Weird Charging Behavior

[Nailhead]

Yesterday, I went for a ride on the RS.

As I was performing my now-standard in-ride ritual of staring at the voltmeter (punctuated by brief peeks at the road), I noticed that I was seeing 11-ish volts indicated at an RPM (3500+-) that a short distance before had yielded 13-ish, indicated. I was forced by traffic to ride in this manner for about 15 miles.

After I exited the highway, I downshifted and was able to ride a couple-three miles at about 4K & saw 13 volts. After upshifting again, I was pleased to see the high side of 12 volts on the voltmeter at about 3300.

What gives here? It's like the alternator (or potato battery, or piezo element, or whatever that panty-waisted device is...) went to sleep for 15 miles, and was only awakened by a high-RPM condition. I realize charging voltage will taper off as the battery is topped up, but it shouldn't drop to 11.5 volts in doing so. Also, after the customary crank-a-thon starting involves, I have trouble believing that the battery had already charged to taper point.

Is this a regulator issue? Perhaps the brushes are at their wear limit and are making intermittent contact? Wouldn't a KLR have been a more sensible choice?

Ideas & commiseration welcome.

[Kurt in S.A.]

On a peak running charging system, the voltage will always be 14-ish at the battery...there is not a taper off over time. If you're looking at the stock in-fairing voltmeter, you are not reading what is happening at the battery. Likely it is at least 0.5v below what is really going on at the battery. 13v at the battery is not nearly enough to keep the battery charged.

That said, I would believe you have grounding issues with the voltmeter for it to change like this. I would rig up a separate voltmeter carried in say your tankbag that reads straight from the battery to verify what you're seeing with the fairing meter. It could be that the brushes are short with little spring pressure on them and/or are getting hung up in their holders and are not making good contact with the slip rings.

Kurt in S.A.

[barryh]

Does a 1980 model still have the mechanical regulator ? I know they had them in 78 or 79. I've seen the mechanical regulators behave in a strange manner so if it is one of those it might be suspect. A fall off in voltage does occur after a cold start as the regulator warms up and that's by design but it's only by a few tenths of a volt and nothing like the variation you experienced.

[Nailhead]

I have measured the voltage at the battery with a Fluke 116 meter, and IIRC the voltmeter reads the better part of 1 volt low with the bike running.

As far as 14-ish at the battery, it's not even close.

I would imagine a Scotchbrite pad & some dielectric grease would probably do battery & ground connections some good, but I'm just not optimistic the voltage deficit will be made up fully in doing so.

[Kurt in S.A.]

As far as 14-ish at the battery, it's not even close.

Are you saying you're not getting anywhere near 14 at the battery terminals? The closer it is, the better. If it's 13.5v, you might be OK but you'll need to put it on tender more often and/or include longer rides in your future.

Kurt in S.A.

[Nailhead]

That is indeed what I'm saying: 13 is what I remember seeing, but I don't remember for sure.

As soon as I can afford it, I think I'm going to install an aftermarket charging system. I have no garage wiring in the house I just moved into, and the charging system in this bike is losing its novelty value. I'd actually like to be able to take a shorter "ride" on it without having to worry about making it into a "road trip".

[daz]

Wouldn't a KLR have been a more sensible choice?

Ideas & commiseration welcome.

Well, a KLR is always a sensible choice. However for some unknown reason we own BMWs. Even some older ones. Just solve this little problem and forget about that other one. In the long run you will be happy you did.

[SteveD]

Give the connections at the vm wiring loom a good clean up whilst you're doing the battery and ground.

[Duane Ausherman]

+ 1 for Kurt. A poor ground could well be the culprit and very hard to find. Also check the battery cables within 2 inches of the connector for expansion and stiffness. The original cables that some contractor made up for BMW were very poorly done. We started finding failures within the first year. Nice for those that are electrically challenged.

You are dealing with too many unknowns to know much of anything useful about your observations. The factory set up for the voltmeter wiring is almost useless. It is only good for showing a change, but nothing about a measurement.

You need to check by testing at the battery posts, not the battery connectors on the posts. For this measurement, I prefer an old analog type meter. The digital ones might make it hard to see what is happening with an intermittent, such as you have.

You don't need a high impedance meter for this circuit, just about any old meter will do. One can get high quality Weston or Triplett meters for very cheap now on eBay. Nobody wants the old good stuff, just the digital type. Just because they give one a number to 3 digits doesn't mean that those digits mean anything.

Using a multi meter effectively isn't so easy.

[Scoots]

I had been getting all sorts of weird readings from my volt meter, especially when traveling down rough roads on my '89 R100RS. Long story short - 3 of the 4 rubber diode board mounts had broken and had caused a strain on all 3 wires that fed thru the timing case cover, wore off the insulation on the wires and had created a crescent shaped arc in the hole the wires fed thru and shorted out on. A pain in the ass to replace the rubber diode board mounts with solid mounts, but necessary!

Good Luck with diagnosing your electrical gremlins.