mz rt 125 blowing fuse

[kev]

Hi all
my friend has an rt 125 on a 03 plate. everytime u push the electric start button, it blows a fuse so wont start. i cant tell u what fuse it is at this point but will have more info soon.
Is this a problem anyone's come across before? my first thought were the reg rec is knackered.

[Trogolodyte]

If its the starter motor curcuit thats blowing fuses, then it will be the main fuse thats blowing, he will need to check the main earth, the thick cable that leads from the negative side of the battery to a bolt into the frame, make sure it is making a good connection, and then follow the starter cable (thick red cable) from the positive side of the battery, up to the starter solanoid (an electrical switch ), then from there follow the thickest cable to the starter motor, and then from there to the starter motor earth (should bolt into either the frame or engine somewhere near the starter motor), all the time make sure that all connections are good and current is getting through.

[kev]

Since i first posted a couple of hours ago, i now own the bike, my friend couldnt be bothered to mess about with electrics and offered it too me at a good price, so hi im now an mz owner. its only covered 3000 miles and most parts on the bike look like new. connected starter motor straight to battery and turns over fine. first thing i need to know is what this little grey box is and what wire i am missing,


at the moment when starter button is pushed it makes a nice clicking noise,

is there an online wiring diagram for this model?

kev

[kev]

just noticed theres not even a main earth on the bike , theres a thick earth straight to the starter and two thin brown ones going to the loom, now off to make an earth up

[kev]

should there definitely be an earth from battery to frame? i only ask as i cant see a previous mounting point for it

NOTE the big black wire from battery minus is the main ground it goes to starter frame
there should be a 2small brown wires also there at the battery unless the wire harness is different than mine
they enters the harness near the negative battery post
dave MOD

[DAVID THOMPSON]

this link will explane the relay and i will say it and bad connections are a big problem
dave
there may be a short to ground in the handle bar switch

viewtopic.php?f=4&t=2085&hilit=relayrelay

this is a nice bike and well worth fixing
mine is at 19,000 miles and running fine
it will make a nice daily driver
dave
any more problems ask

[Trogolodyte]

The part shown is a starter solanoid, its an electric switch it uses a small current (the two small wires) to open a switch that allows the BIG current (all 12-14 volts at up to 14 amp/hours) to go straight from the battery (the Big thick cables) to your starter motor.

[DAVID THOMPSON]

its relay 19 it keeps the starter from running if the side stand is down
it mounted on frame near left cornor of battery on mine
motor start selonid is under front of seat just to right of gas tank mount
pictures this week end asap
dave

[Trogolodyte]

Most bikes use what is called a negative earth system, that is the battery will be connected At the negative side to Earth (the frame or engine) with (usually) either a thick black or brown cable, everything else starter lights, horn etc then comes off the positive side is fed thru wires to a fuse box, then to the switches on handle bars and fairing and then off to the relevent components (again lights, horn etc) then rather than run all those wire's back to the battery they go to a local earth (the frame or engine) which is then connected back to the battery at the main earthing point where the cable from the negative side of the battery is conected, thus making the circuit.

The starter circuit is really simple, you have two circuits, first is the controll circuit, that starts with a positive feed from battery to the starter button - to the solanoid, solanoid to earth. What happens when you press the starter button is you activate the electro magnet that moves the big switch and allows aVery big current to go through the second circuit from the battery to the starter motor to the solanoid switch solanoid to the starter motorthen to the nearest local earth ( the click you hear when you press the starter button is the solanoid switch activating) because you can hear it working, you just need to check that you have continuity thru the switch when you have the button pressed and that it cuts out when you release the button, use a multi meter or basic circuit tester to do this. Then follow the output cable from the solanoid to the starter motor making sure that all connections are good and you have continuity through the cable. if everything else is good then you may have a frazzled starter motor.

[kev]

it has 5 connections actually, maybe its an aftermarket part? ill soon add better pics of this showing what colours go to what connection. ive cleaned up the terminals to no avail and tomorrow i will borrow a meter so i can test said components. starter motor is working fine when connected direct to +v.

[Trogolodyte]

does it have any burn marks like its gotten VERY hot...?

[kev]

first up is a (bad) pic showing the starter motor area, there is one earth wire going to the starter, and this is the only thick earth coming from the battery. i noticed slightly to the right of it is another threaded hole with nothing in it or going to it. does all this sound ok so far?


next up is the relay


as u can see, it still has a free connection, these may even be in the wrong order as i dont really know what my friend had tried with it. it looks ok and dosnt smell like its got too hot at all.
I would love to get this running this weekend and i really do appreciate your help so far guys

[DAVID THOMPSON]

it is the relay 19 as fitted original and
a link above shows how it is wired and replaced (repeated here)
viewtopic.php?f=4&t=2085&hilit=relayrelay

the green wire works with the side stand switch and neutral switch
to make relay close
when the key is turned on red black comes on with power if kill button side stand sw and netural sw are proper
then red white is +12volts also

if the fuse blows when the start button is pressed that's a different circuit than this relay is part of..

the start relay is on right front side under the seat seat near tank mount
it has 2 wires going into it that are small a brown which is ground and the other looks to be purple on mine that's +12v
and has power when the start button is pressed
disconnect the plug connection
if both measure as if there both connected to ground then there is a short in the start button wires
press start button with key on while measuring for voltage on these 2 wires

if you get power then the start relay is probably shorted inside its case
after a look at the wiring drawing item 32 is a diode in the harness some where if it shorted it could be the problem
i am thinking its in the push switch at the handle bar but also diode could be it
dave

[kev]

going on the way im reading that diagram and the diagram on side of relay, id say its current wired up wrong, terminal 4 should be the terminal without the wire going to it, not 3. im going to try and pick up a new relay today and failing that i expect there is a away i bypass the relay for test reasons by touching the wires together. the handle bar switch dosnt look the best either so i will test that later and if needs be, i will remove those wires and bridge the wires there too. hope to solve this soon then i can start on the fun mechanical bits.

[kev]

heres the readings i have from relay+ battery

battery 11.54v

relay- red/black wires 11.30v push start button slight drop in v
blank terminal 0.07--push start--11.30
green 3.00v push start 3.5v (stand is down)
black/white 9.15v push button 0.01v however if remove this wire and test on relay terminal i get 11.30.