'75 R90/6 HEADLIGHT DIMMING after 4000 rpm...

[Rohls]

I guess i could live with it, but when riding in the dark the other night I noticed the HEADLIGHT DIMS when I apply throttle from ~4000 rpm (When I kinda goose it on the highway)...it does this every time I goose it. What could be causing that? Brushes? Crappy diode board? Alternator...rotor...? The battery is good and seems to be charging (never have a problem starting it in the mornings..)...

I feel like my electrical system may be on it's way out...any simple checks I could do?

Many Thanks,

Nick

[hal]

Simple method - put a multimeter on the battery terminals. Fire it up and watch the readings as you throttle up.

[Deleted User 287]

Brushes?
If they or one is too short, it could lose contact with the rotor at higher RPM, then a marginal battery (as hal suggested) has to take up the slack, except it hasn't been receiving a good charge because of a weak system.



How'd I do, Major?

[ME 109]

Brushes?
If they or one is too short, it could lose contact with the rotor at higher RPM, then a marginal battery (as hal suggested) has to take up the slack, except it hasn't been receiving a good charge because of a weak system.



How'd I do, Major?

Would the slip rings need to be out of round for that?
I was thinking the rotor had a broken wire and was flinging the wire out of contact over 4,000.

[barryh]

Rotor going open circuit due to centifugal forces at 4000+ RPM ?

Voltage would drop to low 12's when it happens but don't know how you would distinguish between this and a brush fault. Perhaps the drop from normal charge voltage would be more clear cut with the rotor fault.

[Ken in Oklahoma]

I don't remember the exact symptoms, but I had at least a similar charging problem on my '91RT. The bike just didn't seem to be charging like it should, though I don't remember any actual dimming. But then I ride almost entirely in daylight. I did a lot of the usual things such as swapping in a known good voltage regulator, cleaning connections, etc. Finally, adding a half turn to the brush tensioner fixed the problem. (That sentence might not make much sense if you're not looking at how the brushes are tensioned.)

Apparently insufficient brush pressure allows the brushes to vibrate up and down away from the rotor slip rings. Hence more wear with less pressure, even on smooth, concentric slip rings.

In any event I think it would be worth a good look around on your brush lengths and the tensioning. The brush is, after all, a consumable, and the fact that they are riding on the slip rings, apparently well, doesn't mean actually well.

I like the idea of monitoring the voltage at the battery with a DVM. Harbor Freight makes good DVM's and they don't cost much. In fact you can get them free with the 20% off coupon that appears in a lot of moto magazines, DIY magazines, and even more "buff" oriented magazines.

Addendum, if you're running a "standard" volt meter in your dash or pod, I like to put a dot of paint on the meter face where the needle is when the charging system is well. Otherwise, when looking at the voltmeter, I can never seem to decide where I remember where the needle should be pointing.


Ken

[dougie]

I remember Airhead had one of these and liked it.
http://www.motorcycle-superstore.com/14 ... Gauge.aspx

[Kurt in S.A.]

I thought our electrical system was basically all coming from the battery...that is the charging side simply stuffs the battery with "juice" and the bike's systems pulls the "juice" back out. A car will run without the battery in the system once started...but our bikes need a functioning battery to run at all.

Maybe the load on the battery is too much at higher RPM and the lights dim? Check the cables coming from the battery for corrosion and connections.

Kurt in S.A.

[jagarra]

I guess the problem is can the battery keep up with the load requirements. I like Kens theory of the brush float as a source of low replenishment to the battery and corrosion on the battery cables. But, I suspect bad connections are the problem as Kurt suggested.

[Rev Light]

Perhaps you are going so fast and pulling so much 'G' that all your blood is running away from your head to your feet. The diming of your vision in low light is the result......If you go to the red light you might even black out? If a 'G' suit and compressor is not available, how about tightening your belt by a notch?

Really, I think the boys above might have a handle on it.

I would be interested in know what it turns out to be though. Is there anything that will draw more current as the speed increases? .....on a marginal charging system..

Rev. Light