Mercruiser 7.4. Accidentally reversed battery polarity and now no ignition?

D

Dejay

New Member
Aug 1, 2014
11
Australia
Boat Info
Sundancer 270 2000 model
Engines
Mercruiser 7.4 Brave II I think.
2000 model sundancer 270 with Mercruiser 7.4
Last run 3 weeks ago.

Hi everyone.

Headed out to my boat tonight and went to connect a new charged battery stupidly in the dark.
I swore I had it the right way but didn't. There was a small spark on connection. I then realized and reversed the leads. Everything still works. Lights, blower, horn. But there is no ignition. I don't get the warning beeps or anything. I turn the key and it just turns till the end of the barrel. No clicks, no fuel pump wizzing sound. It's just dead.

I checked the relay fuses near the battery switch.. all good.

Checked the pop fuses above all the dash switches.

Unscrewed the dash section with the key barrel and checked the fuse connected to the ignition switch that has 2 wires go in and out the other side.

I climbed in and checked the engine relay fuse.. the red button on the left hand side of the engine near the harness. When I push and pushed pretty hard, it felt like it was as far as it would go.. Sticks out about 1cm.

I sort of feel like the ignition key barrel may be gone. Or even maybe the starter relay?

If its the starter relay, would I still get the start up alarm and fuel pump wizzing sound?

If it's the key, can I short the barrel and what wires would I cross to get a start?

Thanks for any help and suggestions in advance.

Dave
 
Last edited:
Sorry about your predicament...

Probably not the key switch. More likely still a fuse somewhere. See if there is a fuse right near the starter. While most electronics are reverse polarity protected, I don't know about ignition components.

There are others here that will have first hand knowledge, so you came to the right place for help.

Welcome to CSR!
 
Last edited:
breaker.jpg
 
It's not the starter or any singular solenoid. Main fuse/relay or if you have one, engine PCM, ECM.... It will most likely be in the engine harness and will not effect any other boat operations.

Good luck
 
Check to see if you fried the computer it only takes one little spark to do it in ask scott fergusion.
 
Thank's everyone for your suggestions. I was also worried it may be the computer. No way to check that without replacing it I would think. I'll go through the book and see if there are any fuses I missed.


Aquaholic. Thanks but not sure what you mean under the steps. If you mean the breaker in the engine bay, I checked that. gave it a good push but it feels like it is in.
 
If you dont have the fuse link in post #4 then locate the slave solenoid and short the 2 large lugs, large yellow with red stripe and red with a purple stripe. this should spin the starter at least. It also lets you know the fuse in post #4 is still good.(if you do have it )
Next check for 12v at the purple wire at the key switch, report back with the results.
 
Thanks. Will do that. Sadly, working for the next 10 days. Will try all the above on the next day off and will report back.
 
Thanks Frank. I was hoping thinking it may be that but had no idea where it was. I'm going to try and go out today and have a look. If not, it will be next weekend. It's our winter here and it's rain every weekend.
 
Hate for it to be something as simple as the shifter not quite in neutral.

MB


Sent from my iPhone using Tapatalk
 
fwebster said:
Sometimes Mercruiser uses a regular automotive type fusible link...looks like this:
http://www.finditparts.com/products...Mmuu-K607464&gclid=CJ3Cpo2bi8ACFaVZ7AodOHEA9Q
and, it is located down near the starter. When it blows, your starter and ignition will be DOA but everything else will work.
Click to expand...

I looked for this fuse and any similar looking fuse with no luck.
It is such a tight place to get into with the toilet vacuum pump right in the way. I laid over the engine and looked and even resorted to using my iphone to take pics where I couldn’t quite see. My concern is that when the engine was replaced, the fusible may have been taped up inside electrical tape?


Bt Doctur said:
If you dont have the fuse link in post #4 then locate the slave solenoid and short the 2 large lugs, large yellow with red stripe and red with a purple stripe. this should spin the starter at least. It also lets you know the fuse in post #4 is still good.(if you do have it )
Next check for 12v at the purple wire at the key switch, report back with the results.
Click to expand...

I found 2, what looked like fusible links, but don’t think they are related.
One on the rear right and one on the top middle. Checked them and they are fine.
This is a pic of them.
fuse1.jpg

There is a pop out fuse near the starter. It appears to be in. It looks something like this.
5193_thmb.jpg

I checked the purple wire at the ignition. No power there. I ran a positive wire to the purple and the ignition alarm and engine pump started up. I decided to try turn it over. There was a click that sounded like the starter solenoid but the starter motor didn't engage.
The batteries are fully charged as well.

I'm guessing this is a good thing and it may just be the fuse but where is it? What wire should I follow back and how far? Access again is the problem. I may need to pull the toilet pump out just to get access?

Thanks again guys.
 
Sounds like the starter solenoid engaged but the starter did not turn. You supplied power to click the solenoid on but there was no power to turn the starter motor. The main battery cable to the starter and related harness is not getting power. Do you have a battery selector or isolator switch that may be shorted out?
 
I had a big red button right above the engine harness mounted to one of the engine lift points on my old boat.

I did the same thing though I'm not proud to admit it.
 
I had a second battery mounted loosed on the starboard side of OldSkool. One afternoon I launched and the main battery was low. I instructed my son to jumper the loose battery to the main battery and start the engine while I parked the trailer. When I returned to the dock 2 guys were in my boat with rags on their hands holding a smoldering set of jumper cables. Thanks God they were there. Son did the unthinkable and mixed the cable even though they were clearly marked.

OldSkool has a main red push relay and no computers so the repair was easy.

See, it feels good to admit and cleanse yourself of stupid. :lol:
 
OldSkool said:
Sounds like the starter solenoid engaged but the starter did not turn. You supplied power to click the solenoid on but there was no power to turn the starter motor. The main battery cable to the starter and related harness is not getting power. Do you have a battery selector or isolator switch that may be shorted out?
Click to expand...

There is a switch. Off/bat1/bat2/Both.

It seems to be working as I am getting power to the lights, toilets, etc.

Next time out, I will try and bypass i in case in some way, it is fused.
 
AFD said:
I had a big red button right above the engine harness mounted to one of the engine lift points on my old boat.

I did the same thing though I'm not proud to admit it.
Click to expand...

Pressed this button. That's the one on the left hand side of the engine? I guess I better check if power is getting there as well.

Hopefully BTdoctor can advise with the info I gave back as far as the purple ignition wire being dead.

Thanks again everyone.
 
the main power feed starts at the starter thru a direct connection or the square fuse bock. thru the CB, the engine harness plug and up to the dash. The red/purple is the main feed to the panel, thru a 20A fuse, then to the ign switch.Jumping the 2 large lugs on the slave solenoid will verify the CB and the 90A fuse(if equipped) is good.
Next check for power on the red/purple under the dash,then the purple wire with the key"on" that purple wire goes back thru the harness to the coil and other items
 
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