Strange Voltmeter Behavior

[John Brase]

My '78 R100RS has been exhibiting strange voltmeter readings for quite a while now. The voltmeter needle is constantly bouncing between the 12v mark and a little above the 13v mark. This happens whether the bike is moving or on the stand and whether the headlight is on or off. (When the headlight is off the needle jumps between 12v and about 14v.) This seems to have no real effect on the charging as the batt stays strong. My plan was to take it to the dealer for new tires and to have them check the charging circuit in preparation for some long rides this year but, alas, the dealer (Revard) is no more.

Does this suggest a diode board on its way out? I'm not well versed in electrics but I do have a digital multi meter. I don't know where to connect it or what to look for though. Suggestions?

Thanks,
John

[bikerfish1100]

last time i saw that happening on one of my airheads it was diagnosed as being a faulty voltmeter. charging system (& bike) was running and charging fine, gauge was bad, was advised to fix the meter with tape. over the face. or just disconnect. YMMV.

[James.A]

In my experience, a diode board is an all or nothing event. Battery stays strong?, if it ain't broke, don't fix it.

[GlobalRider]

Does this suggest a diode board on its way out? I'm not well versed in electrics but I do have a digital multi meter. I don't know where to connect it or what to look for though. Suggestions?

Maybe. Do some simple tests. Connect a DMM (put the meter on the 20V DC range) across your battery with the engine running. What voltage do you read (above 12V)? Now take the RPMs up to 2500. What does it read now (above 13V, but not higher than mid 14V)? If it steadily increases, that is OK. Now do that with a load on: high beams, driving lights or heated vest, etc.

If you had a bad diode in your board, that may cause the variations that you see on your voltmeter. Oh, it will bouce if you have your turn signals on, especially while at idle.

[Braddog]

I've got a '77 R100RS. When idling, the voltmeter reads steady something below 12V. When underway, and going up in revs, the needle bounces back and forth, just a hair above, or right at 12V. This is with the headlight on. With the headlight off, it still bounces, only higher, usually up to around 14.

The battery charging seems to be just fine. Just for fun, I put an analog volmeter on the battery, and the readings were steady, and about what they should be for an idling bike (12) and rev'd (up around 13.5 to 14).

I think the voltmeter itself may be funky, but in my opinion, as long as you seem to be charging OK, I think you're good.

[donkey doctor]

Hello; I have a 16 volt reading on my voltmeter just about all the time. I checked it with a digital voltmeter, and it's accurate. It doesn't matter wether the lights are on or if I have an electric vest, it's still 16 volts. I asked the forum what to do about it, but since I had already changed the diode board, I didn't do anything about it. That was two years ago now, and I haven't burned out my battery or any other electric parts, so I've decided to just live with it. After three years there should have been some consequences by now, but nothing.

I'd say "let it develop"

[GlobalRider]

I think the voltmeter itself may be funky...

And too many of them are incorrectly wired into the motorcycle. Ever take a voltage reading across the battery and compare it to the analog voltmeter reading? Usually off, sometimes way off because it was wired into a current carrying wire.

[Braddog]

And too many of them are incorrectly wired into the motorcycle. Ever take a voltage reading across the battery and compare it to the analog voltmeter reading? Usually off, sometimes way off because it was wired into a current carrying wire.

Good info, I'll give this a look as well. I like to use an analog VOM because you can see "bounces" much easier than you can on a digitial. Bottom line, every so often, when I'm working on the bike, I'll just put the VOM on the battery just to make sure that everything's working as it should. So far, so good.

[mrmaico]

Mine on my 86 R80RT was doing something similar only it sometimes would bounce all the way across the range. Guage was bad. I found a used one by placing a wanted ad at www.ibmwr.org.

Barry

[chasgrips45]

In my opinon your voltmeter is toast. If you want to replace the meter & don`t want to spend a fortune, Egauges.com has VDO weatherproof units. It wont match your old gauge ,but you`ll get the info you need. Chas.

[whicks]

Here's another vote for taping over the meter.My "86 R80RT has done about the same thing since new.I just ignore it,or I glance over at when I need something to laugh at!

[CustomSarge]

It seems to me that nominal battery is 12-12.5V & charging is supposed to be ~13.5-14V (16 is high). Granting that the charging action is switched, not proportional, a good battery would "bounce" between charging & not, fairly frequently. The lower load of no headlight would allow for the charging point of ~14V. Proof would be to put an ammeter between the charging line & battery, it should change direction of current in sync with voltage change; voltage high=current into battery, low=out of. Note, ammeter in charge line, NOT in line with starter feed. Good Hunting... <<<)))

[bullit7801]

As listed below, I am also an owner of a 1978 R100RS. My old Motometer voltmeter also got erratic so I chucked it and installed a VDO that I got cheap at an auto parts store. No, I don't remember which one, it was years ago. It was a perfect fit and matched the clock well enough to look stock. I did have to modify the wire connections, though. The Boxer voltmeter, IMHO, has two problems. One is a relatively long, thin wire from the battery with the usual voltage drop. The other is that the guage is a Motometer. They are the company that made all those early K-bike speedos. :doh

tb