1993 Ducati 900SS Fuel & Ignition Fuse blowing.

West Cork Paul

West Cork Paul

Man in a Shed
Subscriber
I had a weird one today, perhaps someone can explain to me why someone would have done this.

1993 900SS kept blowing the 15A ignition & fuel pump fuse as soon as the start button was pressed. Various attempts to trace the issue of the massive current draw resulted in numerous blown fuses 😕

IMG_3430.webp


Eventually I discovered that both wires at the starter solenoid had continuity to ground. Obviously one should and the other should only be live when the start button is pressed. I stripped back the insulation to discover what appears to be a resistor (or maybe it’s a diode 🤷🏻‍♂️) had been connected between the 2 wires.

IMG_3428.webp


IMG_3429.webp


Once I’d removed it all was fine again. The fuse doesn’t blow anymore and the starter solenoid does its job when the start button is pressed.

What on earth was it doing there?
 
It's a diode fitted across the solenoid terminals to drain back EMF spikes to ground so that they don't damage any electronics on the bike. On the 900SS that would be the ignition modules. They are also fitted to the later bikes to protect the ECU, instruments, etc. Sounds like that one had gone short circuit internally.
 
West Cork Paul said:
I had a weird one today, perhaps someone can explain to me why someone would have done this.

1993 900SS kept blowing the 15A ignition & fuel pump fuse as soon as the start button was pressed. Various attempts to trace the issue of the massive current draw resulted in numerous blown fuses 😕

View attachment 156233

Eventually I discovered that both wires at the starter solenoid had continuity to ground. Obviously one should and the other should only be live when the start button is pressed. I stripped back the insulation to discover what appears to be a resistor (or maybe it’s a diode 🤷🏻‍♂️) had been connected between the 2 wires.

View attachment 156234

View attachment 156235

Once I’d removed it all was fine again. The fuse doesn’t blow anymore and the starter solenoid does its job when the start button is pressed.

What on earth was it doing there?
Click to expand...
Was it a bodge to correct a fault that lies somewhere else, or which used to but which was fixed at some point?
 
Derek said:
It's a diode fitted across the solenoid terminals to drain back EMF spikes to ground so that they don't damage any electronics on the bike. On the 900SS that would be the ignition modules. They are also fitted to the later bikes to protect the ECU, instruments, etc. Sounds like that one had gone short circuit internally.
Click to expand...

Thanks Derek 🙏🏼. It didn’t look like it was fitted by the factory though, or was it?
 
Loz said:
Was it a bodge to correct a fault that lies somewhere else, or which used to but which was fixed at some point?
Click to expand...

That I don’t know. The bike was running, allegedly, then wasn’t.

I’ve a 900 Monster which is identical with regards to its electrickery/starting/ignition circuits and it doesn’t have such a diode.
 
Derek said:
These were generally factory fitted to the 900ss. I'm surprised they weren't fitted to the Monster.
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That’s interesting to know, thank you. Maybe the one on the monster has already been removed, I’ll look closer at the connector. I know the diode isn’t present as I had to fit a new starter solenoid a couple of years back and I’m sure I would have noticed the bulge of the diode on the +ve feed wire.

I only discovered this as the short as I stripped back the loom in various places where owners over the years had wrapped insulating tape and effected repairs.

I’m curious to understand though where a possible EMF strike may come from from and why the protection would be placed there and not directly at the ignition modules or coils if that’s what’s intended to be protected.
 
West Cork Paul said:
I’m curious to understand though where a possible EMF strike may come from from and why the protection would be placed there and not directly at the ignition modules or coils if that’s what’s intended to be protected.
Click to expand...
Probably a solution to a problem from the factory either as recall or bodge in production line - 1993 (y)
 
There isn't one showing on the factory wiring diagram, but I wouldn't say that's conclusive from back then. One other possiblity is that at some point the bike may have had an alarm? The very early Datatool used to require a diode being fitted as part of the starter immobaliser wiring?
 
Yeah, I didn’t see it in the wiring diagram either, which, combined with the heat shrink, which doesn’t seem to be anywhere else on the bike, made me think it was a later addition.

@Hugo Magnus have you just copied an AI (not Alan) response up above? I’m still none the wiser. The only magnetic field collapsing would be within the coils, why not fit a diode on the low voltage side of the coil just in case it backfires and doesn’t dissipate via the spark plug.

