1975 850T electric mistery

[Woodrow]

So I have a 850 , great bike , starts great , stops great , does everything.   BUT the other week i turn the key,, nothing,.(no lights on dash)   So ok , roll out another bike and go to work,       Come back from town , check the battery , 12.5  volts .  WOW OK .  turn the key , push the button ,  fires right up.

     So I blamed myself ,  i thought I had turned the key too far and not turned it enouf to start .  so great ,  I rode it the rest of the week.   And it did it agan..    no matter what it wont crank ,  battery is 12v . .  Turn the key on and off and have a spark at plug.    so I start looking it over a day or so later .AND,, lights  oil light , netural light.  and cranks and starts.   

 This bike has a 3 year old wire harnes from G Bender,  all the wires look great .    Today I wiggled and unpluged and repluged all the wires i could .
I put the tank on and ran it up and down my road ,  not as far as I would like to push this thing.   

   Would be nice to have someone follow me around with a trailer.     Good running bike (when it runs )       Woodrow .

[Chuck in Indiana]

Sounds like a good time to disassemble the ignition switch, clean, wire brush the contacts, apply fresh vaseline. Even if that isn't it, most likely it's due.

[guzzisteve]

I second that motion. Good idea unless replaced with harness.

[Antietam Classic Cycle]

Have you checked the charging voltage? Battery resting voltage should be a bit higher than 12.5.

[cliffrod]

I’ll jump in and ask if the fuse box was replaced along with the new wiring harness. Hidden corrosion in my V7 Sport’s original fuse box confounded us for quite a while after I replaced everything wiring harness-related except that because it looked fine.   Once fixed, all was good.

[Stretch]

Good advice on the ignition switch and fuze block; don't forget the kill switch, too.

                                                           -Stretch

[n3303j]

My T3 drove me nuts with a similar problem. .
Found out the battery NEGATIVE cable was making a poor connection at the chassis. Clean it up, tighten it up and it all was history.

[Woodrow]

OK   Thank you ,  I replaced the key switch in the last few years ( this is the only bike I havent kept a record book on and I regreat it ). I found the old switch .    Just how do I take that apart , I got nothing to loose. 

  This morning ,  at rest cold battery is at 13.75 .  Key on , lights, oil.gen  Netural battery still at 12 .75 or so.    Start up , gen light goes out . Voltage  14 volts at 3500 rpm.

 Today Im going to start a record book . also email a print shop to have a wiring diagram printed.

 So Im going to test ride some more ,  taking a cell phone , credit card , fire extinguisher. confortable walking shoes. ect.   

  I do rember having the fuse box off the bike .  I can do that agan just for a look ..    Woodrow

[Woodrow]

Im going to take another closer look at the battery connections.   Why not ?

[n3303j]

Im going to take another closer look at the battery connections.   Why not ?

Pull them apart. Clean them thoroughly. Grease them up. Reassemble.

Had recent problem with headlight shell ground on the T3 (after installing excellent Bender harness). There is a ground tab riveted to the shell. Turns out 50 years of corrosion in that area insulated the tab from the shell. Drill out rivet, clean all metal parts thoroughly, pack assembly with grease then re-rivet and all problems went away.

[cliffrod]

Pull them apart. Clean them thoroughly. Grease them up. Reassemble.

Had recent problem with headlight shell ground on the T3 (after installing excellent Bender harness). There is a ground tab riveted to the shell. Turns out 50 years of corrosion in that area insulated the tab from the shell. Drill out rivet, clean all metal parts thoroughly, pack assembly with grease then re-rivet and all problems went away.

This^^^^ is what can happen with any and potentially all of the riveted connections on these old bikes.  My original fuse panel looked great with no undue or obvious corrosion issues.  After checking continuity and drilling out rivets, the hidden corrosion on every connection was impressive.  Lots of small losses created the larger cumulative problem.

[cappisj1]

I had a pin break inside one of the headlight pugs. Took a summer and a half to find it. That caused my T to immediately lose power to the engine. From rumbling down the road to instant dead silence coasting down the road. I don’t envy your future of tracing wires.

[chuck peterson]

Moto Guzzi, making electricians out of motorcycle riders  since 1925…

Similar situation…dead everything with the key on

Opened the fuse block, spun the fuses in their settings, started right up

Corrosion

Also…

Check both ends of the battery neg ground cable, and up inside behind the sheathing, then scrape, sand and smear the frame. It only happens because it’s a 50 year old bike. Do the same for all your grounds…add a second ground off the diode board to the engine bolt holding the transmission on…

[Stretch]

There is nothing - but nothing -  harder to troubleshoot than in intermittent
electrical fault!

                                                     -Stretch