4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

I have a 1986 INvader with a 4.3 mercruiser I converted from OMC cobra outdrive to Alpha 1. It has alway been hard to turn over, despite great compression and several new batteries, etc. Recently it began stumbling under load (@2500rpm) and I started to troubleshoot. I did the usual checks and swaps - compression check, new coil, wires, spark plugs before realizing that the 12.6 volts measured at the battery was only 4.0 volts at coil.

Here are the steps I have taken so far:
1) checked grounds at engine. The alternator does not have a dedicated ground wire or terminal and I cannot identify the type. I suspect it is grounded through the mount.
2) checked the shift interrupter. Interruptor is wired correctly except that the wiring from the small terminal block on the switch itself does not have a wire to ground. I put in wire to ground and it has made no difference.
3) Checked wiring at starter. purple/yellow wire between starter and coil was missing. I added this wire to an empty post to the left (as you face it) of the large post where the battery cable is attached. The posts on the starter are not labeled but it appears to be the correct location. This change gave me battery voltage at coil (12.6V).

I thought this licked it but the gremlins had more to say:

voltage at coil now drops from 12.6v to 2.0 v while cranking. Started troubleshooting again:
1) Checked voltage at key. 12 volts at key, but noticed body of ignition is hot to touch.
2) Replaced ignition key - no change
3) removed purple wire (resistance wire? leads to alternator) from coil and key - still large voltage drop but (not surprisingly) no heat build up.
4) traced all other wires and all seem okay.
5) wondering if the ignition key needed a ground I jumpered from the ignition key to ground and was rewarded with a nasty spark of electricity, so I quickly pulled the wire.
5) battery is currently disconnected while I figure out my next step.

I am stumped. The boat is old and the wiring is pretty much what you would expect. Chopped, sliced and diced by a succession of owners. I want to rewire but need to know what the problem is before I do.

The issue seems to be the connections between starter-coil-alternator. Is the starter wired incorrectly? How do I check if the starter and alternator is wired correctly? I cannot find any manufacturer info on either the starter or alternator.

Thanks in advance.

Matt

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

mbasura said:

I have a 1986 INvader with a 4.3 mercruiser I converted from OMC cobra outdrive to Alpha 1. It has alway been hard to turn over, despite great compression and several new batteries, etc. Recently it began stumbling under load (@2500rpm) and I started to troubleshoot. I did the usual checks and swaps - compression check, new coil, wires, spark plugs before realizing that the 12.6 volts measured at the battery was only 4.0 volts at coil.

Here are the steps I have taken so far:
1) checked grounds at engine. The alternator does not have a dedicated ground wire or terminal and I cannot identify the type. I suspect it is grounded through the mount.
2) checked the shift interrupter. Interruptor is wired correctly except that the wiring from the small terminal block on the switch itself does not have a wire to ground. I put in wire to ground and it has made no difference.
3) Checked wiring at starter. purple/yellow wire between starter and coil was missing. I added this wire to an empty post to the left (as you face it) of the large post where the battery cable is attached. The posts on the starter are not labeled but it appears to be the correct location. This change gave me battery voltage at coil (12.6V).

I thought this licked it but the gremlins had more to say:

voltage at coil now drops from 12.6v to 2.0 v while cranking. Started troubleshooting again:
1) Checked voltage at key. 12 volts at key, but noticed body of ignition is hot to touch.
2) Replaced ignition key - no change
3) removed purple wire (resistance wire? leads to alternator) from coil and key - still large voltage drop but (not surprisingly) no heat build up.
4) traced all other wires and all seem okay.
5) wondering if the ignition key needed a ground I jumpered from the ignition key to ground and was rewarded with a nasty spark of electricity, so I quickly pulled the wire.
5) battery is currently disconnected while I figure out my next step.

I am stumped. The boat is old and the wiring is pretty much what you would expect. Chopped, sliced and diced by a succession of owners. I want to rewire but need to know what the problem is before I do.

The issue seems to be the connections between starter-coil-alternator. Is the starter wired incorrectly? How do I check if the starter and alternator is wired correctly? I cannot find any manufacturer info on either the starter or alternator.

Thanks in advance.

Matt

Click to expand...

Ayuh,... Welcome Aboard,.... How much of what is Mercruiser, 'n how much is still OMC,..??

Got a serial number from the Mercruiser parts of the motor,..??

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

5) wondering if the ignition key needed a ground I jumpered from the ignition key to ground and was rewarded with a nasty spark of electricity, so I quickly pulled the wire.

What did you jumper to ground? a terminal lug or the casing?

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

I jumpered from the body of the key to a neutral terminal block under the control panel. I was sort of grasping at straws, thinking that the key needed a ground.

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

Bond-o said:

Ayuh,... Welcome Aboard,.... How much of what is Mercruiser, 'n how much is still OMC,..??

Got a serial number from the Mercruiser parts of the motor,..??

Click to expand...

here is the code from the engine block as best I can make out: TC45738898. The numbers are very faint but the TC457 looks pretty clear.

I pulled 6269732 off the intake manifold and 6269732 off the flywheel.

any ideas would be appreciated . . .

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

mbasura said:

here is the code from the engine block as best I can make out: TC45738898. The numbers are very faint but the TC457 looks pretty clear.


