85 Mercruiser 3.0 possibly bad coil??

[Jeffafa]

Hi guys, trying to figure out why my 85 Mercedes has no/ very weak spark. Good 12v to coil + and - when points are open. With points closed (after ensuring they are clean and adjusted), coil positive drops to around 4.5v and negative shows around 1v.
I have bypassed the coil negative by removing the grey wire and the wire to the points, attached a jumper wire to negative and removed positive wires and attached a lead straight to the battery. When grounding the coil negative and watching spark from the tower it is very weak and hard to even see,, when everything is connected, spark will not deliver to the plugs. Next I disconnected all coil wires and tested primary and secondary resistance. Primary: 2.4 ohms, secondary: 11.8k ohms, after doing research these readings look to be in range. Is it possible the secondary windings are good enough for an ohm meter but not healthy enough to deliver high voltage?? In my experience with cars when ohming a coil and it's in spec, it's usually a good part. But seeing that I've bypassed the points and the power supply I don't know what else it could be other than a bad coil.
Also what are the odds someone can attach a wiring schematic as I am having trouble locating one!
Thanks in advance and happy boating!!

 

[Scott Danforth]

did you set the dwell with a meter? (note the meter will also tell you the health of the condenser)

 

[kenny nunez]

If you grab the rotor shaft and push it does it make the points open even when the rubbing block is on the flat side of the shaft? If it moves enough that the points open then it is a good time to have the distributor rebuilt or replaced.

 

[Jeffafa]

did you set the dwell with a meter? (note the meter will also tell you the health of the condenser)

I have not.. but wouldn't I be bypassing all that by grounding the negative side of the coil?

 

[Jeffafa]

If you grab the rotor shaft and push it does it make the points open even when the rubbing block is on the flat side of the shaft? If it moves enough that the points open then it is a good time to have the distributor rebuilt or replaced.

I'll have to check that this evening! This motor only has 80 hrs on it so I don't know that it's very likely that the distributor would be worn out.

 

[Scott Danforth]

I have not.. but wouldn't I be bypassing all that by grounding the negative side of the coil?

Since the coil is fired by switching the ground, NO

 

[Jeffafa]

Since the coil is fired by switching the ground, NO

ok I should be more specific.. when I say I'm grounding the negative side of the coil I'm tapping a jumper wire from the negative to a good ground.. basically mimicking the points...or am I going about this all wrong??

 

[Scott Danforth]

Did you check the condenser? This is where the dwell meter comes in handy. Or remove them for the test

Coils either work or they don't. Fact is about a million good coils get swapped out every year due to faulty diagnostics.

 

[Jeffafa]

Did you check the condenser? This is where the dwell meter comes in handy. Or remove them for the test

Coils either work or they don't. Fact is about a million good coils get swapped out every year due to faulty diagnostics.

It's not the condenser, it's not dwell... AGAIN I am bypassing everything in the distributor. The coil is grounded by the points which I am eliminating from my diagnosis by creating my own ground switch. I am well aware coils get shotgun replaced all the time as a result of poor diagnosis, happens all the time in the car world. But the tests I have done to eliminate everything else, it 100% has to be the coil. Just throws me off because the coil ohms out in spec. So I'm wondering if the secondary windings are good enough for an ohm meter but not stable enough for a high voltage load. Much like how a chafed wire may test ok when testing resistance but can't deliver proper voltage through the circuit. If a new coil fixes it, I will cut the old one open to inspect.

 

[nola mike]

Yes, that test should spark. You're not testing the coil under load, so yeah, you can have an internal short under some conditions. At this point I would certainly spend $20 on a new coil. Here's a somewhat generic diagram for a 3.0, dunno if that's what you're after?

 

[Jeffafa]

Yes, that test should spark. You're not testing the coil under load, so yeah, you can have an internal short under some conditions. At this point I would certainly spend $20 on a new coil. Here's a somewhat generic diagram for a 3.0, dunno if that's what you're after?
View attachment 399557

I appreciate that diagram man, that will for sure help later on!
I ended up changing the coil last night, first crank she fired right up, I was honestly surprised how quick it started but probably cause all the fuel in the cylinders from cranking before.
I appreciate the help, next step is to take her out for a spin.