Rectifier overcharge fix

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duke900
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model: 750 Paso
year: 1988
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Rectifier overcharge fix

Post by duke900 »

After have bought a new rectifier and it still overcharge i found that the original rectifier have a Sense Wire (it reads the battery voltage from the harness that comes from the ignition Key switch)
I cleaned all earth and conections also disasemble ignition switch and cleaned all.
But after this problem still remains, i found that this Sense Wire was reading low voltage (less 1 to 1.5v) with bike off battery full charged 12.6v and i was getting about 11v in the Sense Wire that gets the information to rectifier so when started the bike i get at 3000/5000rpm 15.5v or more!
I fit a relay with fuse from battery to Sense Wire and to rectifier (to get real voltage)
Job done with the old rectifier i get steady 14.3v.
Cheap solution and i keep the new rectifier ir this fails.
Went for a Ride and all OK, teste again and 14.3 at 5000rpm.
What do you guys think of this solution.
Relay in the - circle.
Rectifier Sense Wire - Arrow.

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Mc tool
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Re: Rectifier overcharge fix.

Post by Mc tool »

The real issue is the missing volts. Our beloved bikes have a few issues with the wiring as they get older.there will be one or more poor connections . Another solution is to simply stick the sensor wire straight to the battery+ . 1.5v doesn't seem a lot but as you can see its enough to cause issues .
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duke900
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Re: Rectifier overcharge fix.

Post by duke900 »

MC tool

If you Wire directly from battery +12v it Will Turn on other parts (fuel Pump etc...) and you end up with a dead battery.

I have tried that first, thats the reason i used a relay it Will only give 12v to rectifier when you Turn on the ignition Key. :thumbup:
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Mc tool
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Re: Rectifier overcharge fix.

Post by Mc tool »

If you wire the voltage sensor wire from the regulator/rectifier directly to the battery it will turn on the fuel pump?
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Derek
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Re: Rectifier overcharge fix.

Post by Derek »

The voltage drop is happening at the ignition switch. On my 907 I fitted a relay, switched by the ignition switch, that supplied power to the fuse box and all the ignition switched circuits, bypassing the ignition switch. A similar mod would fix your problem and also supply the correct voltage to the ignition, headlights etc.
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paso750
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Re: Rectifier overcharge fix

Post by paso750 »

That's exactly what I thought when I read this. ;) This mod is like the coil wiring mod. It will certainly work but it's a fix that will only address this issue. It won't effect anything else and more importantly it will not eliminate the root cause of the voltage drop.
I also see using a relay which is switched by the ignition switch as the better choice as that will positively effect the entire electrical system. Of course this requires cutting into the main wiring harness and I guess many don't want to do that.

How a wire directly to the battery would make the fuel pump run I also don't understand.
There's 12V switched on the yellow wire (which is normally green). If an additional connection to the battery was made (which makes no sense) the fuel pump and everythig else would of course be on as this would bypass the ignition switch.
If the yellow wire was disconnected from the 'C' connector instead and replaced by one connecting directly to the battery the fuel pump would only run if the ignition key is turned.
(Personally I wouldn't do that as I don't know what the regulator/rectifier does if there's constant 12V on 'C'. 'B+' is the charging output of the reg/rect. It's my understanding that if 'C' measures a voltage below a certain level it switches 'B+' which will then allow the battery to be charged. If the voltage measured is above that level the 'B+' output will be cut off to prevent overcharging. If 'C' is connected to the battery directly it would measure the output of 'B+'. Or am I getting something wrong?).

On a different note. The fuses in the fuse holder should be a 15A and two 7.5A (according to the wiring plan. Interestingly the same owner's manual on P.36 says two 15A and one 7.5A which I've never noticed before). A 30A on the relay is overkill. Before that blows the wires would melt as they can only handle about a third.

G.
duke900
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Re: Rectifier overcharge fix

Post by duke900 »

This is how i conected the relay.

Sense Wire only get +12v when ignition Key is ON, that Will switch the relay and give the correct +12v to rectifier.

The only Diference is i conected the +12v (BAT in drawing) to the starter solenoid +12v feed.

PS: I did not cut any original wiring.

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Sense wire before relay fix (GR-BK)
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Connection +12v feed (solenoid)
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higgy
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Re: Rectifier overcharge fix

Post by higgy »

So it sounds like there is a short in your ignition switch or wiring or the fuel pump relay has a short in its wiring.
Or the typical loose or bad ground is feeding back through one harness or the other
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duke900
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Joined: Mon Jul 21, 2008 5:51 pm
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year: 1988
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Re: Rectifier overcharge fix

Post by duke900 »

higgy wrote:So it sounds like there is a short in your ignition switch or wiring or the fuel pump relay has a short in its wiring.
Or the typical loose or bad ground is feeding back through one harness or the other

Tested this again last night and it do not started the fuel Pump!!!

I had the ideia that it did that... but i tested this a long time ago and maybe i was testing another part off wiring and get confused. :wacko:

To get things cleared.

IF YOU CONNECT THE SENSE WIRE TO A +12VvFEED IT DOES NOT START THE FUEL PUMP.

Sorry for my confusion. :-P
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paso750
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Re: Rectifier overcharge fix

Post by paso750 »

higgy wrote:So it sounds like there is a short in your ignition switch or wiring or the fuel pump relay has a short in its wiring.
Or the typical loose or bad ground is feeding back through one harness or the other
You have a fuel pump relay? :huh: :)
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higgy
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Re: Rectifier overcharge fix

Post by higgy »

Sorry G,missed it’s a 750 My bad
Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
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