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Voltage drop

English Mike

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Hi all

I have a problem with my headlight on my CB750 F1 , bike starts ok but the headlight wont come on yep I tested the bulb and the fuse and the switches etc if I remove the bulb and put a test meter across the earth and ethier thre low or high beam I measure aroung 12 to 13 vdc if I then put the bulb back in the voltage drops to 0v . If I try a led bulb this will light but the voltage still drops to around 5.5vdc, I know the LED is pulling a lot less current so I am thinking the the fault is the regulator ? or rectifier ? Both ? the bike waqs working fine until last time out , any help very much appreciated thanks
 
I believe the problem is not with the regulator or rectifier, hence you said you're getting 12-13 VDC. The problem you're experiencing is a poor connection somewhere in the circuit. This poor connection is causing a resistance that is allowing what the electrical trade calls a parasitic amount of current to flow, but not enough to light the regular bulb. This problem connection can be either the ground connection as Cyclebuster suggested or in the positive as well. If you have access to a digital VOM, use the resistance scale and attach one end to a known good ground (like battery or engine) and trace the ground lead from the head light bucket till you find the culpret. If you don't have a VOM, you can run a temp ground from the headlight directly to the negative battery terminal to isolate whether the problem is on the ground side or the positive side. If it lights well with a temp ground, them its a ground lead issue. If no differance, then the problem is on the positive lead (like the HI/LO switch) and/or the lead suppling juice to that switch. Conversely, you could also run a temp positive lead from the battery to the head light to diagnose if the ground leads are good. Look for corrosion in the switch contacts or the soldered connections. Lightly sand switch contacts and all bullet connectors with emory cloth. Please report what you find to help us all.
 
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Thanks Guys

I believe the problem is not with the regulator or rectifier, hence you said you're getting 12-13 VDC. The problem you're experiencing is a poor connection somewhere in the circuit. This poor connection is causing a resistance that is allowing what the electrical trade calls a parasitic amount of current to flow, but not enough to light the regular bulb. This problem connection can be either the ground connection as Cyclebuster suggested or in the positive as well. If you have access to a digital VOM, use the resistance scale and attach one end to a known good ground (like battery or engine) and trace the ground lead from the head light bucket till you find the culpret. If you don't have a VOM, you can run a temp ground from the headlight directly to the negative battery terminal to isolate whether the problem is on the ground side or the positive side. If it lights well with a temp ground, them its a ground lead issue. If no differance, then the problem is on the positive lead (like the HI/LO switch) and/or the lead suppling juice to that switch. Conversely, you could also run a temp positive lead from the battery to the head light to diagnose if the ground leads are good. Look for corrosion in the switch contacts or the soldered connections. Lightly sand switch contacts and all bullet connectors with emory cloth. Please report what you find to help us all.

Thanks guys ,spot on Friartuck, I pulled the electrics to bits and ran a continuity test on the cables, problem for me is the bike is an import into the Uk it spent its early life in sunny california, so the wiring is a little different, anyway I traced the fault to the joint box on the main frame mounted under the tank, the fault was indeed a poor connection here ,out with the soldering iron problem fixed. I also took the switches to bits and cleaned them all up. Thanks again guys this is a great site.
 
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