wild voltage fluctuations - Team Camaro Tech
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post #1 of 10 (permalink) Old May 8th, 12, 08:11 AM Thread Starter
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wild voltage fluctuations

Motorcycle question here. Got my bike out of winter storage and it runs like crap. Been fiddling with and testing the carbs, plugs/wires/caps, coils etc. Everything seems fine.

Last night checked the voltage across the battery terminals with the engine running and got some crazy readings. 14.2----4.6----12.8----3.0 and even some negative voltage readings thrown in the mix. I'm guessing that means the regulator/rectifier is fried? Would all the crazy readings screw up the voltage at the coils and cause weak spark?

Just had another thought. Maybe the alternator brushes are worn down and not making constant contact?

Mark

68 327 4-speed 3.42 posi and a sunroof
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post #2 of 10 (permalink) Old May 8th, 12, 08:41 AM
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Re: wild voltage fluctuations

Either, as you suggest, brushes not making good contact, or the meter leads not making good contact across the posts, or a bad meter or leads themselves. Meter will never read negative across the battery posts, regardless of the battery charge, or lack of charge.

Might remove battery clamps and wire brush the posts and clamps and same with the other end of the cables.

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post #3 of 10 (permalink) Old May 8th, 12, 10:02 AM Thread Starter
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Re: wild voltage fluctuations

Quote:
Originally Posted by Everett#2390 View Post
Meter will never read negative across the battery posts, regardless of the battery charge, or lack of charge.
Even if the rectifier is fried and the meter (set on DC volts) is reading voltage coming from the alternator? A negative reading popped up at least 3 or 4 times. I hope my new meter isn't defective...

Battery terminals and all cable ends were cleaned when I put the battery back in for the season. Frame grounds scraped clean also. New digital meter. Readings taken at the battery posts, attaching bolts and cable ends. Same results all three times.

I guess my main question is will the crazy voltage fluctuations affect the voltage at the coils making the bike misfire, or will the battery compensate for it and provide constant voltage? The bike idles great but misses and backfires when you give it gas. Plugs are black and sooty like weak ignition.

Remembered an old joke...

Rectifier: A motorcycle accident in which the rider falls off the bike at speed and skids down the highway in the sitting position.

Mark

68 327 4-speed 3.42 posi and a sunroof
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best so far... 13.25 at 106mph

78 Firebird 350/T350 3.23 posi
65 Mustang convert. 6 cyl. 3-speed
66 Mustang coupe 6 cyl. auto
80 Chevy C10 406 4-speed

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post #4 of 10 (permalink) Old May 8th, 12, 11:38 AM
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Re: wild voltage fluctuations

Quote:
Originally Posted by mbrekke View Post

Rectifier: A motorcycle accident in which the rider falls off the bike at speed and skids down the highway in the sitting position.

Mark
Humm.... I always thought that was called a rectum-fryer Atleast on the east coast anyway

Kev
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post #5 of 10 (permalink) Old May 8th, 12, 06:01 PM
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Re: wild voltage fluctuations

If you believe all other variables are eliminated, meter, cable cleanliness, etc., then try another rectifier. Battery should regulate the system, aka, become the filter.

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post #6 of 10 (permalink) Old May 8th, 12, 08:38 PM
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Re: wild voltage fluctuations

If the battery had the charge to start the engine, it seems unlikely that you can read 3V or 4.6V. The battery can supply more current than the alt can, so even if the alternator or something else is shorting, you'll still be reading near 12V at the battery.

What do you read with the engine off?
What do you read with the engine off and the key in the run position?

Bad meter lead? New doesn't always mean good...
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post #7 of 10 (permalink) Old May 8th, 12, 08:50 PM
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Re: wild voltage fluctuations

Probably the rectifier. If it's bad you're actually getting AC ripple riding on top of the DC. If it's a newer bike with EFI and an ECM that'll confuse the hell out of it. Set your meter to read AC and measure at the alternator B+ terminal and alternator case with the bike revved up some and anything over 0.5V should indicate a failed rectifier. You want to measure at the alternator since the battery will smooth it out some. Also what bike is it? I used to have a VTX (sold it today actually) and they had a recall for bad engine ground when they first came out.

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post #8 of 10 (permalink) Old May 9th, 12, 06:43 AM Thread Starter
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Re: wild voltage fluctuations

Thanks guys. The battery read 12.63 volts, if I remember correctly, before I started messing with anything. Didn't check it with the key 'on' though.

I used the meter to test the resistance on the coils and spark plug caps. Everything seemed to work o.k. so I don't think the meter has a bad lead. I'll do some more testing tonight. Didn't get to work on it last night. Mowed for a couple hours instead.

It's an old Yamaha XS650 and is still pretty much all original. Original regulator, rectifier, alternator assy. etc. so I'm sure something needs attention. I did replace the alt. brushes on it many years ago when I owned it the first time. The guy I bought it back from had some pretty 'creative' ideas when it came to wiring and such though, so who knows what will surface. I spent most of last summer fixing what he screwed up. Never should have sold it...

Mark

68 327 4-speed 3.42 posi and a sunroof
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best so far... 13.25 at 106mph

78 Firebird 350/T350 3.23 posi
65 Mustang convert. 6 cyl. 3-speed
66 Mustang coupe 6 cyl. auto
80 Chevy C10 406 4-speed

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post #9 of 10 (permalink) Old May 10th, 12, 06:41 AM Thread Starter
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Re: wild voltage fluctuations

Took the regulator apart last night and it actually looks like new inside. Did a little resistance testing on it and then moved on to the alternator brushes.

One brush looked new and was twice as long as the other one. The shorter one was about halfway worn according to the specs. The longer one was cockeyed in the holder, about 45 degrees from how it's supposed to fit. Don't know if that had it stuck to where it wasn't free floating like it should be and that's why it wasn't worn like the other one, or if the previous owner only replaced one brush. Anyway, it has a much steadier reading at the battery now that increases with revs. The meter still throws out a single digit volt reading now and then though.

Checked the battery again with the key off. Got a reading of 12.4x last night and then turned the key on and it dropped to 11.85. Seems like quite a drop. Not sure what that means. Also checked it on the AC voltage setting with the bike running and got a steady 26 volt reading. Don't have a clue if that means anything either.

Still have the misfire problem though. Gotta move on to the next thing...

Mark

68 327 4-speed 3.42 posi and a sunroof
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best so far... 13.25 at 106mph

78 Firebird 350/T350 3.23 posi
65 Mustang convert. 6 cyl. 3-speed
66 Mustang coupe 6 cyl. auto
80 Chevy C10 406 4-speed

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post #10 of 10 (permalink) Old May 11th, 12, 06:30 AM Thread Starter
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Re: wild voltage fluctuations

Misfire problem solved last night. Almost embarrased to admit what it was.

Bought some new pod type air filters and installed them a week or two ago. Found out there's a lip inside the rubber mounting part that covered the small air holes on the outside edge of the carb inlet. Took the filters off and it runs great.

Mark

68 327 4-speed 3.42 posi and a sunroof
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best so far... 13.25 at 106mph

78 Firebird 350/T350 3.23 posi
65 Mustang convert. 6 cyl. 3-speed
66 Mustang coupe 6 cyl. auto
80 Chevy C10 406 4-speed

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