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Discussion Starter · #1 ·
Hello all,

I have a 2006 Mercury EFI 4-stroke with about 400 hours on it, and it's in great shape (when it runs). It has not been abused. It is manual pull start, no battery attached, tiller version.

Last year the motor quit on me and the Dealer diagnosed a bad ECU unit. I replaced the ECU and the motor ran great for 4 trips on the water. The motor quit again, and again the shop diagnosed a bad ECU unit and advised that I pursue a warranty claim (the part was less than a year old). I got a warranty claim filed, and replaced the ECU again, and guess what... 6 days of salmon fishing and the ECU failed AGAIN!.

The only thing failed in the ECU is it stopped sending power to the high pressure fuel pump. If I connect the fuel pump directly to a 12V battery with wires and gator clamps, the motor runs fine. So my question... what could possibly cause the ECU to burn out multiple times?

Maintenance performed last year: New ECU, new fuel injectors, new high pressure fuel pump, new low pressure fuel pump, new IAC valve, fluids and sparkplugs.
Dealer also performed a stator voltage diagnostic and found nothing unusual.

Thanks in advance.
 

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Did you check your fuses? I'm pretty sure the ECU has it's own fuse on my 60hp Mercury. If that's been it all along your mechanic has some explaining to do.

Sent from my SM-G781U using Tapatalk
 

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Discussion Starter · #3 ·
Did you check your fuses? I'm pretty sure the ECU has it's own fuse on my 60hp Mercury. If that's been it all along your mechanic has some explaining to do.

Sent from my SM-G781U using Tapatalk
There is no fuse on the pull-start model, just the electric start version.
 

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Classic inductive kickback symptoms: when your DC fuel pump turns off, a huge voltage spike occurs (any DC motor or solenoid does this: basic electronics: V=-L * di/dt). Probably exacerbated by a grounding/negative connection that is not quite right. Make sure your grounding is right (all grounds to a central ground bus bar) and put a diode across the fuel pump leads so the negative kickback does not keep blowing up the power transistor or relay that runs the pump.

Pretty close to 100% probability that inductive kickback is the culprit and the diode is the sure fix - but this is very likely a symptom of a grounding problem on your vessel that you should definitely find and fix.
 

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Discussion Starter · #5 ·
Classic inductive kickback symptoms: when your DC fuel pump turns off, a huge voltage spike occurs (any DC motor or solenoid does this: basic electronics: V=-L * di/dt). Probably exacerbated by a grounding/negative connection that is not quite right. Make sure your grounding is right (all grounds to a central ground bus bar) and put a diode across the fuel pump leads so the negative kickback does not keep blowing up the power transistor or relay that runs the pump.

Pretty close to 100% probability that inductive kickback is the culprit and the diode is the sure fix - but this is very likely a symptom of a grounding problem on your vessel that you should definitely find and fix.
Roger that. I'll start digging into it. Thank you.
 

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Classic inductive kickback symptoms: when your DC fuel pump turns off, a huge voltage spike occurs (any DC motor or solenoid does this: basic electronics: V=-L * di/dt). Probably exacerbated by a grounding/negative connection that is not quite right. Make sure your grounding is right (all grounds to a central ground bus bar) and put a diode across the fuel pump leads so the negative kickback does not keep blowing up the power transistor or relay that runs the pump.

Pretty close to 100% probability that inductive kickback is the culprit and the diode is the sure fix - but this is very likely a symptom of a grounding problem on your vessel that you should definitely find and fix.
Dang that's a great response! I don't know a lot about that electrical magic in these things, but I do know poor grounding can cause all kinds of weird stuff. What I don't understand is why the "Techs" at these shops don't don't tap resources that can solve problems like this before the customer has spent how much? How much is the bill for the ECU and labor? Twice!?
 

