here is a wiring diagram of all wiring side components .
Aprilia RS 125 parts in the USA
so on the electrical side you have ( click on each to see tests posts all will be done within next week )
battery , key- ignition switch , start button , kill switch , stand switch , starter relay , starter motor .
Bendix , bendix cover , starter ring ( flywheel ) .
So if i came to a bike that would not start or turn over No crank issue .
firstly id charge the battery or use my jump starter kit , watch this video to see how the starter bendix fails to
engage when the battery is not charged enough .
Aprilia RS 125 blog flat battery jump start battery boost pack ( portable , small )
Next try to start does it click or try to turn but stop or clunk this can give you a good idea of problem but for this post ill just go step by step according to our list above .
Low amp side ( control side )
the low amp or control side is used via relay so that you can control and major or high amp circuit with a small switch or push button at the handlebars , the low amp control circuit close the contacts of the relay to
operate or touch the 2 large starter motor circuit wires together .
I would jump straight to starter relay turn key on and test at relay for 12v to earth on one of the small connectors ( yellow/red) if you get positive here this verifies the Key - Starter button - test the other small connector now ( green / yellow ) this should have continuity to earth .
here i show tests with my 12v light tester
First plug the connector end to negative on battery and the other probe end into the Yellow / red wire one of the small wires going to starter relay ( key / starter button side ) .
when you press button the test light should light up to show a good path through the key to the starter button and then onto the starter relay .
connect scope on a rope (test light ) connector to negative terminal
connect probe end of test light to yellow and red wire
( 1 of small connector at relay )
operate button and it should light .
This tells me that the Key and the starter button work if it did not light i would check fuses although this would of been first on the list , test key and starter button and then look at any connectors inbetween the key and starter relay .
Next i connected to positive terminal of battery as this side of relay should be going to earth .
and at first it did not light i moved the stand up and down and nothing
so i disconnected the stand switch and eventually go it to light up .
( So my problem here was stand switch not earth the
low amp circuit side of starter relay )
( though it was simple now half way through 2 hours in and im still rambling on )
High amp direct battery to starter motor side
OK so looking at the wiring schematic for the stater system the 12v high amp starter motor circuit goes from
an earth battery down via the positive ( red ) high amp wire to one of the large relay posts ( red wires )
One of these larger red wire should be permanently live so connect test light ( scope on a rope ) to the negative terminal of the battery and test both wire the one that lights up is the permanent live feed direct
from battery if non light up you have wiring problems from battery to replay post ( or an earth problem )
Now set your test light on the opposite side cable - large post red cable this is the wire going direct to
the starter motor , press the relay button and you test lamp should light up meaning that relay is in fact
working and closing the internal high amp contacts , it might be good to read this post of testing relays
now so you understand how it should operate .
Now if the test light lights you can move on to the starter motor now , if it does not light then
we know the control circuit is ok so it must be relay failure ( replace this and move on to starter motor )
Testing at starter motor , so we now know that the control side of relay is ok and the high amp direct
power supply to the relay is ok and that its passing through the relay ok too .
So testing at the starter motor now begins , test for 12v at starter motor terminal with button pressed for starter operation if not 12v is found then check wiring from starter relay too the starter motor and dismantle the starter connection and rebuild cleaning it .
If you do have 12v here its time to do a volt drop test watch this excellent video to understand voltage drop and how its important regards starter motor testing
to be continued
further to mention , relay test - voltage drop test ( damaged high resistance cable ) - test starter motor
mechanical ie bendix - worn stater ring , engine siezed , high compression .