Ignition problems - 1987 3.0L Mercruiser

[ExUSCG-QM]

I've been reading threads for days trying to figure this one out, so if you know of one, please point me in the right direction.

I have a 1987 Imperial 18'4" with 3.0L GM (#B486968) and Alpha 1.
It's in fair condition for it's age and I've had it for two years with no problems.

Now I have an intermittant problem starting it. The engine is firing while I crank it, but when I let go of the key, it quits. I've gone through all of the troubleshooting items that I can find and this what I've come up with.....

The coil is only getting power from the starter while cranking. When I let go of the key to the "run" position there is no power to the coil any longer. I put the test meter on the coil + side and this is what's actually happening. So I tested the key switch and it works fine. Is there an inline fuse someplace that is stopping the power flow? As I said, this is intermittant so a fuse doesn't seem like the problem. I can start it five times and the sixth time I lose the voltage. I'll take some readings for an hour and all of a sudden the voltage is back without me making any changes!

One last point to make....the previous owner replaced the points with a Pertonix Ignitor. I didn't know how old it was so I replaced it yesterday. This did not fix the problem, but thought it could have something to do with it.

Thanks for any help,
Bill

 

[Don S]

Re: Ignition problems - 1987 3.0L Mercuiser

Re: Ignition problems - 1987 3.0L Mercuiser

First, disconnect the purple wire from the + terminal of the coil,Turn the ignition switch to the run position. You should have power to the purple wire you just disconnected from the coil. If not, start tracing the wiring from the purple wire on the ignition switch to the purple wire on the coil. Somewhere in there a connection is lost. Also make sure the emergency kill switch lanyard is in place (and working).

 

[ExUSCG-QM]

Re: Ignition problems - 1987 3.0L Mercuiser

Re: Ignition problems - 1987 3.0L Mercuiser

Thanks for the feedback Don.

I have tried removing the purple lead from the + coil and I get the same results...sometimes voltage, sometimes no voltage. I've also tried tracing the wire to the key switch, but stopped to post this and see if something simple (haha) jumps out at the experts. There's not a kill switch/lanyard on my rig.

There are two wires on the coil + and looking at my wiring diagram the purple/yellow is from the starter (powers at "start" on switch) and there is a purple resistor wire. Does that purple wire act as the resistor and supplies the primary voltage in the "run" position too? I didn't think a resistor wire would be used to provide power.

 

[Don S]

Re: Ignition problems - 1987 3.0L Mercuiser

Re: Ignition problems - 1987 3.0L Mercuiser

A resistor wire is in the wiring harness of a Merc engine of your era, with a points type distributor. The resistor wire was used to replaced the ballast resistor used on some motors, but does the same thing.
If the resistor wire goes bad or you have a bad connection you will not have power to the distributor. And yes, it's used by the Pertronix ignition also.
The purple/yellow wire is the wire from the starter, which is why it runs till you let off the key.
A quick test is to hook a jumper wire from the batteries positive terminal to the + terminal on the coil. If it starts and runs with the jumper, they you have a wiring problem between the engine and helm.

PS: If it starts and runs with the jumper, you have to remove the jumper to shut it off.

 

[rodbolt]

Re: Ignition problems - 1987 3.0L Mercuiser

Re: Ignition problems - 1987 3.0L Mercuiser

I am with DonS on this one as well.
another quick check is while you have 0V at the coil with the key in the on position check the red wire at the switch for 12V.
if you have 12V at the switch WHILE having 0V at the coil your issue is going to be between the keyswitch and the coil.
when that hull left the factory heading to the dealership for sale it was equipped,by law, with an emergency stop lanyard.
if its now missing then someone has spliced around it.
the emergency switch,like the key switch, opens the circuit on the purple wire heading to the engine harness connector to shut the engine off.
some lanyard switches are located on the dash and others were located on the control box depending on the box used and who rigged it.

 

[ExUSCG-QM]

Re: Ignition problems - 1987 3.0L Mercuiser

Re: Ignition problems - 1987 3.0L Mercuiser

I do have 12v at the switch with 0 at the coil so I was hoping that I would find the problem at the splice of the removed kill switch. However, it's a tight connection. One odd thing that I have come across is continuity between purple, yellow/red and ground on the engine side of the wiring. I thought it was at the ignition switch, but I disconnected the pigtail and it isolated the problem to the engine side. I get 0.00 resistance between these wires and ground....any combination. Also, I get 0.00 res between ground and the terminal for the yellow/red wire on the starter (with wire removed). Since there is positive voltage coming into this post shouldn't it have resistance with ground and only the starter body would be grounded?

In my novice opinion, it would appear that under all of that electrical tape wrap I have a mispliced or bare wire to find. But I have know idea what the deal is with the starter and continuity with ground.

 

[Steveu23]

Re: Ignition problems - 1987 3.0L Mercuiser

Re: Ignition problems - 1987 3.0L Mercuiser

A resistor wire is in the wiring harness of a Merc engine of your era, with a points type distributor. The resistor wire was used to replaced the ballast resistor used on some motors, but does the same thing.
If the resistor wire goes bad or you have a bad connection you will not have power to the distributor. And yes, it's used by the Pertronix ignition also.
The purple/yellow wire is the wire from the starter, which is why it runs till you let off the key.
A quick test is to hook a jumper wire from the batteries positive terminal to the + terminal on the coil. If it starts and runs with the jumper, they you have a wiring problem between the engine and helm.

PS: If it starts and runs with the jumper, you have to remove the jumper to shut it off.

How long can you run it like that ? Asking for a friend

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