Possible ECU or throttle solenoid problem

On today’s ride on by Bonneville Speedmaster I had the cruise control stop working, then I noticed the engine was running a bit rough which gradually deteriorated. Thinking now, the rough running was accompanied by a laggy throttle where it seemed to lose power, and upon giving it more throttle it ‘overshot’ and I had trouble maintaining steady running. I found a convenient place to pull over and the engine died once stopped at idle. Upon trying to restart, the starter would crank over but engine wouldn’t start.

There were no fault lights on the instrument panel and no error codes shown after checking with TuneECU and the OBD2 reader I carry. But that did show the battery voltage was only just over 6 Volts!

To cut a long story a little shorter, the battery was good, reading 13V, and the AA roadside assistance I had called removed my battery and connected in their own which also wouldn’t start the bike. Thing is, with ignition on, the instrument panel works, the fuel pump ran, and the full beam headlight would work fine. So seems a lot of the electrics have a good power supply but the bikes computer reckoned battery was in the 5-6V range.

Once bike and I were brought home by the AA I did a little experimenting and things seemed the same as before. Then the throttle solenoid had a seizure banging against the end stops and has now thrown an error code and stopped working. Fuel pump also doesn’t come on. Voltage according to CAN bus has now dropped even further to 5V, so that could be the cause of the above stopping working.

I’m hoping that there is not a fault with the ECU, just some corroded or broken connection. Another possibility is something like the throttle solenoid failing and causing a partial short, bringing down the voltage.

As my ‘workshop’ is a conservatory which is too hot from a day of sun I am going to wait until tomorrow before starting to dismantle bike for access to wiring. I will try disconnecting things like the throttle solenoid, and wiggling things to see if I can get the reported voltage to change, so giving me a clue to the problem area.

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Good luck finding the issue soon, @BrownMouse , so your continental trip is not jeapardized… Fingers crossed!


How are you accessing the data from the ecu or the ecu voltage read outs?

TuneECU on my phone with a Bluetooth OBD2 dongle. Used this for a few years for various servicing and diagnostic jobs.

Edited to add…

The app that came with the dongle says battery voltage is 12.6V, perhaps they are getting voltage from different devices? I’m 99% sure that TuneECU has displayed sensible voltages before and I’ve not updated the app since installing it.

I managed to measure the voltage to the ECM/ECU as that also goes to easily accessible ignition coils, and voltage is good. Damn, was hoping for a corroded relay on that supply.

Perhaps TuneECU’s ‘battery voltage’ is red herring.

I would get a volt meter and check the reg/reg and stator to make sure they are working properly. Also, check the earths on the bike.

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You probably thought of this, do you know anyone with the same bike?

No, Don’t know anyone with same bike. (And TuneECU is licenced to a specific ECU serial number so couldn’t be used on bike.).

Regulator and stator shouldn’t come into this problem as fault occurs without trying to start engine, and battery level is good.

Yes, dodgy, earth could be involved somewhere, one of the first things I did was clean and re-tightened the main earth connection which joins the batter, wiring loom and engine. Any other dodgy earth connection would be inside the loom and associated connectors.

Does the bike have an alarm/immobiliser?

Yes, has an immobiliser. There’s no immobiliser enabled light on dash though.

As well as the anomalous voltage reading from TuneECU, the error codes for throttle servo motor are persisting.

After lunch I am going to set about removing airbox and manually push butterfly valves in case they got jammed when the device had it ‘seizures’. I’m hoping it isn’t really the throttle servo because the throttles come a single unit costing £1861 to buy!

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Have you managed to get to the ecu and checked the plug(s) into that? Could just be a bad connection there causing can bus problems?
I’m no electronics engineer so sorry not much help, but I believe can bus operates at around 2.5v

That’s what I’ve been trying to do this afternoon. Unfortunately, rather being under the seat like a lot of bikes the ECU is down at the bottom back of the engine requiring swingarm removal, which requires suspension disconnection, which requires airbox removal.

I’m now stuck getting the swingarm out because where the spindle bolt goes through is a ‘frame adjuster sleeve’ requiring a precisely sized tool. I had cobbled one together last time I had to do this, but that got all bent and twisted, so have just splashed out £45 to get the proper tool delivered tomorrow. (As I’m on a tight deadline I didn’t want to risk struggling tomorrow to muddle through only to have to order tool anyway and so have wasted another day.)


Good luck, hope you get it sorted in time!


Go to dealer and have a chat to a mech.

From your description it sound like the ecu is in a vunerable position, so fingers crossed for you that it’s just a dodgy connector that can resolve the problem. :crossed_fingers: :crossed_fingers:

What a sensible piece of design. :astonished: :grimacing:

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I feel for you mate. I hope you get sorted for you trip :grinning:


So, I got access to ECU…

Despite the state, the connectors were immaculate with no signs of corrosion or muck, thanks to the pins sitting below a 15mm shroud and the connector having an o-ring. All power and ground lines read as effectively zero ohms resistance to where the should connect, as did the lines to the throttle actuators.

If the power line is good and I measured it previously at 12-and -bit volts the it sure looks like the ECU is getting good power, so it reporting a voltage of around 5-6 volts seems to highly suggest an ECU problem.

Someone on the TuneECU forum reckoned that the voltage TuneECU shows is coming ECU and not somewhere else, though pointed out circuits measure voltages in reference to another one to my mind doesn’t completely rule out some circuit fault outside ECU.

Tomorrow will try and find out if I can find any faults. May also speak to local bike mechanic and see if he has the tech to program a new ECU for my bike should I decide to roll the dice on a very expensive gamble to get bike working in a week. (ECU needs to be programmed to work with my bike’s immobiliser, and I could possibly wave enough money under his nose to bring his laptop here and do the job should I fit a new ECU, if I could get one in time.)

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It’s beginning more and more to sound as an elusive issue - hoping you get to the bottom of it soon…

Did some more experiments today, trying to triple check battery was good and wiggling cables and connectors to see if the ECU report of only getting 5ish volts changed, it didn’t. Even scraped off a tiny bit of insulation on wires next to ECU connector to measure voltage and confirms it was getting more than 12V. (I couldn’t see how it wouldn’t as I’d already measured the near zero resistance on wires back to battery.)

So, I’ve decided to go the route of getting a new ECU, hopefully tomorrow. As I said before, this will need someone to program it for my bike to tell it my VIN and pair with the immobiliser.

I did find the app for my OBD2 dongle can show the dodgy voltage, its under the heading 'Control Module Voltage '. This means that I can immediately see if things have changed when I plug new ECU in. I can’t use TuneECU because it’s licence is locked to VIN number, or possibly even the specific ECU?

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