Tach and Coil Problems

Ryanlewis2010

Ryanlewis2010

Petty Officer 1st Class
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Oct 4, 2011
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314
Hi guys so my 75 merc 140 has one problem but two possible fixes, My coil Keeps goin bad like spewing liquid bad and extremely hot. So whats causing it, the system is not on the original harness so should there be a resistor and how would i test to see if there is one like how many volts need to be at the + terminal. OR could it be my tach being hooked up wrong the tach works fine when hooked into the neg side now ive ben told thats the wrong way to wire it but thats how its been wired on both motors (one original harness other not)

Thanks Ryan
 
Bt Doctur

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,111
Re: Tach and Coil Problems

if it uses points, then it should have a ballast resistor or resistor wire to limit the coil voltage
 
Ryanlewis2010

Ryanlewis2010

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Oct 4, 2011
Messages
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Re: Tach and Coil Problems

ok so what should the ballast resistor change the voltage to i.e 12.5 v to 8v
 
D

dubs283

Vice Admiral
Joined
Jul 27, 2005
Messages
5,096
Re: Tach and Coil Problems

you should only see roughly 9 volts at the coil with the engine running

if its 10 or higher, you run the risk of frying the points/coil
 
smokeonthewater

smokeonthewater

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Re: Tach and Coil Problems

and yes the tach should be connected to the negative side of the coil
 
Ryanlewis2010

Ryanlewis2010

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314
Re: Tach and Coil Problems

so i should prolly replace the points to because ive blown two coils with out having this resitance wire i just cut it out of the old harness and ill put it on tomorrow thx
 
Ryanlewis2010

Ryanlewis2010

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Messages
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Re: Tach and Coil Problems

Ok Thanks guys got it all figured out i just pulled the wire and put it on i got it running in the right range 6.5 key on engine off and a 9.5 key on engine on now i here the extra wire is suppose to be hooked into the starter so that it starts easier which i find is true but engine dies like it was grounded out right after the starter disengages do i have it wired to the wrong side of the starter?
 
smokeonthewater

smokeonthewater

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Dec 3, 2009
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9,838
Re: Tach and Coil Problems

the extra wire goes to the extra "I" terminal on the starter solenoid. NOT the same one "S" as the start wire.

If your starter only has two terminals then you can't hook the "extra" wire up.
 
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