External Resistor on '77 Mercruiser 120

[mgilker]

i am not getting spark to my boat engine. i'v just recently tested the coil, so i know that is not my problem. i do beleave it is the External Resistor thats the problem. dose anyone know where it is? and how to fix it if it can no longer be bought? help on the would be appreciated.

 

[Bondo]

Re: External Resistor on '77 Mercruiser 120

Ayuh,... Welcome Aboard,...

Have you checked to condition of the points,..??

The resistor is the wire goin' to the coil...
What are you gettin' for voltage, at the (+) side of the coil,..??

 

[mgilker]

Re: External Resistor on '77 Mercruiser 120

i have checked the points, the apper to be good, but i am not even getting spark from the coil. as of the amount of voltage on the postive side of the coil, i am not sure; only that it DOSE have power.

 

[Don S]

Re: External Resistor on '77 Mercruiser 120

You can't tell by looking at points if they are bad or not. If they are used and over a year old. Replace them.
If you are going to work on electical items, you need a DVM, Even walmart carries usable meters for less than 20 bucks.

 

[Bondo]

Re: External Resistor on '77 Mercruiser 120

You can't tell by looking at points if they are bad or not. If they are used and over a year old. Replace them.
If you are going to work on electical items, you need a DVM, Even walmart carries usable meters for less than 20 bucks.

Ayuh,... Exactly,...

Read this, it'll help ya understand how it's supposed to work...

 

[mgilker]

Re: External Resistor on '77 Mercruiser 120

i'll check on that.. in the mean-time, do anyone know where the External Resistor is, and for that matter; what it looks like? im about 98% postive the coils good because i tested it in another boat with the same coil setup and got good spark, and since im not getting spark from the coil after i tested to make sure its good.. it the only thing i can currently think is wrong.

 

[Fishermark]

Re: External Resistor on '77 Mercruiser 120

i'll check on that.. in the mean-time, do anyone know where the External Resistor is, and for that matter; what it looks like?

As Bond-o mentions above - the external resistor is the actual wire leading to the coil. The wire ITSELF is the resistor. It is a special wire that drops the voltage.

I doubt seriously that the resistor is your problem. I too vote for the points being the issue. You can wire a hot line directly to the coil (+ side) straight from the battery and try to start it. That will bypass the resistor circuit. It will not hurt to run it for a short time that way.

 

[Bondo]

Re: External Resistor on '77 Mercruiser 120

i'll check on that.. in the mean-time, do anyone know where the External Resistor is, and for that matter; what it looks like? im about 98% postive the coils good because i tested it in another boat with the same coil setup and got good spark, and since im not getting spark from the coil after i tested to make sure its good.. it the only thing i can currently think is wrong.

Ayuh,... As I said above, The WIRE is the resistor...

When you check for power at the (+) on the coil, it'll be less than 12V, except when the starter is crankin'...

That's WHY We're trying to get ya to Check things insteada throwin' parts at it...
The Points fire the coil...

 

[Fishermark]

Re: External Resistor on '77 Mercruiser 120

What he said. ^^^

 

[Bondo]

Re: External Resistor on '77 Mercruiser 120

What he said. ^^^

Ayuh,... It's the Same thing you said,...

At the Exact same time,....

 

[Bt Doctur]

Re: External Resistor on '77 Mercruiser 120

no power to the coil
points grounded/oxidized (not making electrical contact)
bad condensor
wire from coil(-) to dist (points plate)
bad tach or tach wire
shorted shift interupter switch

 

[mgilker]

Re: External Resistor on '77 Mercruiser 120

so iv checked the volts at the coil and its about 4.5 volts and jumps up to about 12 volts when turning it over.. how do u test points and/or condenser to see if one or the other needs to be replaced?

 

[Bondo]

Re: External Resistor on '77 Mercruiser 120

how do u test points and/or condenser to see if one or the other needs to be replaced?

Ayuh,... Ya Don't,... They're "Throw-away" parts....

A couple of the usual "Throw-away" parts that are used in an Annual Maintenance service...
Like filters, 'n such...

 

[elkhunter338]

Re: External Resistor on '77 Mercruiser 120

If it was the resistor wire the motor should start then die when you swap from 12v to reduced voltage.
My problem with no spark was a failed cam gear in the engine, it took me awhile to figure that you.
Verify your distributor is going round/round when you crank the engine over.
Otherwise you are probably looking in the right area coil/points/condensor/ plug wire from coil to dist.

 

[mgilker]

Re: External Resistor on '77 Mercruiser 120

so i have gotten around to changeing the points and condensor.. still no spark from coil. anymore ideas??

 

[Bondo]

Re: External Resistor on '77 Mercruiser 120

so i have gotten around to changeing the points and condensor.. still no spark from coil. anymore ideas??

Ayuh,... Did you read the link I posted way up at the top in post #5,..??

 

[mgilker]

Re: External Resistor on '77 Mercruiser 120

so iv got my boat firing and running now. turns out that all i had to do was to take off the brown wires from the negative (-) side of the coil and then got good spark. thanks for all the help!!!

 

[Don S]

Re: External Resistor on '77 Mercruiser 120

so iv got my boat firing and running now. turns out that all i had to do was to take off the brown wires from the negative (-) side of the coil and then got good spark. thanks for all the help!!!

Well, that means you also lost your tachometer, and your shift interrupt system. All you have done is a single step in troubleshooting.
Now you have to hook one at a time back up and find out which one is causing you to loose spark. Then you have to fix that item.

 

[mgilker]

Re: External Resistor on '77 Mercruiser 120

well im just glad i have been able to get it started for now.

 

[Leedanger]

Re: External Resistor on '77 Mercruiser 120

i'll check on that.. in the mean-time, do anyone know where the External Resistor is, and for that matter; what it looks like? im about 98% postive the coils good because i tested it in another boat with the same coil setup and got good spark, and since im not getting spark from the coil after i tested to make sure its good.. it the only thing i can currently think is wrong.

Are you talking about the external resistor for the alternator? Many of 72-78 model 120's that I have come across have the 3 wire externally resisted alternator. Stick with the advice you've been given by all the others, they are right, but this might be where you are getting confused with your terminology.