Mercrusier 470 coil voltage question

[carguy001]

I have a 1984 celebrity with the good old mercruiser 470 in it. the last two years I have experienced all 470 issues. This year as I was preparing the boat for the season I went to start it and it would not start I did all the checks and found I only had 4.5 volts at the coil with the key on points closed and measuring at the the coil. But if I check battery ground to coil positive I have battery voltage. I ran a jumper wire from + battery to the + coil and she fired right up

So my thinking is the resistance wire is dropping the voltage too much I am wanting to run new wires for this circuit one wire to supply full 12 volt while cranking and another wire that will run thru a ballast resistor to drop the volts to about 6-9 volts.

Is this the right thing to do?

Thanks in advance you guys help my a bunch last year.
Scott

 

[stonyloam]

Re: Mercrusier 470 coil voltage question

You already have a wire that supplies 12V to the coil during cranking. It is the purple/yellow wire coming from one of the small terminals on the slave solenoid. Check that it is secure and clean, and supplying battery voltage during cranking.

 

[carguy001]

Re: Mercrusier 470 coil voltage question

I will get my wife to crank it while I check for 12 v at the purple/ yellow wire and at the + side of the coil tomorrow

Thanks

 

[nola mike]

Re: Mercrusier 470 coil voltage question

If its just the ballast that's bad, the engine will catch while cranking, but die as soon as you let off from the start position.

 

[carguy001]

Re: Mercrusier 470 coil voltage question

I was able to work on the boat a bit today since it is raining here, and traced down the wiring for the coil and ignition key switch and found someone else had ran a new wire from the start position on the back of key switch to the coil. But they ran the wire thru the cigar lighter fuse and that fuse was blown. I replaced the fuse and cranked the motor over and it fired right up hopefully i can get it on the muffs and make sure that was the issue.

I check the voltage at the purple/yellow wire at the slave solenoid and I had 12.45 volts and the battery volts are 12.69 volts. But with finding someone added a new wire means troubling shooting is going to be a chore.

Thanks for the help on this one.
Scott

 

[stonyloam]

Re: Mercrusier 470 coil voltage question

OK something is not quite right here. When that fuse was blown, did it fire up when the starter was turning and then quit when you released the key? It should have, because the P/Y wire from the slave would have been feeding 12V to the coil. If it did not then it sounds like you have a problem with the connection. If you are feeding 12V directly from the ignition switch then the coil is seeing a higher voltage than it was designed for and that could cause problems.

 

[carguy001]

Re: Mercrusier 470 coil voltage question

It only ran for a few a few seconds, I was not expecting it to start so did not pay attention to if I quit when I released the key. The non-factory wire is only attached to the start side of the key switch at the dash so it is only seeing 12v when it is cranking. If turn the key to run with engine off it only has about 4-5 volts at the coil.

Hopefully the rain will let tomorrow and i can verify if it is only running while cranking.

Thanks
Scott

 

[stonyloam]

Re: Mercrusier 470 coil voltage question

OK things are beginning to clear up here. It sounds like your p/y wire from the slave is not making good contact. If you have 12v at the terminal of the slave solenoid, that means you have a broken or badly corroded p/y wire. To troubleshoot try this: Remove both ends of the p/y wire (from the slave and coil) and replace it with a new wire directly from the slave to the coil +. Remove the fuse from the wire from the ignition and give it a try. It should start. If you have an ohmmeter check the continuity of the p/y wire, it may just need to be cleaned up and have new ends put on. Put on your muffs to supply water to the engine and see if it will run once it starts. The other thing to do is remove the purple resistance wire from the coil and with the ohmmeter check the resistance, should be 1.5 to 1.8 ohms (between the choke connector and the coil end).

 

[carguy001]

Re: Mercrusier 470 coil voltage question

Thanks stony I have been looking for the ohms on the purple wire I will give that a try

 

[stonyloam]

Re: Mercrusier 470 coil voltage question

Thanks stony I have been looking for the ohms on the purple wire I will give that a try

Slight brain cramp here should be 1.8 to 2 ohms. That is what I get for relying on my memory:facepalm:

 

[carguy001]

Re: Mercrusier 470 coil voltage question

Thanks stoneloam and nola mike for the tips and ideas I got the boat outside today and was able to get some trouble shooting done. Here is what I did I cleaned every connection point between the key, solonids, coil ,starter ,dizzy, choke. I checked the resistance wire and it was 2.1 ohm's not too far off. I made sure everything was tight, hooked the hose up, gave the throttle one pump and she fired up and idled a little rough for a minute but smoothed out, after letting it idle for 10 mins or so I revved it up to 2000 rpm and brought it back to idle and then stopped and started it 4-5 times no issues. The voltage running at the coil was 8.3volts.

For others tht read this take the time to do the free stuff first and don't over look the little thing like cleaning the connections these motors are pushing 30 years of service and corrosion does build up over time. I think I am adding this to my yearly maintenance to do list.

Thanks again
Scott

 

[stonyloam]

Re: Mercrusier 470 coil voltage question

she fired up and idled a little rough for a minute but smoothed out

Perfectly normal, she'll run a little rich until the electric choke fully opens. Glad things worked out.