VFR 750 charging

[1-up]

Got an absolute bargain today, VFR 750 FH with 10 months mot, FSH and new tyres for £400. Pleased as could be, bought cheap as sometimes it won't charge the battery, put a new battery on and it gives 12.2 volts on tick over, 13.1 revved up to 5k but as soon as the lights are switched on the voltage drops to 8.3 regardless of revs.

Are there any electrical engineers that could offer some advice? doubtful it's the rec/reg as it charges when lights are off.

[RhynoCZ]

Find 3 yellow wires coming out of the stator. Set your multimeter to AC and meassure the VOLTAGE between the 3 yellow wires, all combinations, with the engine running. You should be getting anything between 30V and 60V. All three readouts should be very similar. Preferably, at high revs: 60V-60V-60V.

The out comes:
1) No power at any combination of wires = DEAD STATOR
2) No power at one of the combinations = DEAD STATOR
3) Very reduced power output = SOON TO BE DEAD STATOR, if not alredy
4) High voltage at running engine = THE STATOR IS ALRIGHT, move on to the next thing = the REG/REC.
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Charging being present suggests the Rectifier (the AC to DC convertor) works just fine, but there might be a problem with the Regulator. Both being a one unit, try a different Reg/Rec = you may use any Reg/Rec for a short test, that has the same plug/socket.

IF this won't help, then there's a huge short circuit. Find all the earth points, check all the wiring and plugs for oxidation (white and green stuff) and clean them if necessary.

PS: There is a very unlikely possibility, the rotor of the alternator lost it's magnetic properties, but I've never, ever experienced that before.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[1-up]

Thank you for your help RhynoCZ, I shall disappear into the shed shortly and have a look.

[RhynoCZ]

If you can't do the AC Voltage test, you may also measure the resistance at those wires, when the engine is off. You may also check whether the stator is not shorting to the ground. I'm not going to give you any Ohm values, as that depends on your multimeter. My multimeter reads less than 1 (10ohms, being 01.0 on the display?), so I'm getting numbers like 00.8 - 00.7 - 00.8. Anyway, I preffer the AC Voltage test.

When doing the AC Voltage test, be careful not to electrocute yourself.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[1-up]

Ok, also I have borrowed a friends Rec/reg off his Jellymould CBR 600 to try that.

[RhynoCZ]

Great, do a short test with the CBR Reg/Rec. I once did a test with 919RR Reg/Rec in my CB-1.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[1-up]

That is going to be my first port of call, followed by the things you have suggested RhynoCZ, thanks again.

[RhynoCZ]

I'm not sure whether your VFR has a Mosfet type Reg/Rec (2 plugs) or the ''regular'' unit though.

Either Way, the AC Voltage test will give you the answer.

EDIT: You can also come to a conclusion, the charging system is alright. Then you're going to have to check the wiring harness of the motorcycle and I would definitely start at the lights switch. A drop in the voltage this big would suggest a huge short circuit.

By the way, a proper charging system should give you about 14v at idle (could be lower/higher) and no more than 15v at high revs. 14.5v would be ideal.
____________________
'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[1-up]

Ok, the VFR Reg/rec has green mould on the contacts and a PC fan wired directly to the battery which had burnt out (the fan). Mates CBR Reg/rec on and all is good. Need to get one for myself now. eBay has some likely suspects, so I'll get one ordered. Thanks for your help RhynoCZ.

[RhynoCZ]

Cool, yes the PC fan mod was quite popular to keep the VFR's Reg/Rec unit temp down. I'd aim for an Reg/Rec unit with a heatsink and maybe use a PC thermal paste to mount it. I bet it's bolted down to a flat piece of aluminium or steel (part of the frame), just like on any other motorcycle, that doesn't have the unit exposed.

Glad to help.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[P.]

Buy a MOSFET one, put it in a position where it gets airflow and hardwire it.

[1-up]

Thank you Paddy. I'll give it a go.