10 cent charging diagnosis please...

MacMcMacmac

MacMcMacmac

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First ride of the season. Started off nice. Fully charged battery cranked the motor over nicely. Ran well for most of the ride until I began to get random misfires. Bike dies in the middle of a four lane road. Nursed it home by pulling the headlight fuse. Colorful language ensues. Have 8.5 volts at the brushes. Rotor ohms out at 3.9 ohms, still in the bike, brushes pulled, measured by touching the rings with the probes. Kick starts ok. 11.8 volts and dropping, regardless of revs. I am thinking if my regulator was toast I would not be getting any charging current, and if my rotor was toast, I would get infinity or very low ohms. Sill, 3.9 ohms isn't great, considering it is all new windings. Any thoughts?
 
Brush lengths a-ok. Slap test, not much happening with the motor off and the ignition on. Would not pull a wrench dangling next to it. I figured 3.9 ohms would still work. Dagnabbit...
 
Are the slip rings nice and clean? If dirty you can get bad (or wrong) readings and poor output. Cleaning them is free, might as well give it a shot.
 
I've never gotten an accurate reading of the slip rings on rotors by holding the probes against the slip rings. For an accurate reading I virtually drill the probes into the rings at a place where the brushes don't ride, sometimes I sand these divots flat again. Just holding the probes against the slip rings can give a wide variation of readings, even if the slip rings are clean.

Scott
 
the charging system is only as good as the function of each of the various components . 3.9 ohms is on the low side but my rotor measured exactly the same and still charged the battery adequately.

One test you could try is to connect a bypass ground wire from the inner ring (ground ) at the brush and take it straight to ground either on the frame or the battery negative then check the charging voltage or do a slap test .

Personally I would take this opportunity to go right through the entire charging system and clean the rotor rings and all the cable connections and test all the voltages from the fuse block ,ignition switch ,stator brushes and rectifier regulator . Oh and make sure your battery hasn't sulphated over Winter and is still capable of holding a good cranking charge
 
Paul Sutton said:
My rotor at 3.9 Ohm was still charging well also but the epoxy was cracking badly so I replaced it.
Click to expand...

yes that would be the best bet for sure but its an expensive item for some of us to replace Paul

I am not so sure that the Rotor is the issue here though.;)

Mac has said that the slap test he carried out was unsucessful so there must be another issue somewhere involving the 12v+ and or the ground .
Not sure which year Macs bike but either the regulator regulates a 12v+ supply to the rotor with permanent ground or it regulates the ground with a permanent 12v+ and that doesn't appear to be happening.!....

This is why I suggested Mac checks the fuse ,wiring ,connectors and brush assembly for continuity and voltage/ground
 
Amps equal volts/ohms the lower the ohms the more amperage the rotor is drawing, 3.9 ohms indicates the insulation is breaking down and shorting out sections of the winding, this makes a shorter current path, increases the load AND decreases the magnetism, that creates current in the stator windings. It's a cascading failure mode, the shorter the winding the more amperage it draws, the hotter it gets, the more the insulation breaks down. There's your 10 cents and maybe gusting 2 bit worth of advice. What mr. motowheel says is true it's not easy to get accurate readings from VOM's at low ohms but bad connections will show higher (fluctuating) ohms not lower. Most VOMS will read a good rotor at about 6, 6.5 ohms this reading will gradually drop for several seconds. When it stabilizes that's pretty close to the real ohm's (if your VOM battery is fresh, LOL)
 
Well, that was easy. Rotor measured out at 2.8 ohms.

So I took it off, ohm'd it again, and got 5.0. So I thought, damn, I'm never this lucky, and, well, I was right, because as soon as I put it on again, it went to 2.8, where it stayed, on or off the bike.

Pretty disappointing considering all of the work I put into this thing last year. Didn't even get one ride out of it before it went for a dump. Let me state again, DO NOT BUY AN RM STATOR ROTOR! THEY ARE JUNK! The salvaged windings didn't even last. Who sells a QUALITY aftermarket rotor? Most I see are the same junk rotor I bought but under a different brand name. I would rather not go PMA, as I would like to keep the TCI, but I might if it proves hard to find a decent replacement. I saw Mr. Riggs do something with a salvaged Toyota ignition plate and reluctor, but I can't find any used parts and I don't think it's the kind of signal the TCI would use anyway. Buying these parts new is not a cheap option.

It was not all for nought today however...Got my 137.5 mains and larger pilots in to work with the new Uni filters. Put on new intake rubbers as well. I'm sure the old ones weren't doing airflow any favours. They both had a "tongue" of rubber sitting out into the airstream.
 

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I only buy rotors from Rick's Motorsports, great reputation. They sell from their website and on ebay. I get them in Toledo for $140.

You could try cleaning it with brake cleaner, depends on where the short is. I've done this, cleaned out the carbon from the brushes. That only seldom works with a short to ground.

Scott
 
Last edited:
Sad thing is that bargain $99.00 there is probably north of 150 to the door in Ottawa in our currency.
 
Greaaaat. I bought a RM rotor off of ebay last month. so far so good.
 

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