dam you rectifier...

Dr_Dispronounce

Dr_Dispronounce

New Member
So, 1975 ST90, changed the rectifier after the stock one died. All was ok until recently 1 - 2 months of riding and now the bike seems to be killing batteries, two dead. Thinking that the replacement rectifier could be bad? I checked the connections are flowing correctly and it is. I purchased the one below. Is it possible it lets too much flow through or something?Anything else to check? Raidoshack time? (I am in NZ and shipping is dumb). Thanks

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Dr_Dispronounce

Dr_Dispronounce

New Member
I have the same battery as stock. With the dual wire. Lead.
I ran a volt meter. Battery is holding at 6.3 well. has been sat off charger for a week on bench.

When bike is running at idle:
5.6v on red wire
5.9v on red/white wire

when lighting is on:
5.6v on red wire
0.5v on red/white wire -Hmmm

So, something not right. I imagine it should have greater charging than that, somewhere in the 7v range? Will see what I can locate online while some hopeful wisdom in replies return to me.

Also- the red wire runs super hot.
 
B

b52bombardier1

Well-Known Member
Flip the knob on your multimeter with the bike running to AC volts. Hope that you don't see more than about 0.1 volts AC - generally much less. Your DC volts number with the bike running should be 7.0 or so. The red wire being hot is also crazy wrong and my suspicion is a bad rectifier.

Rick
 
J

joel1234567

Active Member
At first I suspected wrong battery or possibly gone dry. But sounds like more issues at play.
Definite wiring and/or charging issues. As Rick mention, red wire def shouldnt be hot. And red/white shouldn't drop to .5v with headlights on.
The only thing you need powered for the bike to run is power to the coil. Try disconnecting everything else and add things back in one at a time... maybe start with rectifier and see if you can get a charge and/or if the "hot" red wire goes away. See if you can isolate the problem.

Also note that idling won't charge the battery, it will actually slowly drain it. So you prob won't see an any excess of charging voltage with the bike at idle with your voltmeter.
 
Dr_Dispronounce

Dr_Dispronounce

New Member
I pulled under the seat apart and found some loose connections as well as the stock battery fuse holder was terrible. Re installed a new one and soldered up the loose connections which ran to the rectifier. I re tested the rectifier and its operating as it should. In a retest the battery is charging under revs. The red wire is no longer hot and gives a normal range with volts. I need to go for a test ride but hoping that it is resolved.
 
H

Honda Hornet

Member
I had a parallel experience of deteriorated wiring on a 1978 CT90, which resulted in strange voltage readings that could not be explained until I started testing for resistance and ran some supplementary wiring to get more volts to where they were needed. I am sure it is not as common as selenium rectifiers and old flooded lead acid batteries needing to be replaced, but it seems to be another problem to be considered.

I remember owning a mid 1970s C70 in the mid 1980s and the challenge of dealing with the sub-optimal 6V battery and its "load balancing" design. To me it seems a much more fundamental challenge for these little old motorcycles than having to learn how to fettle with rusty tanks, carburettors and breaker points. Getting spark is of course what everyone thinks about in dealing with a newly obtained old Honda, but getting it to run so that it does not run the battery dry (or in my nightmares turn a sealed battery into an IED) or have lighting that is too weak to get home in the dark. These seem to be the real challenges of these old bikes staying useful for transportation.
 
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