R100/7 Tachometer Troubleshooting

John0313 · Post by **John0313** » Mon Jul 02, 2012 11:08 am

I'm trying to run down a few "gremlins" on "Klaus", my '78 R100RS, recently awoken after a long nap. The tach worked fine prior, but now is best described as "lazy". Sometimes it shows maybe 1/2 the RPMs I'm actually running, or sometimes just hovering around 1200. The bike's been convereted to electrnic ignition, and has the later elec. tach.

Are there any "usual suspects" in the way of tests/fixes that I should look into before I resign myself to boxing it up, and shipping it out for rebuild?

Thanks in advance!

Eck

schrader7032 · Post by **schrader7032** » Mon Jul 02, 2012 12:59 pm

So you have the standard/original tach but have converted from points to electronic ignition? But you still have the twin coils under the tank? If that's the case, the tach doesn't really care about anything but the wire coming from the coil. It takes the rate of collapse of the coil field and internally that's converted to an RPM which is then converted to the swinging needle.

Not sure if it helps much, but Snowbum has an article on testing the tach:

http://bmwmotorcycletech.info/tach.htm

My guess is that somethings wrong or dragging down the needle and that a rebuild will be necessary. This is really about the odometer repair, but it helps understand how to get inside the pod and look around. Might be worth a look:

http://www.pbase.com/dwerbil/1978_r1007_odometer_repair

Be careful with the small flexible circuit seen in figures 3 and 4. They're a bit touchy and costly to replace.

There are a number of places that do this work if it comes down to it.

John0313 · Post by **John0313** » Mon Jul 02, 2012 7:14 pm

Thanks Kurt, they both gave me some ideas on things to check!