[GrahamH]

I have just finished rebuilding the engine on a CB175 K4 (UK model) and whilst it runs nicely and all the electrical parts, lights, indicators etc are working I have found there is a 5A draw on the battery when the ignition is turned on, the battery is new and reads 12.5 volts all the time. The alternator appears to be pushing out a good voltage but as it should only supply 1.8 A even when in top condition it is loosing the battle with whatever is stealing my electricity. It still has the original selenium rectifier which l believe is working OK as the output waveform has been checked by my neighbour with an oscilloscope, (interesting but not fixing anything). So far I have disconnected all the wires easily got at with no change to the current draw. Note the UK bike does not have a voltage regulator but should switch the alternator windings when the lights are used.


So have you seen this sort of thing before and any thoughts on where the electric thief is hiding?

 

[mike in idaho]

That 5 amp draw with the engine stationary and the points closed will be less when it's running, when you figure in the points dwell duty cycle the average current draw will be somewhat less.

 

[GrahamH]

That 5 amp draw with the engine stationary and the points closed will be less when it's running, when you figure in the points dwell duty cycle the average current draw will be somewhat less.

Thanks for that but the current draw with the engine running is still around the 5A and the voltage over the battery stays at around 12.2 to 12.5 V even at 5000 rpm.

 

[Scramblr]

Honda service manual on page 64 says the battery voltage at 5000rpm should be 14v, at 2400rpm should be 13.2v. Lower voltage would result in a discharge from the battery. How are you measuring the current off the battery with the engine running?

The rectifier can be tested for resistance; spec says 5 to 40 ohms normal direction and over 100 ohms in reverse. See page 65 of the Honda service manual. Common Motors and other vendors sell new rectifiers ($65) with a more efficient design.

Aftermarket ignition coils with lower than spec resistance will pull a higher current during use. I did not find a resistance spec for the coil in the manual.

 

[GrahamH]

I have been away from home the past week so have only been able to research and think about the problem.
We measured the current using a multimeter by removing the battery fuse and measuring across the connections.
It was not possible to reach the rectifier during the first checks but now I will remove it and give it a full check. It an original selenium rectifier which have a poor reputation, so that is the first check.
I will probably replace it anyway although the CM unit is not readily available in the UK. It is tricky to select one with an assurance of good quality, they all look alike and I suspect they all come from the same source.
The UK version of the bike does not use a regulator but switches in additional stator coils when the lights are switched on. Maybe the switch is not connecting so to check it I plan to bypass the switch to see if there is anymore current available.
The ignition coil looks as though it may have been replaced, I will try to test it, but am I correct in thinking the coil should not draw any current if the CB points are open?

 

[GrahamH]

Things are becoming clearer....one of the diodes of the rectifier is faulty and it is the one which handles the extra two alternator coils when the lights are switched on. So this probably explains the low charging voltage. I also measured the coil resistance at about 3 ohms and after checking my old electrical science notes then V=IR or Current = 12/3 = 4 amps.

This has been a learning curve for me as I didn't expect the coil to take so much current.

Anyway a new type rectifier is on order and hopefully will fix it.

 

[mike in idaho]

Have you checked the resistance on the primary side of the ignition coil? 5 amps of current draw, at 12.4 volts, figures out to a primary resistance of approximately 2,5 ohms. That makes for a nice hot spark but will affect points life and heat up the coil, not to mention draining the battery. Is it possible some previous owner has installed a 6 volt coil? You should see 4 to 4.5 ohms of resistance in the primary windings.

 

[GrahamH]

Have you checked the resistance on the primary side of the ignition coil? 5 amps of current draw, at 12.4 volts, figures out to a primary resistance of approximately 2,5 ohms. That makes for a nice hot spark but will affect points life and heat up the coil, not to mention draining the battery. Is it possible some previous owner has installed a 6 volt coil? You should see 4 to 4.5 ohms of resistance in the primary windings.

I have double checked the primary resistance of the coil, 2.6 ohms.

How would I know if it is a 6V coil? The CD175 runs with a 6v system, perhaps a CD175 6v coil has been used or are they interchangeable with the 12v CB175?

From other internet research the general consensus is that the coil should have between 3 and 5 ohms, (exactly as Mike has suggested). The coil looks brand new but as the previous 'restorer' made so many bodges and errors in other areas I have little confidence he has fitted the correct item. It looks as though a new coil may be needed.

 

[mike in idaho]

You could put a ballast resistor in the circuit and use the same coil. Norton Commando uses 6 volt coils with a ballast resistor, some Honda Goldwings do also. A lot of points-era automobiles also use them, should be available in the auto parts stores for a reasonable cost.

 

[GrahamH]

You could put a ballast resistor in the circuit and use the same coil. Norton Commando uses 6 volt coils with a ballast resistor, some Honda Goldwings do also. A lot of points-era automobiles also use them, should be available in the auto parts stores for a reasonable cost.

Thanks Mike, I have decided to replace the coil, rectifier and the CB points......will report back when they arrive and are fitted. The ballast resistor will be my fall back option if the new coil does not work out. I haven't quite worked out how the 6V and 12V coils exactly differ. I think the 12V have more windings so more magnetic flux but as they have a greater resistance then the current will be lower and the magnetic flux will therefore be lower. This is all a bit too subtle for me so at least I will be assured to have the correct parts to start from.

 

[GrahamH]

I’m pleased to report the new coil and rectifier appear to have fixed the problem.

The old coil had a resistance of 2.6 ohms and the new coil has been tested at 5.5 ohms. So using V=IR the current drawn has been reduced from 4.6 to 2.3 amps (based on a nominal 12v). Was it a 6V coil? I don’t know how to differentiate between 6 and 12v coils but at least it looks much better.

The rectifier has also been replaced with a silicon type bought cheaply from Ebay. I first tested each diode and then it plugged straight in. The old selenium rectifier had one dead diode so although there was some power going to the battery it was only from one winding of the alternator, now all three windings should be connected when the lights are switched on, (UK type - no regulator).

As an aside I also bought a quick connect for the under tank fuel pipe so it could be removed without draining. In theory a good idea but in practise there was not enough access between the carb’s, wires etc to connect the two ends. I think I will stick with a simple fuel pipe and drain the tank if need be.