Baffled By Voltage @ Coil 888 Merc...

[Haut Medoc]

This is a '73 Mercruser 888.....
I had it running in the driveway, shut it off & when I came out the coil was hot & leaking oil....
I replaced the key swich & slave solenoid after determining both had failed.....
Now, on to the problem at hand....
After this incident, I put on an old coil that came with the boat, it is marine & says use a resistor, (It actually has 'Mercruiser" printed on it, I suspect it is the original...
Anyway, I now have 12v at the coil.....
It should be 4-9v, running or not, correct?.....
Is it possible to fry the resistor wire so that it loses resistance?.....
Are these resistor wires still avaiilable?.....
It is an auto part or a marine part...
I could easily run a new wire bypassing the harness.....
Any help out there?.....

 

[EddiePetty]

Re: Baffled By Voltage @ Coil 888 Merc...

...if the engine is cold and the points are open, you will ready battery potential from the coil's positive terminal to ground. After the engine has run sufficently to open the choke the reading should drop to the 7-9 vdc range.

 

[bruceb58]

Re: Baffled By Voltage @ Coil 888 Merc...

With the points open you will have 12V. With the points closed, you will have the drop across the resistor since now there is current going through the coil so you will read the lower voltage at the coil.

 

[bruceb58]

Re: Baffled By Voltage @ Coil 888 Merc...

After the engine has run sufficently to open the choke the reading should drop to the 7-9 vdc range.

Coil voltage has nothing to do with choke or the temp of the engine.

 

[EddiePetty]

Re: Baffled By Voltage @ Coil 888 Merc...

Coil voltage has nothing to do with choke or the temp of the engine.

Didn't mean to imply that it did!!!

Having the engine run long enough to open the choke would provide ample time for the resistance wire to heat sufficently to reduce coil supply voltage.

( Gosh Lucy...I gotta 'splain everything!!!!)

 

[Silvertip]

Re: Baffled By Voltage @ Coil 888 Merc...

By golly I always thought resistors heated "because" of the resistance, not that they needed to heat up to "become" a resistor. Must be "modern" electronic theory.

 

[Haut Medoc]

Re: Baffled By Voltage @ Coil 888 Merc...

So which is it?.....
I was under the impression that the resistace wire should resist immediately, or the "I" terminal of the solenoid would not be necessary.....
How about some real answers for ME, & quit quibbling amongst yourselves.....
Can the wire lose resistance or not?.....
Should I bypass the harness with a new resistor or not?.....
What voltage should be at the + terminal with the key in the "I" position?....

 

[Haut Medoc]

Re: Baffled By Voltage @ Coil 888 Merc...

With the points open you will have 12V. With the points closed, you will have the drop across the resistor since now there is current going through the coil so you will read the lower voltage at the coil.

It will drop slightly while running, but not below the 9 volt threshold..........

 

[bruceb58]

Re: Baffled By Voltage @ Coil 888 Merc...

So which is it?.....
I was under the impression that the resistace wire should resist immediately, or the "I" terminal of the solenoid would not be necessary.....
How about some real answers for ME, & quit quibbling amongst yourselves.....
Can the wire lose resistance or not?.....
Should I bypass the harness with a new resistor or not?.....
What voltage should be at the + terminal with the key in the "I" position?....

The resistance wire has a fixed resistance and could vary slightly over temperature but would be fractions of a percent. Its just a resistor. Just as a resistor often is made up of a resistance wire wrapped up in a package so is this one. Allmost all power resistors are wire wound.

The wire can't lose resistance.

If you want to bypass your resistance wire and wire in a regular ballast resistor, you can do that but need to make sure it is the proper value called out in your manual.

Like I said before, it should be 12V with the point open and less(close to 9V but also possibly called out in your manual) with the points closed.

 

[Haut Medoc]

Re: Baffled By Voltage @ Coil 888 Merc...

The resistance wire has a fixed resistance and could vary slightly over temperature but would be fractions of a percent. Its just a resistor. Just as a resistor often is made up of a resistance wire wrapped up in a package so is this one. Allmost all power resistors are wire wound.

The wire can't lose resistance.

