1981 R65LS Charging Problem

[napanorton]

I'm working on a friends 81 R65LS and believe I have a charging problem.

Symptom - If I put a voltmeter across the battery and rev the bike up to over 3500 RPM, the indicated voltage only gets to 12.6v. (I also think the battery has been discharging, but since I've been working on it - it goes on the charger enough I really don't have proof positive.)

What I've done - I pulled the diode board and checked all the diodes with my DVM. All seems good. Checked the resistance of the rotor - 3.2 ohm - Good. Checked the winding to winding resistance of the stator - all are 0.6 ohm - Good. I checked resistance through brushes - zero - Good. Checked for stator and/or rotor grounding - Good.

So I figure all the components look good on the bench, what happens if I short the D+/Df terminals in the regulator connector to bypass it and look for max output. Pull the plug, jumper the blue and black wires, start the bike, rev it up. Well try to rev it up. At around 1200/1300 rpm - dies. Can't get it to rev any higher than that. Pull the jumper - no problem! WTF?

Next I tried a variation on this, I attached my DVM to one of the alternator windings, set it on the AC scale. Then I attached a lead to the B+ terminal of the diode board. Voltage regulator is unplugged. Start it up, rev to 3500. DVM is reading about 1.2v, touch the other end of the lead to the Df terminal of the brush holder. There's a nice little spark, the engine starts to falter, but I can see the DVM shoots up to about 15V.

My research shows that jumpering D+/Df should allow a max alt output reading at around 3500 RPM. It's a standard test, so the fact the bike is dying means somethings not right. Right?

Since I can't find any weird continuity problems, what do you think is going on??

-- Thanks, David

[SteveD]

Does the GEN light come on with ignition on, but not started? It should. Does it go off if you get the rpm above 1500-1800 rpm? It should. Otherwise charging is the issue, finding what's failed is the problem.

[SteveD]

Ohh and welcommen to the forum. General rule, if you think it's the charging, it's probably the battery. If you think it's the carbs, it's probably the battery. If you think it's the battery, it's probably a loose negative cable.
If you think it's the ignition, it's probably the carbs....and on and on it goes.

Rest assured, you'll work it out systematically.

Someone else will be along with more specific thoughts. I'm thinking mine haven't been much help.

Have you substituted the battery?

[napanorton]

Yes, charge light goes on with ignition, and goes off above 1500 or so.

[napanorton]

No, I haven't tried a different battery - I don't really have one to try. The one that's in there is about 2 years old and seems to hold a charge OK. Voltage at rest is about 12.3v.

Thanks for your help!

[napanorton]

I was wrong. Just retested the battery, after an over night charge it was sitting at about 13.2v. Lights on and cranking went to 11.3 in 30 sec. After cranking - no start, goes back up to 12.9. That looks good to me.

[barryh]

So I figure all the components look good on the bench, what happens if I short the D+/Df terminals in the regulator connector to bypass it and look for max output. Pull the plug, jumper the blue and black wires, start the bike, rev it up. Well try to rev it up. At around 1200/1300 rpm - dies. Can't get it to rev any higher than that. Pull the jumper - no problem! WTF?

.....My research shows that jumpering D+/Df should allow a max alt output reading at around 3500 RPM. It's a standard test, so the fact the bike is dying means somethings not right. Right?

It is the standard test and just puts full current to the field windings which should make the alternator go to maximum voltage output. I can't think why it should do this but my best guess is it is somehow reducing the voltage to the ignition system hence the unwillingness to rev. How about monitoring the voltage at the coil terminals to see if it falls when D+/DF is jumpered.

The bike hasn't by any chance got an after market ignition system that might be sensitive to under voltage or over voltage come to that.

[napanorton]

I just put a new EME ICU in. My understanding is it is more resilient to voltage excursions than the stock unit. Good idea on monitoring the voltage at the coil. I'll give that a try. Thanks! David

[napanorton]

I spent some more time on this and here's the latest. I'm 99% sure it's NOT the battery. After an overnight charge, cranking the engine for 30 sec, the voltage went down to about 11.9v. Immediately returned to 12.8 after. I call that good.

I then jumpered the D+/Df at the regulator connector again, started the bike, rev'ed to 1400 or so and it died just as before (just to make sure nothing had changed...). Then I checked the voltage at the coil during this, and saw no issues - i.e. stayed at batt voltage, so I don't think it has much to do with the ignition system. Then I disconnected the 3 output leads on the alternator - and lo and behold, was able to rev bike up fine. I think that rules out the rotor and stator as the issue as they were in the mix (except that there was no feedback from the diode board). Then I reconnected the 3 leads and removed the B+ at the diode board. Couldn't rev beyond 1400 rpm again. I think that indicates the battery and rest of the bike isn't the problem as they weren't even connected. So with diode board in and connected to the alternator, I get it fails. When the diode board is disconnected, works fine. So I pulled the diode board again and double checked all the diodes. They all tested fine (I was using my Fluke 73 in diode test mode). WTF??

I can only guess at this point that the diode board is bad somehow, but doesn't show under testing with low voltages. Anybody have any suggestions?? A real stumper for me.

-- David

[barryh]

This is one weird electrical problem.

If we step back and say what can cause the engine not to rev it has to be fuel or spark. There is one other possibility and that is mechanical load from the alternator but I think we can rule that out as even at full output it can only absorb so much power.

It goes without saying that fuel isn't the problem so regardless of battery voltage being available at the coil the fault, whatever it is, must be influencing the ignition system. This is not very helpful but I'd like to bet the problem wouldn't exist with points ignition.

We could theorize about how a faulty diode board might somehow generate electrical interference that would impact on the electronic ignition system but what you really need is a solution.

Seems time to substitute parts. Do you still have the original ECU ? I guess you don't have a spare diode board or you would already have tried it.