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Discussion Starter · #7 ·
Dang that's a great response! I don't know a lot about that electrical magic in these things, but I do know poor grounding can cause all kinds of weird stuff. What I don't understand is why the "Techs" at these shops don't don't tap resources that can solve problems like this before the customer has spent how much? How much is the bill for the ECU and labor? Twice!?
Yeah no kidding. The ECU is a bolt-on single plug install. $600 part and less than 5 min install and easy to do myself. The painful part was deciding between 6-8 month wait for a new motor (lead time at the moment of decision) and $5k and missing the 2021 salmon season or spending $1200 and fish this year. Chose the $1200 now, sell the Merc, then get a Honda for 2022. Didn’t expect it to blow another ECU so fast. Current situation is I ordered the new motor and I’m trying to decide what to do with the Merc. Sell for beans as-is or fix and sell for more.
 
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Yeah no kidding. The ECU is a bolt-on single plug install. $600 part and less than 5 min install and easy to do myself. The painful part was deciding between 6-8 month wait for a new motor (lead time at the moment of decision) and $5k and missing the 2021 salmon season or spending $1200 and fish this year. Chose the $1200 now, sell the Merc, then get a Honda for 2022. Didn’t expect it to blow another ECU so fast. Current situation is I ordered the new motor and I’m trying to decide what to do with the Merc. Sell for beans as-is or fix and sell for more.
Ain't technology great. I run a 20 YO Merc 75 2-stroke with about 800 hours. Still going strong. Stories like yours make me mighty glad. I'm close enough to quitting all together. Not sure if this motor blew if I'd hang something else on the boat.

Good luck. Hope for the best for you.
 

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What frosts me is that manufacturers are too cheap to include a 20 cent diode across the pump coil to shunt and damp inductive kick or a 5 cent cap across the electric trolling motor power input to reduce conducted and radiated RF interference. Most boaters or riggers don't have access to the necessary test equipment to detect and prevent these problems. Profit over quality. /end grumpy old engineer mode/
 

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What frosts me is that manufacturers are too cheap to include a 20 cent diode across the pump coil to shunt and damp inductive kick or a 5 cent cap across the electric trolling motor power input to reduce conducted and radiated RF interference. Most boaters or riggers don't have access to the necessary test equipment to detect and prevent these problems. Profit over quality. /end grumpy old engineer mode/
My 40hp merc does have a diode attached to the ecu, it's actually hidden inside the main harness, but is a serviceable item. The newer 40-50-60h efi motors all have this per my service manual.
it's common to fry these if the user hooks up the battery cable backwards and you will get a no start. It's a pretty expensive oops, 56$ for the diode with harness.
Now it does not have one in the negative trigger on the pump as you said per the wiring diagram.
 

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Wonder if the regulator is not regulating, as in when motor is running it's allowing more than 13.5 or 14v to go into ecu. The ECU will have a built in regulator, but it can only step down so much before overheating.
 

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Interested in the timeline of the parts replacements and why the fuel pump was changed the first time. Was it changed prior to the ECU failures, or in response to the ECU failures?


“Maintenance performed last year: New ECU, new fuel injectors, new high pressure fuel pump, new low pressure fuel pump, new IAC valve, fluids and sparkplugs.”
 

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Classic inductive kickback symptoms: when your DC fuel pump turns off, a huge voltage spike occurs (any DC motor or solenoid does this: basic electronics: V=-L * di/dt). Probably exacerbated by a grounding/negative connection that is not quite right. Make sure your grounding is right (all grounds to a central ground bus bar) and put a diode across the fuel pump leads so the negative kickback does not keep blowing up the power transistor or relay that runs the pump.

Pretty close to 100% probability that inductive kickback is the culprit and the diode is the sure fix - but this is very likely a symptom of a grounding problem on your vessel that you should definitely find and fix.
So....just to appease my curiousity or more likely my ignorance, are you referencing an internal grounding issue with the motor? OP stated no battery attached, so I was under the impression that the motor was isolated from the hull and had a floating ground? I guess it would be good to know whether the hull material is aluminum, wood, rubber, fiberglass or painted to help understand if there is even a path to the actual boat's ground to know if a grounding problem on the vessel is the culprit?
 

