Thursday, 20 December 2018

It Never Rains but it Pours

On this occasion I'm not talking about the Scottish weather.

After getting my CBF250 back on the road after some engine work another problem emerged. The battery stopped charging.

I first suspected the battery, it's 7 years old so maybe a replacement is required. I attached my charger and it charged up ok. The bike then ran for a while but the battery soon went flat.

Now before I go any further I'll freely admit that I'm not good with electrics so forgive me if any of the following is rubbish.

To continue….the battery seemed to be discharging. If the charger puts juice into the battery then so should the bike's charging system.

It's easy to check whether a battery is being charging. Put a voltmeter across the battery, it will show about 12 volts, start the engine and this should rise to about 14 volts. I did this but the voltage didn't change - so no charging.

A quick description of the CBF's charging system. It has a permanent magnet alternator. A rotor with magnets is attached to the end of the crankshaft and spins around wire coils (the stator) This produces AC (alternating current) this goes to a regulator/rectifier unit. This changes the AC to DC (direct current) and regulates the voltage to about 14 volts. This goes to the battery to charge it.  
CBF's charging system - quite simple really
I then suspected the regulator/rectifier. So just replace the unit? Well, a check with my usual supplier gave a cost of £173.70. A bit steep for a bike in the twilight of its years. I couldn't find a used item (the CBF is fairly rare here) so I hoped that I could find something I could use. I bought a pattern unit intended for the CBR600 for a mere £7.89. The pin positions were different but I lashed up cables with spade connectors to connect the unit to the plug on the bike's wring loom.

Cheapo reg/rect
Unfortunately there was still no charging. I checked the AC output from the alternator and it was low - about 8 - 12 volts. It should be about 50 volts so it looked like the stator was at fault. There was continuity between the phases of the stator so there was no break in the wires. This indicates a short.

I removed the stator and it looked ok. I was expecting the wires in the coils to be completely burned out. There was a little "crustiness" in some bits of wire otherwise it looked ok. Worryingly the insulating resin on the stator was flaking off in small chips. A sign of overheating? The stator is inside the engine and submerged in the engine oil. These chips will contaminate the oil but are hopefully caught by the filters.  
The stator looks ok. Just a little discolouring on the odd wire.

These particles are loose in the engine - not good.
Now I had to make a decision. A replacement stator is £210. My CBF, well used at 65,000 miles isn't worth much so should I spend money on it or buy a new bike? Recent engine work revealed that the motor was in good condition and it seems a waste to scrap the bike. So I looked for used item on eBay.

CBF250s are fairly rare here so there aren't a huge amount of parts. But I did found one at £80 so I bought it. Fitting the stator is very simple. Just bolt it onto the engine casing, replace the casing and engine oil and plug it in.  
Stators old new & old

I fitted the new stator…..and I got charging voltage! 
Engine off....

....and running

The CBF ready to take on winter!


  1. Well done Stuart, the CBF is saved again! Happy Christmas!

    1. Thanks & seasons greetings to you too.

      The CBF's engine is in good condition with little signs of wear despite its miles so I thought it was worth trying to fix it.