VT750 voltage regulator on a VT1100 - Honda Shadow Forums : Shadow Motorcycle Forum

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Old 02-28-2010, 02:14 PM   #1 (permalink)
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Default VT750 voltage regulator on a VT1100

I am trying to adapt an 80s something regulator from a VT750 that I had lying around to use on a 1987 VT1100 Shadow whose regulator is fried. I have one issue. The VT750 regulator has an extra wire that the VT1100 does not. Along with the three yellows from the alternator and the two red and two green wires, it also has a wire that is either black or blue. It's hard to tell the color exactly because of the age.

The original regulator from the VT1100 does not allow the charging system to work. Voltage goes no higher than 12.5 bolts. The VT750 regulator charges the VT1100, but it charges way too high - as high as 16.8 volts. I believe the 750 regulator is good, but I suspect that extra wire may do something on a 750 to limit charging. Can anyone tell me what that extra wire attaches to? Is it something that could limit alternator output so my battery does not fry?

I don't have a wiring diagram for a 750.

Thanks,

Patrick
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Old 02-28-2010, 03:50 PM   #2 (permalink)
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I haven't seen the schematic on the VT1100 but I do know the VT750 Voltage Regulator HAS to be mounted to the frame or it WILL charge sky-high and cook your battery. Do you have it mounted to the frame of your 1100 when you check your charging rate.
The VR of the 750 routes excess voltage to ground.

On my 750, there's 3 yellow wires (AC in) , a Red/WHITE wire that goes back to BATT POS basically, a GREEN (ground), and a BLACK that goes to the ignition. So it only has 6 wires. Doesn't sound like the 83-85 early 750/700 regulator to me unless maybe aftermarket ?????? Good luck.
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Old 02-28-2010, 05:14 PM   #3 (permalink)
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I'm going to have to look closer at the mount. I assume this RR grounds through the mounting bolts. I'll check and make sure the contact points are clean.

Thanks for that suggestion. I am open to any other observations as well.

Patrick
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1969 Honda CB750
1975 Honda CB750K
1978 Honda CB750F3
1979 Yamaha XS11
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Old 03-01-2010, 07:54 AM   #4 (permalink)
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Someone else posted about this a while back, but dang if I can't find the thread!

Some of he regulators have a black wire that's not found on other regulators. If I remember right, that black wire is an extra ground, but it's for voltage sensing. It's supposed to be connected as close to the negative cable from the battery as possible.

On a voltage regulator, it's internal voltage reference in always in relation to ground. if you have high current draw or any corrosion in the connectors, its ground reference can actually "float" slightly higher than true ground. As such, the voltage regulator always puts out power voltage than it should and that leads to problems.

This black wire (and someone please verify me on this, because I'm going from memory here) is connected to the frame where the negative from the battery connects. This allows the regulator to get a "true" ground reference.

--Justin
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Old 03-01-2010, 10:05 AM   #5 (permalink)
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Default Regulator

Before you go replacing the regulator make sure your stator is working properly, should be putting out 50-60vac.
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Old 03-01-2010, 10:54 AM   #6 (permalink)
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I did a resistance check on the stator and all three pairs of yellow wires are showing .4 ohms of resistance.

I tried just grounding that black wire to the frame and the voltage actually went higher, up to close to 20 volts at the battery. I am really leaning toward a ground problem, but everything in the wiring looks fine and when I connect a green wire from the RR directly to the frame I get the same readings.

This is confounding.

Patrick
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1987 Honda Shadow VT1100
1969 Honda CB750
1975 Honda CB750K
1978 Honda CB750F3
1979 Yamaha XS11
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