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Discussion Starter · #14 · (Edited)
Interested in the timeline of the parts replacements and why the fuel pump was changed the first time. Was it changed prior to the ECU failures, or in response to the ECU failures?


“Maintenance performed last year: New ECU, new fuel injectors, new high pressure fuel pump, new low pressure fuel pump, new IAC valve, fluids and sparkplugs.”
Well, originally we took it to the shop because the idle would surge nonstop and cause the motor to die (but only at idle speed) about 20 times a day. It could always be restarted but was intensely frustrating when trolling and/or motoring at idle speed. The shop first diagnosed a bad Low Pressure Fuel Pump. Replaced that, no dice. Then the High Pressure Fuel Pump. Replaced that, no dice. Then the fuel injectors. Replaced those, no dice. Then finally the ECU. Replaced that. Still had the idle surging and dying issue. Did some googling and then I replaced the IAC valve, which solved the idle surge by about 95%. Still died occasionally (about twice a day), but I figured that's gotta just be a crappy Mercury quirk for this motor. After 6 trips on the water, the ECU stopped sending power to the High Pressure Fuel Pump. Took to shop and they connected the fuel pump to a 12V battery and the motor started and ran fine. Plugged back into ECU and it wouldn't power the HP fuel pump, so I replaced the ECU again and filed a warranty claim for the failed one.

After all that, thought I was all good. Motor ran like a top aside from the occasional and infrequent idle-surge-die combo. Then fried another ECU on the 6th trip out this year.
 

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Discussion Starter · #15 ·
So....just to appease my curiousity or more likely my ignorance, are you referencing an internal grounding issue with the motor? OP stated no battery attached, so I was under the impression that the motor was isolated from the hull and had a floating ground? I guess it would be good to know whether the hull material is aluminum, wood, rubber, fiberglass or painted to help understand if there is even a path to the actual boat's ground to know if a grounding problem on the vessel is the culprit?
Correct, no battery.

It is mounted on a 2007 16ft Smokercraft Lodge, rivited aluminum painted gray. I see ground wires running from the motor to the transom bracket. The mounting bolts that run through the transom are directly bolted the bracket. Maybe no ground between motor and boat? Would this cause an issue?
 

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Your motor is a tohatsu, they make 2.5-30hp for mercury, pretty decent motors.
You said it is battery less, it's charging the capacitor to start it when pulling. Wonder if that capacitor has reached its service life of charges and discharges like any battery.
Is there cables or means to attach a small 12v hell battery to test.
 

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So in the simplified order of things, the fuel pump was changed and now the circuit in the ECU that powers the fuel pump burns out after little use repeatedly.

I’d think putting the original fuel pump back in would be a good test.

You need a different shop. Based on your description they are just blindly throwing parts at the motor without any real diagnosis.
 

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Discussion Starter · #18 ·
So in the simplified order of things, the fuel pump was changed and now the circuit in the ECU that powers the fuel pump burns out after little use repeatedly.

I’d think putting the original fuel pump back in would be a good test.

You need a different shop. Based on your description they are just blindly throwing parts at the motor without any real diagnosis.
This sounds accurate unfortunately. The OEM fuel pump is $500 and the ECU is another $620, so I guess if I want a running motor that's the next step.

Thanks everyone for your input.
 

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Do you, or the repair shop, not have the removed parts?
Sounds like a fairly expensive troubleshooting method to just throw more new parts at it.
 

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Discussion Starter · #20 · (Edited)
Do you, or the repair shop, not have the removed parts?
Sounds like a fairly expensive troubleshooting method to just throw more new parts at it.
Started with the shop repairing. Transitioned to me repairing. Spent a total of $2400 on this since the beginning. Had I known it would be this much I would have just ordered a new motor on day 1. The repair shop is a certified Mercury service center, so not saying their troubleshooting method was ideal, just wanted to establish that I didn't take it to rinky dink operation.
 
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