Why fit it at the starter solenoid trigger connector where, if/when it fails, it just shorts direct to the battery. Imagine if the infinite voltage in your example were to just go direct to the battery, surely that would possibly blow the battery open?
 
West Cork Paul said:
Yeah, I didn’t see it in the wiring diagram either, which, combined with the heat shrink, which doesn’t seem to be anywhere else on the bike, made me think it was a later addition.

@Hugo Magnus have you just copied an AI (not Alan) response up above? I’m still none the wiser. The only magnetic field collapsing would be within the coils, why not fit a diode on the low voltage side of the coil just in case it backfires and doesn’t dissipate via the spark plug.

Why fit it at the starter solenoid trigger connector where, if/when it fails, it just shorts direct to the battery. Imagine if the infinite voltage in your example were to just go direct to the battery, surely that would possibly blow the battery open?
Click to expand...
It's only high voltage, very low current I can't see it harming the battery.
 
West Cork Paul said:
Yeah, I didn’t see it in the wiring diagram either, which, combined with the heat shrink, which doesn’t seem to be anywhere else on the bike, made me think it was a later addition.

@Hugo Magnus have you just copied an AI (not Alan) response up above? I’m still none the wiser. The only magnetic field collapsing would be within the coils, why not fit a diode on the low voltage side of the coil just in case it backfires and doesn’t dissipate via the spark plug.

Why fit it at the starter solenoid trigger connector where, if/when it fails, it just shorts direct to the battery. Imagine if the infinite voltage in your example were to just go direct to the battery, surely that would possibly blow the battery open?
Click to expand...
The coil within the solenoid creates a magnetic field that pulls the armature in, bridging the 2 heavy duty terminals to energise the starter. When the starter button is released the magnetic field collapses creating a high voltage, low current spike. This is what can damage electronic components within the bike.
It has nothing to do with the ignition coils.
 
Derek said:
The coil within the solenoid creates a magnetic field that pulls the armature in, bridging the 2 heavy duty terminals to energise the starter. When the starter button is released the magnetic field collapses creating a high voltage, low current spike. This is what can damage electronic components within the bike.
It has nothing to do with the ignition coils.
Click to expand...
Aha, that makes sense, thank you 🙏🏼
 
West Cork Paul said:
I had a weird one today, perhaps someone can explain to me why someone would have done this.

1993 900SS kept blowing the 15A ignition & fuel pump fuse as soon as the start button was pressed. Various attempts to trace the issue of the massive current draw resulted in numerous blown fuses 😕

View attachment 156233

Eventually I discovered that both wires at the starter solenoid had continuity to ground. Obviously one should and the other should only be live when the start button is pressed. I stripped back the insulation to discover what appears to be a resistor (or maybe it’s a diode 🤷🏻‍♂️) had been connected between the 2 wires.

View attachment 156234

View attachment 156235

Once I’d removed it all was fine again. The fuse doesn’t blow anymore and the starter solenoid does its job when the start button is pressed.

What on earth was it doing there?
Click to expand...
Hmmmm..... my 97 ss is doing just this. I've not got around to fault finding as i know pretty much diddly about electrics. Maybe its the same issue. How would one test this diode?
 
Nelly said:
There isn't one showing on the factory wiring diagram, but I wouldn't say that's conclusive from back then. One other possiblity is that at some point the bike may have had an alarm? The very early Datatool used to require a diode being fitted as part of the starter immobaliser wiring?
Click to expand...

Yep, according to a label on the bike it had a Datatool alarm at some stage in its life, perhaps the fitter installed it when wiring in the alarm.
 
West Cork Paul said:
Yep, according to a label on the bike it had a Datatool alarm at some stage in its life, perhaps the fitter installed it when wiring in the alarm.
Click to expand...
That may be it then. The later bikes that use the ECU to switch the solenoid use a diode to prevent spiking. I think that’s why the alarm needed one.
The diode might have failed and is shorting the switch circuit? Cut the diode leg to isolate the earth and switched feed and give it a try?
 
You need to explore why the diode failed. If it is just ‘old age’ then replacing it should cure your problems. If it was from a high voltage spike then you need to find out why. A possible cause is a large reverse spike from the coils. That is the reason for the diode being in place, it protects the likes of the ECU from being damaged by reverse voltage spikes. Hopefully the diode and the fuses have protected things. Do not run without one in place.
 
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