Fitting a starter relay to a T3.

[injundave]

Following a meltdown in my (cheap chinese) switch block, presumably because of too much amperage going through small wiring and contacts, I bought this small car relay which, I hope, will assist.





Can anybody advise me where to splice it into the T3 loom, please?

[Diploman]

The Marelli starter relay Guzzi fitted to bikes of the T3 era is an archaic device that looks like it was designed prior to WWII. I suggest you consider updating to a modern relay (at least 30 amp) like the Bosch linked below.  I am not certain that the auto relay you have pictured has the same pin arrangement for easy adaptation to your bike.

https://www.amazon.com/Bosch-0332019150-Mini-Relay/dp/B004Z0W1LM/ref=sr_1_9?keywords=bosch+relays&qid=1564060955&s=automotive&sr=1-9

[Kiwi_Roy]

Instead of running the 12 Volts through the ignition switch as it shows run it direct from the battery through an in-line 20 Amp fuse to the relay contact.
Run the wire from fuse 2 to the neutral switch so it goes switched fuse 2 to neutral switch to start relay, this way the starter will not operate if the key is turned off but the full battery Voltage is available for operating the starter solenoid.
The starter solenoid will pull 40-45 Amps for a split second but the 20 Amp or even a 15 Amp can easily take that.
http://www.thisoldtractor.com/guzzi007/schematics/1980_T3_Europe.gif

Plus 1 for the Bosch relay Diploman suggested, they look the part on an older bike and no need for a base.

[injundave]

Instead of running the 12 Volts through the ignition switch as it shows run it direct from the battery through an in-line 20 Amp fuse to the relay contact.
Run the wire from fuse 2 to the neutral switch so it goes switched fuse 2 to neutral switch to start relay, this way the starter will not operate if the key is turned off but the full battery Voltage is available for operating the starter solenoid.
The starter solenoid will pull 40-45 Amps for a split second but the 20 Amp or even a 15 Amp can easily take that.
http://www.thisoldtractor.com/guzzi007/schematics/1980_T3_Europe.gif

Just to clarify, Roy, are you saying to disconnect the wire from fuse 2 to the relay contact and replace that with the wire from the battery positive through the fuse, then take the wire from fuse 2 to the neutral switch then back to the same relay contact as the fused wire from the battery?

[Kiwi_Roy]

At the moment Fuse 2 is supplying the contact and also the coil via the neutral switch
This is a weak supply, too much resistance hence the ignition switch melted if not that it will certainly be prone to Startus Interruptus
The relay coil only draws about 1/4 Amp not a problem for the ignition switch.
 
The solenoids have 2 coils, the Guzzi factory don't seem to know that because they never show the second coil on their schematics
The one they show draws 10 Amps, the one they don't show will draw 40 - 45 if its wired properly, at best wired through the switch you might get 25 and it's working half heartedly, but its fried your switch.
This high current only lasts until the solenoid has pulled the gear into place and closed the contact.

To answer your question directly
The new fuse feeds the relay contact, fuse 2 feeds just the relay coil via the neutral switch.
This is something Guzzi have done every now and then on some models for example 2000 era spine frames and early Grisos
I took the time to open up a Bosch solenoid to show the two coils, The Valeo is similar.


This is a Valeo on a 2001 VII Sport but the Bosch solenoid is almost the same

Take no notice of how the relay coil is wired keep that as per the T3 diagram.

[injundave]

Thanks to all who answered this question and special thanks to Kiwi Roy. I now understand what I need to do.

[injundave]

I bought a new Hella relay, an inline fuse and rewired as per Kiwi Roy's instructions today. It was a very quick and easy job and works fine. Now to fit a new switchblock.

[Kiwi_Roy]

Im not sure I picked the right diagram
http://www.thisoldtractor.com/guzzi007/schematics/1974_850T_USA.gif
Most Guzzi ignition switches have a separate contact for the park lights, since nobody uses them you could combine the Brown and the Green wire at the fuses or simply move the Brown wire at the switch.
Now you have taken most of the load off the switch it should last.

Actually the T diagram doesn't show the park position properly, the contact 16 is missing on Pos 0 (Park), it would be closed.

[Chuck in Indiana]

Gotta love Guzzi schematics..