8.7 Volts at the Coil

[drewm3i]

At my wit's end with a boat. 1997 5.7L Vortec Mercruiser. 2 BBL carb. Thunderbolt V ignition. After doing a top end rebuild, it won't start...VERY weak spark that dies out immediately due to the low coil voltage of about 8.7 with key on and 6.5 while cranking.

Batteries are both new and read 12.7 volts. I have bypassed the tach, key switch, and shift interrupter and all reads the same. I have 12.6 volts at the key switch. 11.4 with the key turned to the on position and 11.3 at the I terminal. I also bypassed the key switch with a jumper wire from the B to the I leads and there was no difference at the coil.

I have replaced the ignition sensor, cap, and rotor, but still the issue persists. I have tried two different coils, both of which are assumed to be working. I tested the purple lead from the ignition module: also 8.7 volts.

I want to know anything else to check other than the red/white lead from the ignition module? Maybe I should just run a new wire to the coil from one of the many ignition wires and be done with it?

One other thing to note: the purple barrel connector terminal on the alternator was loose, but I put it back on. It could still be a bit loose though, but I believe that's related to the charging circuit?

 

[tpenfield]

Ouch . . . sorry to hear about the issue.

The first thing that comes to mind is the drop in voltage with the key switched on. I'd be looking for something shorting out the electrical system. Maybe get the wiring diagram for the engine, if you don't already have one and start chasing the circuitry. It's got to be a pretty hefty current draw to bring the voltage down that much. Maybe with an IR thermometer, look for the hot spot in the wiring. Got access to a thermal imaging camera?

 

[dubs283]

Is the ignition module properly grounded?

 

[Bondo]

Ayuh,.... Clean both ends of all the battery cables, including the grounds,....
You've got a bad connection,.... somewhere,.....

 

[Scott Danforth]

agreed, start at the battery and work your way to the helm cleaning every connection with 150 grit sand paper, making them shiny enough your mother would eat off them. then seal them with marine electrical varnish.

 

[nola mike]

Likely 2 wiring issues. First is drop with key on, either bad ground connection to helm and/or bad connection along r/p wire. Loss across fuse would be a suspect. #2 is a bad connection along purple wire from key switch. Connections to test are at the tach where the purple (usually) branches. Other trouble spots are at the mob switch, and cannon plug. Possibly also at the choke depending on your wiring. R/w at the ignition module is powered by purple, so not the issue.

 

[drewm3i]

Ouch . . . sorry to hear about the issue.

The first thing that comes to mind is the drop in voltage with the key switched on. I'd be looking for something shorting out the electrical system. Maybe get the wiring diagram for the engine, if you don't already have one and start chasing the circuitry. It's got to be a pretty hefty current draw to bring the voltage down that much. Maybe with an IR thermometer, look for the hot spot in the wiring. Got access to a thermal imaging camera?

I have it but it's so confusing and hard to discern.

 

[drewm3i]

Is the ignition module properly grounded?

I don't know. How do I test?

 

[drewm3i]

Ayuh,.... Clean both ends of all the battery cables, including the grounds,....
You've got a bad connection,.... somewhere,.....

I think I'll check the main engine grounds and the connections at the perko switch. I agree something such as this could be up.

 

[drewm3i]

Likely 2 wiring issues. First is drop with key on, either bad ground connection to helm and/or bad connection along r/p wire. Loss across fuse would be a suspect. #2 is a bad connection along purple wire from key switch. Connections to test are at the tach where the purple (usually) branches. Other trouble spots are at the mob switch, and cannon plug. Possibly also at the choke depending on your wiring. R/w at the ignition module is powered by purple, so not the issue.

I'm going to check the main bus bar connections behind the engine. The ones behind the helm are all snug. I will double check the tach, but like I said, I bypassed it to rule out a fault.

I also tested the kill switch by bypassing it and it's fine. The wiring there is all heat shrunk and is clean as I did it myself last year. Canon plug? Aka main engine harness? That is suspect and I did see some corrosion inside of the harness pins.

 

[dubs283]

I don't know. How do I test?

No test, just make sure the ring terminal for the module ground is attached to a proper ground location.

Common to see it attached to the module mounting stud at the elbow/riser or at the main ground stud at the flywheel cover or manifold for distributor mounted models

 

[nola mike]

No test, just make sure the ring terminal for the module ground is attached to a proper ground location.

Common to see it attached to the module mounting stud at the elbow/riser or at the main ground stud at the flywheel cover or manifold for distributor mounted models

This isn't the issue. Module has nothing to do with his problem.

 

[drewm3i]

No test, just make sure the ring terminal for the module ground is attached to a proper ground location.

Common to see it attached to the module mounting stud at the elbow/riser or at the main ground stud at the flywheel cover or manifold for distributor mounted models

It's attached to the main engine ground.

 

[Scott Danforth]

It's attached to the main engine ground.

Clean the ground connection and ever other connection in the boat

 

[dubs283]

This isn't the issue. Module has nothing to do with his problem.

Have seen this before with valve job reassemble/long block swap before

Module struggles for solid ground through its casing when someone misses the ground wire connection upon reassembly

 

[nola mike]

Have seen this before with valve job reassemble/long block swap before

Module struggles for solid ground through its casing when someone misses the ground wire connection upon reassembly

Weak spark, sure. But he's getting low voltage to the coil, which is coming from the ignition. If he gets that fixed and still has weak spark then go chasing the module

 

[dubs283]

he's getting low voltage to the coil, which is coming from the ignition.

Right. Purple/ignition lead from the module attaches to the coil, as such it is part of the entire ignition circuit. Not saying the module or its wiring is the issue but it is something to consider.

OP says the module is grounded properly and I believe them. Also says there is corrosion at the ten pin connection. I'd focus on that next

 

[nola mike]

Also says there is corrosion at the ten pin connection. I'd focus on that next

Certainly more likely imo, along with corrosion across the ignition fuse and ground from the helm back (not clear if the voltage drops at the red wire when ignition turned on, and if he's measuring voltage using helm ground or battery)

 

[drewm3i]

Clean the ground connection and ever other connection in the boat

It's brand new.

 

[drewm3i]

Have seen this before with valve job reassemble/long block swap before

Module struggles for solid ground through its casing when someone misses the ground wire connection upon reassembly

How is the ignition module grounded?