748 alt/charge

Yep, should have 140v at that rpm, your stator is fried by the sounds of it, search for 2 wire alternator. I have just been through all this with a mates bike, the stator had a dead section and and the reg rec was failing meaning it could 'just' charge at idle and then as it revs it would drop the voltage to more like 11v at the battery

 

I don't know how much people charge to rewind these things but maybe an option

 

Myself and a couple of others have used West Country windings who were brilliant. I think the complete job with a new 'electrex' reg rec and connectors etc was around £200

 

Okay, sorry, I'll try and help more now that I am on a PC. Stupid phone!

http://ducatiforum.co.uk/forum/threads/1997-748-2-wire-alternator-output.17730/

that's the thread I started, Bootsam and Nelson were a big help amongst others, also speak to West Country Windings, here:

http://www.westcountrywindings.co.uk/

 

There are many reports of similar voltage readings to yours, I tested mine 3 years ago and got readings very close to them.
Here's another after a new alternator had been fitted.Taken from another forum. He went on to fit a new reg/rec and all was well. All i am saying is check everything before wasting money.
Steve



New Stator Tested... Regulator Bad???
I'm getting frustrated. The expertise of the group is still needed..

After installing the new stator I test the VAC from the stator while running. It is apprx 24.8 VAC at 2000 rpm and 30-35 VAC at 5000-6000 rpm. I expected to see more (stator resistance is 0 ohms).

Then I check the values of the VDC across the batter and see < 13 VDC. So, I'm getting no charging and it seems like the regulator is shot.

The kicker is, the regulator/rectifier is basically brand new (installed by a shop when they misdiagnosed my charging ailments). Is it possible that the brand new reg/rec got fried immediately after installation? It was zipped to the frame close to the vertical cylinder (a pretty hot spot). Is there an easy way to isolate and test the regulator?

And here's another with similar measurements. First couple of posts,
http://www.ducati.ms/forums/80-hall-wisdom/6288-916-748-charging-problems-easy-fix.html

 

I'm not familiar with the bike. If the alternator is a Saprisa and is a permanent magnet type, which I think it probably will be, there are 3 things that can lower the generator output as measured AC on the Yellow wires.

1) Magnet feked................ie rotor problem. If its old the magnetic could have become a lot less magnety over the years hence its still working just your output is reduced because the excitation is reduced.
2) Rotor open circuit............rotor problem.
3) Stator open circuit............stator problem

The latter 2 result in no volts, the first one to me seems a possibility.

It wouldn't be reg/rec as all that's been tested is the Alternator itself (unless it was an electro magnet type like the Bosch type used on Guzzis and BMWs that I know about).

I would reckon that a decent auto electrical shop would be able to test it for you pretty cheaply

John

 

I would get a puller, it's a big case to get off and worth a tenner for piece of mind. also u don't want to screw up the bearing in there

 

Here's another taken from another forum.
Steve


#14 (permalink)
02-16-2005, 04:27 PM
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Re: help with alternator problems... (DukeBoy)
Do a simple output voltage check of the stator itself, before going any furthur.

Take the multimeter set to AC VOLTS, connect it to the stator wires and disconnect the voltage regulator. Run the bike at 4 to 5K rpm. Check the AC VOLTS output. It should be anywhere from 50 to 75 AC VOLTS. If you get a reading in that range, the stator is doing it's job and the problem is elsewhere.

I've been through this several times with my own '95-916 over the last eight years, and it's always been the voltage R/R or the wiring. For a stator to actually go bad is very rare.........YMMV

 

I bow to your superior knowledge on the model Birdie, as I know sod all about them.

My point was simply if you open circuit (i. e. pull off the connector that has the yellow wires) then check AC volts across the tabs on the stator where these wires connect onto, then the rest of the charge circuit is out of the test. If however you leave the plug connected, then I'd expect AC volts to be lower but by how much I have no idea as I DON'T KNOW the circuit or components. So anything I'm saying here should be double checked by somebody more intimate with the bike (like Birdie).

On a permanent magnet Alternator, the output of the alternator is independent of the regulator and rectifier, as the field excitation is being powered by a magnet whose output is fixed (you cannot make it more or less magnety).

Larger electrical machines and some bike alternators have the output regulated by varying the field. In the bike example an electro magnet controlled by the regulator. This is a more efficient design, but also more complex and has its own problems.

The reason for the usual rash of reg/rec failures on permanent magnet alternators is they have to work harder. They are not controlling Alternator output (like the variable field type) but have to sink all the excess electricity to somewhere.

That electricity is simply energy by another name and the sink is the frame, this means a lot of current going through the reg/rec to frame this equates to heat. Which means a hard working life.

Right that's it I'll get ma anorak

John

 

No superior knowledge here John, all i am pointing out to the OP is that there are plenty of people out there including me that have similar readings to him and have found the problem to be the rec/reg. I am trying to be helpful and suggesting the OP does as many checks as possible before splashing out on a new stator.
It could well be the stator at fault.
Steve

 

For what it's worth I have just been out to the garage and tested my 1998 748, it now has a different engine which was fitted 15 months ago, at 2000 rpm it reads 20v at 4000 rpm 45v and at 6000 rpm it reads 77 and rising with more revs, these are very similar readings to my previous engine, all the electrics involved have been upgraded and I have NO charging problems.
Steve

PS Has anyone tested a single phase and actually got 140v ?
Is everyone getting this information from the Haynes Manuel ?
Haynes do make mistakes iv'e found others, is the 140v a mistake?
I am as interested to know the answer as anyone.

 

TBH we didn't when we put it all back together, just checked that we are getting a better reading at the battery and that it doesn't drop off, which it doesn't.

My advise would be to check the stator first as it's so simple to check, if your getting output then check Reg/Rec, although I have to agree that I would be suprised if anything with any wear on the rotor/stator was putting out anywhere near 140V AC, our problem was really obvious, at 3/4RPM we were getting like 50Volts....

 

3/4 thou rpm 50v is more than I am getting.

The OP has checked his stator and has similar readings to me.
Perhaps my stator is on it's way out :Arghh:

 

I've always been lead to believe that about 15Vac / 1,000rpm is what to expect. So that would be around 75Vac at 5,000rpm

 

Personally, now that its out I would get it checked out by an auto electrician, if ok then just solder new wires on and away you go.
You say the wire cover just picked off if so was the cable exposed before you started picking and any sign's of it shorting.
You do say in your first post that you have similar voltage readings to me which would suggest that the wires were not shorting.
If it wasn't shorting this may not have been your problem although it does need attention and you may still need a reg/rec after the stator has been replaced.
Just my opinion I am no expert.
Steve