Reversed polarity 40 hp merc -now it won't start

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RobRudolf

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May 20, 2012
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I foolishly reversed the polarity on the battery and now I have trouble.

A 5 feet section of 16 gage ground wire from the ignition switch to the tach actually melted. I replaced the damaged wire and now the instrument panel and other miscellaneous electrical items work.

The problem is the motor won't start. It turns over, just won't start.

I checked the fuse in the engine cowl and it is fine.

Where do I look next?

I can't believe I was so stupid! I was in a hurry and just didn't see the big fat red wire on the negative terminal!
 
roscoe

roscoe

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Re: Reversed polarity 40 hp merc -now it won't start

Welcome to iboats.

They guys here will get you fixed up.


Check for spark on all sparkplugs.

After you get it running, you need to check out the rectifier, as it probably fried as well.
 
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RobRudolf

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Re: Reversed polarity 40 hp merc -now it won't start

No spark on any cylinder.

I assume this means I need to replace the power pack. I'm trying to get this finished before the week-end, but don't want to replace more than necessary. Any way to test the power pack? Is it possible I didnt' see a fuse I should be looking for?

What about the voltage regulator, if only that were fried, could the no-spark issue still exist, with a good power pack?


roscoe said:
Welcome to iboats.

They guys here will get you fixed up.


Check for spark on all sparkplugs.

After you get it running, you need to check out the rectifier, as it probably fried as well.
Click to expand...
 
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aussieflash

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Re: Reversed polarity 40 hp merc -now it won't start

Your rectifier will be fried.You can test with an ohmmeter.Disconnect battery before you test.
Ohmmeter can be used to test Trigger and Stator as well.To test switch box you will need a DVA and a volt meter.Procedure and flowcharts will be in your merc manual.
Check all wiring including your kill switch first.
 
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RobRudolf

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Re: Reversed polarity 40 hp merc -now it won't start

I did the small rectifier test - from the FAQ page and yes. It failed. I got ohm meter readings after switching the input leads from black to red. If I read the instructions correctly, you should only get a reading in one direction, not both. I was getting readings in all permutations.

I don't want to replace more than necessary, but I'm not sure how to test the trigger and stator. Any links to a test method?

Is it possible that a bad rectifier would kill all spark? Is it possible that replacing only the rectifier will fix my problem? That doesn't seem reasonable. My thought is there must be something else.

Any directions for how to test the switch box or power pack? The owners manual doesn't have any flowcharts you referred to, so you must be speaking about a service manual. I don't have a service manual.
 
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RobRudolf

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May 20, 2012
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Re: Reversed polarity 40 hp merc -now it won't start

Mercury
2002 40ELPTO
sn 0T476915

3 cylinder with EFI
 
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RobRudolf

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May 20, 2012
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Re: Reversed polarity 40 hp merc -now it won't start

Thanks for the service manual link. This should get me in the right direction. I was able to read the flow charts and I think I can work through this tomorrow. Thanks again.
 
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1nebel0

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Nov 28, 2005
Messages
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Re: Reversed polarity 40 hp merc -now it won't start

Try cranking it with the yellow rectifier wires disconnected...no spark now try disconnecting the yellow/blk wire on the switch box.. this is the key kill circuit and if you get spark when disconnected you have a switch problem...keep in mind you won't be able to kill the motor if you start it without this circuit hooked up...please post your results after these two tests before you continue....
 
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RobRudolf

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May 20, 2012
Messages
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Re: Reversed polarity 40 hp merc -now it won't start

OK gonna start over after the last post. I tried removing the yellow rectifier wires and found that #1 cylinder does have spark. It is just weak. #2 & #3 cylinders do not have spark. I never actually checked #1 previously because #2 & #3 were both bad, so I assumed #1 was also bad.

Just for kicks I checked disconnecting the yellow/black wire and same thing. #1 has spark, #2 & #3 don't. As a third check I confirmed continuity to ground on black/yellow from keyswitch wiring harness to ground when in off position. Open when in run position.

By "switchbox" above, I assume you mean Rev Limiter, because the service manual doesn't have anything labeled switchbox.

I'm going to follow the flowchart from the service manual and stick with this. My gut is either CDM #1 is bad, which also affects #2 & #3 due to 2&3 getting ground path through #1. or the trigger coils are malfunctioning. Hopefully I can get out of this with just replacing 1 CDM.

Thanks for the help thus far. I'll keep you posted.
 
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