First day of Guzzi ownership and I know it’s a ride restore bike, but as it runs well I took it out to the petrol station to fill up. The bike wouldn’t start again after filling and I had to push it home….bloody hell it’s heavy!!
I know the battery is quite old but it seemed to charge ok (I’ve ordered another anyway) and so I thought I’d check the alternator for charge. I’ve ckecked the resistance across the three yellow wires and they all show a reading of 1.2 Ohms and this seems larger than the specified 0.62 Ohms. Does this mean that the stator is dead? Are there any other checks I can do to confirm?
Really appreciate your help
Jamie
Go throught the process in the “Bosch Charging System” PDF in the attached link
This should identify the fault for you
Russell
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The resistance is high but as they are all the same i would look elsewhere to start with.
If i recall correctly the wire that excites the rotor via the commutator is very long (about 2.5m from the regulator) with several connections so it is worth check you are getting 12v and enough amps to light a small 12v bulb when it spins.
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Thanks guys, I’ve ordered a new battery as a precaution and I will test all again when it arrives. The wiring is very dirty, so there could well be something amis.
Thanks again
Jamie
Guys just looking at the D- terminal and shouldn’t that spade connector with nothing on it go to earth?
Yes D- should connect to the D- terminal on the regulator and that is earthed.
From my own Le Mans wiring diagram
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That spare spade on the D terminal is already earthed through the aluminium body of the alternator although you can add another earth if you want to. The wiring diagrams may show a connection but on all my Guzzi’s that wire doesn’t exist on the bike unless you add one
I haven’t seen any mention of checking the rotor winding which should be about 3.4 ohm’s and the carbon brushes, usually the first thing I’d look at.
This is quite a useful guide to older Guzzi electrics 
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That’s great Chris thank you. I have checked continuity on the D- to earth and it’s all good. I’ll have a look to see if the D- on the regulator goes straight to earth and doesn’t connect to the alternator wiring anywhere. Thanks again
It states
“On the casing the negative field terminal D- has a lug on it for a wire, but is not connected to anything as it has a path to the frame via the stator casing.”
But the Guzzi workshop diagrams show a wire, but not connected as I have drawn.
It also shows the wire between the DF terminals as violet and not brown as in my diagram, I will have to change this!
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Thanks Keef, I’ve checked the windings. My meter has an internal resistance of around 0.5 and the winding reading give 3.7, so around 3.2 result. Is this ok?
I’ve checked and there is no continuity between the windings and the shaft.
Thanks again
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Yes, 3.2 is fine, to be honest as long as it’s not open circuit the actual reading isn’t that important
Is the red charging light going out ?
Two of my bikes have an additional resistor in parallel with the charging lamp (pale blue wire) so will charge even if the bulb blows, this was standard fitment on both my LM4 & LM5 which is a good idea, my G5 doesn’t have it though, it talks about it in the “charging around the circuit” guide
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Yes the generator light is going out and my next job is to check to see if I have the resistor 
I doubt very much that you have the additional resistor as it was only fitted by Guzzi to bikes from around 1988, which is why my 1979 G5 doesn’t have one, have you checked what you are getting across the battery terminals with the engine running, I’d hope to see around 14.5 volts with a fully charged battery but even with a slightly discharged battery you should see the voltage increase as you rev it up
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Strangely yesterday I was seeing a voltage drop as I revved the bike, but that is with the old battery. I should get the new battery today and I will repeat the test.
I just did the resistor check as explained by John noble and got a strange result. I did the test with the battery out of the bike and not sure if that would make a difference or not?
All fuse outputs showed open circuit apart from one that showed just over 14 Ohms. Hopefully I did the test ok with one end on the D+ on the rectifier and one on the fuse terminal.
Any ideas?
“It would be a good idea to check if you have that elusive resistor. With the ignition off, remove all the fuses and check the resistances individually between the fuse output and D+ on the rectifier. There should be one fuse that will return approx 120 Ohm, that’s the bulb. Only one other should return a value, approx 80 Ohm, if it does then you have the resistor. If all the others are open then you don’t”.
Ok just got new battery and fitted. What a difference to the cranking power and speed!
Bike started straight away and showed 12.5 at slow ticker. I revved it to 3-4K and it only showed 12.7-12.8 volts and didn’t get over 13. The good news is that the voltage is increasing but not to a very high level.
Does this indicate anything specific or am I back at square one with testing everything.
Thanks aain
Jamie
Well, it’s definitely not charging is it !
It’s worth checking that it has the right wattage bulb in instrument cluster, it needs to be 1.2 w
After that it looks like you will be checking the regulator and rectifier, following the instructions in the Gutsibits sheet from earlier in this post should help
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List of bulbs from G5 1979 manual.
It won’t charge properly if a 3 w bulb has been fitted, it needs to be the 1.2w bulb specified
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Just out of interest, you can usually tell if the bike also has a charge resistor fitted, the D- terminal will have 2 wires coming off the rectifier, (light blue) But I wouldn’t worry if it only uses the bulb
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