If you want to bypass your resistance wire and wire in a regular ballast resistor, you can do that but need to make sure it is the proper value called out in your manual.

Like I said before, it should be 12V with the point open and less(close to 9V but also possibly called out in your manual) with the points closed.

That must mean I am somehow getting 12v from another source, possibly a chafed wire or bad harness plug....
I took took the tape off the engine harness & all looked good, but I can't see inside the plug, I guess I'll look for continuity where it should not be, (like Bat touching "I")...
Is the resistor wire part of the engine side or boat side?....
Should I read resistance with the Ohm meter betweeen the proper pin to the coil?.....

 

[bruceb58]

Re: Baffled By Voltage @ Coil 888 Merc...

You need to remove the wire from the coil to measure the resistance of the wire. Take the wire off and measure its resistance to the positive terminal of the battery with the ignition switch on.

There are usually 2 wires going to the positive side of the coil. One is the resistance wire and the other goes to a terminal on your starter solenoid which gives you a full 12V(not through a resistance wire) to the coil during start up.

The resistance wire is on the engine and part of the engine harness.

 

[bruceb58]

Re: Baffled By Voltage @ Coil 888 Merc...

Having the engine run long enough to open the choke would provide ample time for the resistance wire to heat sufficently to reduce coil supply voltage.

( Gosh Lucy...I gotta 'splain everything!!!!)

The amount that the resistance wire heats up will insignificantly affect the resistance value.

 

[Haut Medoc]

Re: Baffled By Voltage @ Coil 888 Merc...

Thanks Bruce, I have to go to work, I'll post back when I get the results....

 

[frank246]

Re: Baffled By Voltage @ Coil 888 Merc...

On a 888 or 188 the resistor wire runs from the choke pos side to the coil pos wrapped in the harness. Its the only wire with massive core so you know what to look for.
good luck!

 

[Haut Medoc]

Re: Baffled By Voltage @ Coil 888 Merc...

On a 888 or 188 the resistor wire runs from the choke pos side to the coil pos wrapped in the harness. Its the only wire with massive core so you know what to look for.
good luck!

Thanks!.....

 

[bruceb58]

Re: Baffled By Voltage @ Coil 888 Merc...

You can them just measure from the end of the wire up to the choke terminal. You still need to disconnect the wire from the coil though.

 

[Haut Medoc]

Re: Baffled By Voltage @ Coil 888 Merc...

I was able to get aresistor wire from another harness & patch it in, but it still reads too hih, (over 9v)....
Is it possible the alternator has some role in this?....
It reads 13v at idle but will rise as RPMS increase, not in the red, but higher than normal....
I figured maybe the battery was run down a little, but it reads 12v....
Any suggestions or input?.....

 

[bruceb58]

Re: Baffled By Voltage @ Coil 888 Merc...

So just to get this straight...you have a little over 9v with the engine not running, the ignition on and the points closed yet you have 13V with the engine running?

 

[Haut Medoc]

Re: Baffled By Voltage @ Coil 888 Merc...

So just to get this straight...you have a little over 9v with the engine not running, the ignition on and the points closed yet you have 13V with the engine running?

Now with the resisor wire, I get 5.6v with the key on, 11.2 running & it will go higher if I rev the engine...
I'm gonna pull the alternator & take it in...
I'm guessing bad diodes?.....
How big a role does the ground play in this scenario?...

 

[frank246]

Re: Baffled By Voltage @ Coil 888 Merc...

I was able to get aresistor wire from another harness & patch it in, but it still reads too hih, (over 9v)....
Is it possible the alternator has some role in this?....
It reads 13v at idle but will rise as RPMS increase, not in the red, but higher than normal....
I figured maybe the battery was run down a little, but it reads 12v....
Any suggestions or input?.....

Try remove all the wires on the pos side of the coil, reconnect the resistor wire only. The other wire (now disconnected) from the pos side of the coil should only read 12v while cranking, with the engine running or ignition only there should be no + voltage on this wire!!
Do the same on the neg side of the coil and only reconnect the wire comming from the breaker points, now measure again...also with engine running.
Good luck!