How do you know your ECM has a problem?
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From: Virginia
How do you know your ECM has a problem?
My 2000 Ultra Classic, which has the Magneti Mareli throttle body and ECM that nobody likes, has some electrical problems that only happen when the engine is running. For example, the radio cuts out or makes a fart noise every 5-10 seconds when the engine is running but works great with engine off (regardless of whether key is on IGN or ACC). Also, when the engine is running the Tach drops 1,000 rpms or so in sync with the radio cutting out but I do not have a way to test the tach with the engine off. Maybe, I am jumping to conclusions here but I would like to rule out the ECM. I believe the ECM shares a ground with the radio (not sure because radio is not on the service manual wiring diagram) and the ECM also has a signal wire to the tach (pink wire, which the wiring diagram labels "not used").
So, what are the symptoms of a bad ECM? I ask because my bike runs pretty strong through the gears and the rpms aren't actually dropping when the tach needle drops. The bike does not seem to hesitate or lunge forward like you would expect if the rpms actually dropped by a 1,000. Can the bike still run strong through the gears with a bad ECM? I always assumed you knew you had a bad ECM because if you did the bike would not run. Has anyone actually had a bad ECM?
So, what are the symptoms of a bad ECM? I ask because my bike runs pretty strong through the gears and the rpms aren't actually dropping when the tach needle drops. The bike does not seem to hesitate or lunge forward like you would expect if the rpms actually dropped by a 1,000. Can the bike still run strong through the gears with a bad ECM? I always assumed you knew you had a bad ECM because if you did the bike would not run. Has anyone actually had a bad ECM?
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From: Honah Lee
There are some very good YouTube on the system that walks you thru it.
Find if tac is OEM where it's pick up is and make sure power and ground wires are good . That last wire is the light.
Your processor I think only controls the engine.
It's simple once you understand it. Parts may be the real issue. Doesn't sound like anything major however.
My oem tac on my 04 when cold once it hit 2500 would jump a 1000. It was the tac itself's little stepper motor that spins the needle.
Find if tac is OEM where it's pick up is and make sure power and ground wires are good . That last wire is the light.
Your processor I think only controls the engine.
It's simple once you understand it. Parts may be the real issue. Doesn't sound like anything major however.
My oem tac on my 04 when cold once it hit 2500 would jump a 1000. It was the tac itself's little stepper motor that spins the needle.
Last edited by Jackie Paper; Jan 12, 2026 at 06:34 PM.
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No history of ownership, unknown if any changes from stock and unkown mileage.
From a distance it does not read like an ECM problem.
I would review battery terminals for tightness.
Review all 4 four ends of battery cables.
I would also review wire loom under rear tire since lowered rear end can cause wire loom to get scrubbed by tire and cause damage.
I would also review brake light function with front and rear brake.
I would also review ground wires within fairing near radio since a slight ground wire break can cause weird stuff to happen.
I would also check if problem is at wire loom by front neck by turning bars full right and full left during testing...bars are ussually turned left when parked so problem might only happen when turned straight.
I will take a wild guess that you have already tested battery voltage during cranking and high idle and charging system is ok..done with a digital meter at battery terminals.
From a distance it does not read like an ECM problem.
I would review battery terminals for tightness.
Review all 4 four ends of battery cables.
I would also review wire loom under rear tire since lowered rear end can cause wire loom to get scrubbed by tire and cause damage.
I would also review brake light function with front and rear brake.
I would also review ground wires within fairing near radio since a slight ground wire break can cause weird stuff to happen.
I would also check if problem is at wire loom by front neck by turning bars full right and full left during testing...bars are ussually turned left when parked so problem might only happen when turned straight.
I will take a wild guess that you have already tested battery voltage during cranking and high idle and charging system is ok..done with a digital meter at battery terminals.
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From: Honah Lee
.. ...
.. Always Think First or just grab the pop corn Smiley. >> 
<< for your post point. ...
Last edited by Jackie Paper; Jan 12, 2026 at 06:33 PM.
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From: poway
It definitely sounds like the ECM is ordering the radio to make fart noises.. When the Radio simply stops at times. it's because its all out of farts.. The ECU is bad and needs to be removed and set facing a corner for at least 2 hours..
If that doesn't work I'd check power and grounds to fairing.. Tug the wires. If they stretch, they are broken inside..
If that doesn't work I'd check power and grounds to fairing.. Tug the wires. If they stretch, they are broken inside..
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From: Virginia
I went back to the manual, which recommended that I clean the ground for the voltage regulator. After I did that I retested everything. Now every time the tach drops and the radio goes static I get a voltage spike, up to 15.1 V at idle at the tach and battery terminals. So, I think I have voltage regulator problem. Not sure whether cleaning the grounds helped get a more accurate reading or whether I just botched the first reading, but now I think I know what the problem is. Thanks for all the suggestions.
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