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Faulty ecu/cdi unit fried by faulty alarm?

Guest asqn34

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Hello there,

I'am having a FI light ON, due to a faulty ISC valve (Again :x ) on my DR125sm 2009. Here is the background story.

Earlier this year my FI light came ON. The bike would start and run if I kept the rev up, otherwise it will stop. Would start again with difficulty. I made the dealer mode switch and checked the code. It gave me C40, so the ISC valve. I checked it according to the repair manual and found nothing wrong with it. As I had electrical problem with it 2years ago (battery completely flat and beyond acceptable level of charge and stator on the way out) I suspected a bad connection within the wiring loom so I bought a second hand harness, checked all connections and fitted it. Same C40 error code. According to the repair manual the next "thing" was a faulty ECU. I bought a second hand one, fitted it and no more C40 code. Fitted new stator as well while at it and bought a second hand fuel injector just in case.

Few happy months riding done the line :D , so last week end I decided to put back the original harness which has a datatool alarm fitted by dealer when I bought the bike, thinking my ECU was the culpright for my bike troubles.

Well... that did go well!

First the battery optimiser was giving me a reading of 17.9V and the ECU a bloody C40 error code again. Did not took me long to swith it off and unplug it!

Harness swapping again but no luck with both ECU. Changed the ISC valve too but still C40 error. The good thing is the battery optimiser not going to 17.9V lol now with no-alarma-harness fitted.

So how possible is it the alarm fried the ecu, and could the alarm have giving me all the electrical problems I had with the bike?

Any help really appreciated here as I'am at lost.

Many thanks

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Sorry to hear of your problems..... :(

I guess it's possible that the alarm is stuffing the ECU.....not something I've heard of before though..... :? Do you have an alarm bypass plug?......some alarms have a plug that you can put in place of the alarm if you don't want it in circuit anymore..... :)

Sounds like you need to replace the ECU again.....and try it again...... :wink:

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Hi Tango, thank for the reply.

Yes the alarm has a bypass plug but I fitted the second hand wiring loom after frying the 2nd ecu. At least no risk for the 3rd one that I will need to buy. I send a e-mail to datatool today explaining what had happen even so I am not awaiting anything from them as the alarm is past the 3 years warranty. However I felt good expressing my frustration with their product.

Will post their answer...if I get one.

And the solution to my problems when it is sorted :!:


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  • 1 month later...

Got a reply from datatool, very promptly I must say. Just me taking time to post it on the forum. :roll:


Hi Christophe,

Thanks for the additional information and for clarifying that. The Datatool security system does not output any voltage to the bike other than the voltage that is input to the unit initially, for indicator, immobilisation connections etc. Therefore the additional voltage you have observed is unlikely to have been generated via the Datatool ECU, instead either trickle charger or the bike battery / alternator itself, and the over-charging/voltage here would indeed possibly take down any 12v rated product on the bike, such as the OEM ECU, the Datatool ECU etc.

I suggest the best course of action would be to let an auto electrician have a look at the electrics on the bike to ascertain the exact cause of the problems, as just to reiterate the Datatool ECU does not output any voltage other than the regulated supplies that are already present to the unit.


Technical Team

Mike Nolan BA (Hons) PSM

Technical Support


The thing is I have now a brand new ECU and the bike is working fine and the battery get a weekly trickle charge :wink:

So I firmly believe the datatool fried my ECU and it is now a no go whatsoever on my bike.

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I dunno mate.....as they say, if your reg/rec went and were getting nearly 18v at the battery, it could well have fried some of the electronics....which in turn could take out other stuff..... :shock:

I hope that the bike is now working......so put it down to experience..... :wink:

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Don't quite know where to start on this...

I'm reading it that you get ~18V at the battery when on charge on the datatool harness but not on the other one. If you have a quality charger e.g. optimate then it is perfectly normal for the voltage to go as high as 20V during the varification stage. You might want to read the instructions for your charger if this is the case...

Usually it only goes this high for long periods when de-sulphating the battery or if it's dead flat. If your alarm is shagged or if there is a high resistance section in your loom e.g. corrosion then this can cause the charger to enter the aforementioned mode. Many chargers recommend disconnecting alarms/immoblisers when charging anyhow.

Long story short, get a multimeter and check the whole loom section by section then do a draw test on the alarm system to see what current it's taking.

Also, why do you charge it every week? Unless you're average journey is 10 feet you really shouldn't need it. Definitely check the condition of your battery and do a load test.



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  • 2 weeks later...

Thank for the reply.

I did a weekly trickle charge for few weeks because the bike is immobilized in the garage until I fix the ratling from cam chain/valves clearance and also to make sure it was not the battery optimiser being faulty. I have noticed a bad electric wire on the optimiser which is fixed now. Could not say if it was there when the surge happen, but I did not happen after swapping harness anyway and it was not fixed, so that rule out the optimiser. There is a 3amps fuse from the optimiser so I though it will blow before doing any damage to the bike?

The battery is 6 months old and in good condition/working order. It this just weird they dont last long.

Before I fitted the spare loom (the no alarm one) I have tested a alls connections as per the electric diagram.

I did the alarm draw test when I fitted the new battery and everything was with datatool spec.

I would not bother putting the alarm back on as I want to see how long my battery last without it.


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