uncommon for regulators to 'burn out."

I believe the diode board you replaced is the rectifier portion of the circuit, so it sounds like the regulator portion may need replacement. FYI, an aftermarket "reg/rec" that combines the regulator and rectifier is available for your bike (see: The advantage to switching to such a unit is that it's probably better engineered than the OEM unit (more efficient/better heat dissipation), and might last longer.

If your battery is more than a few years old, you may want to take it to a dealer or auto parts store to have checked (many will do this for free), and consider replacing it. Often one bad component in the charging system will cause other good components to fail.

Finally, here is a generic fault finding diagram that is very well put together:

Good luck!

Posted on Sep 30, 2009


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