Ducati Monster Forum

So I've got my bike (somewhat modded 695) going with a new custom loom. There's one loom screwup, where I transposed the side stand and clutch connectors, which is easy to rectify. All the gauges and controls work nicely. The engine runs nicely (like a demon), I've got gobs of power and (thanks to the DP ecu) it doesn't appear to be running lean any more. I've connected ducati diag to it, and thus far all looks extremely happy. All the displayed values (battery volts, air temp, pressure, oil temp, tachometer, etc) look sane, and even the TPS came up spot on.

Except.

My check engine light stays on. I'm perplexed. I can't find anything in ducati diag to turn it off, or that's telling me its on, for that matter. I'm getting pretty close to disabling the light with a pair of sidecutters.

What exactly does the check engine light tell me anyway? I mean if one of the sensors (crank, tps, air) wasn't reading right, it'd be pretty obvious. Similarly if an injector or coil isn't firing, that's pretty obvious too.

If you get no trouble codes (assuming Ducati Diag works as claimed) I would think back feed through ground from another circuit. 

Learning to drive ducatidiag helped.

Turns out the ecu is too smart for its own good. It was upset because the fuel pump relay isn't drawing any current. Given that I don't have a fuel pump relay, that's reasonable. A 470 ohm resistor between the fuel pump relay output and +12v shut it up :)

What??

An engine management fault suzyj doesn't know how to fix? 

Really??

Oh wait....
Nope.

Not really  [thumbsup].

Great detective work!

Why did you decide to not run a pump relay?

I don't have any relays. I do the fuel pump drive and headlight with power MOSFETs. The circuit driving them has relatively high impedance (compared to relay coils, anyway).

Can you use one of those for the starter also?

Keep in mind I only know enough about electricity to incompetently free the smoke. ;D

I toyed with doing a MOSFET starter contactor. Indeed I have a design for one using four IRF3006 power MOSFETs in parallel. thats oodles to supply the 120 odd A that the starter draws. What stopped me from doing it was more not really knowing what the back-emf from the starter would do when it switched off.

I am buffering the input to the standard contactor tho, which has completely cured my original electrical issue of occasionally not starting.

Does the back-emf result from the starter acting like a generator when spinning on over-run?

It's actually from the inductance in the starter motor windings. When you suddenly try to interrupt the current, the winding inductance generates a large negative voltage to try to keep current flowing. On a mechanical contactor it causes arcing across the contacts. On a transistor switch the large negative voltage can cause destructive breakdown and let the smoke out.

Now the smoke is coming from my ears.  :P

Can you do the same diode bridge across for the back EMF in your set up?