I pulled 6269732 off the intake manifold and 6269732 off the flywheel.

any ideas would be appreciated . . .

Click to expand...

***correction***
I pulled 6269732 off the intake manifold and 14088553 off the flywheel.

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

you need to replace the ign switch , power never should be found on the key or the casing.

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

I replaced the ignition switch and now there is no voltage coming off the casing, but the problem persists. The voltage at coil reads 0.8v at the + post on the coil once the key is switched on, 12.0v when key is off. Once the purple resistance wire is removed from the + post on the coil, voltage is back at 12.0v.

Problem appears to be the wiring at the engine. The connections at the starter and alternator are not labeled. How do I check if the wiring is correct? I jumpered a fresh wire between coil and key and it made no difference, still 0.8v at
coil.

Do I simply remove the purple resistance wire? Would changing the distributor to a pertronix system remove the need for the purple resistance wire?

Thanks in advance; all theories are welcome.

Matt

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

Any ideas on this issue? Will a Pertronix II that doesn't use a resistance wire help me?

Matt

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

Do you have points and a condensor? Any chance of a photo of the ignition system?

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

The engine has points and a condenser. I did a few checks per the shop manual and the condenser seemed to check out. Here is a overview of the engine. I'll shoot a better video shortly.

Invader 4.3 - YouTube

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

mbasura said:

I converted from OMC cobra outdrive to Alpha 1.

Click to expand...

mbasura said:

... 14088553 off the flywheel.

Click to expand...

mbasura said:

The engine has points and a condenser. I did a few checks per the shop manual and the condenser seemed to check out. ...

Click to expand...

This is a 1986 engine block (by the casting number), and by then Mercruiser were using their Thunderbolt IV, electronic ignition system on all there V engines.

Conclusion: This is an OMC engine, not a Mercruiser.

You need to troubleshoot this problem as you would any other Kettering (points and condenser) ignition system...

Remove all wires from the coil- except the wire that runs to the points.

1. With the points OPEN, check the coil+ voltage (should be 12 volts).
2. Still with the points OPEN, check coil- voltage (should be 12 volts).
3. With the points CLOSED, check the coil+ voltage (should be about 7volts).
4. Still with the points CLOSED, check coil- voltage (should be 0 volts).
5. Crank the engine with the dissy cap off and watch the gap between the points (and there needs to be the right gap). You should see a small spark.
6. Remove the high tension lead from the centre of the dissy cap, connect an old spark plug, with the gap opened up to about 1/4", to it and lay it on the engine block. Crank the engine. You should see a health spark.

Test result interpretations.
1. No 12 volts. Check power to key, and all wires between key and coil, including resistance wire/ballast resistor.
2. No 12 volts, open circuit coil primary.
3. Volts at 0, check wiring to coil (as per#1). Voltage at 12, points not closed or broken wire between coil and points.
4. Volts not 0, points not closed, or broken wire between coil and points.

Chris.........

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

Also you could try hooking up a 12v light up to the + and - side of the coil to see if you have a consistent full power going into the + side of the coil and consistent blinking power coming out of the - side of the coil as your cranking.

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

Fun Times said:

Also you could try hooking up a 12v light up to the + and - side of the coil to see if you have a consistent full power going into the + side of the coil and consistent blinking power coming out of the - side of the coil as your cranking.

Click to expand...

Always one for the quick and easy answers, ah? :facepalm: :thumb:

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

Dear Chris,

Thanks for the help on this. I visited the boat and ran the tests and the readings are below. It failed tests #3, #5 and #6. I have checked the wires to the key, ran a new wire purple yellow wire, replaced the key switch and can find nothing obvious. The wiring from the alternator (both the purple wire and the red/purple wire) appear to have been spliced at some point. Can you provide any guidance what is the most obvious place which wires may be the culprit?

Matt

achris said:

This is a 1986 engine block (by the casting number), and by then Mercruiser were using their Thunderbolt IV, electronic ignition system on all there V engines.

Conclusion: This is an OMC engine, not a Mercruiser.

You need to troubleshoot this problem as you would any other Kettering (points and condenser) ignition system...

Remove all wires from the coil- except the wire that runs to the points.

1. With the points OPEN, check the coil+ voltage (should be 12 volts). Reading: 12.6V
2. Still with the points OPEN, check coil- voltage (should be 12 volts). Reading 12.6V
3. With the points CLOSED, check the coil+ voltage (should be about 7volts). Reading .3V
4. Still with the points CLOSED, check coil- voltage (should be 0 volts). Reading 0.3V
5. Crank the engine with the dissy cap off and watch the gap between the points (and there needs to be the right gap). You should see a small spark. No spark visible
6. Remove the high tension lead from the centre of the dissy cap, connect an old spark plug, with the gap opened up to about 1/4", to it and lay it on the engine block. Crank the engine. You should see a health spark. No spark visible

Test result interpretations.
1. No 12 volts. Check power to key, and all wires between key and coil, including resistance wire/ballast resistor.
2. No 12 volts, open circuit coil primary.
3. Volts at 0, check wiring to coil (as per#1). Voltage at 12, points not closed or broken wire between coil and points.
4. Volts not 0, points not closed, or broken wire between coil and points.

Chris.........

Click to expand...

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

Hey Matt,

Run test #3 again, but with a wire run directly from the battery+ to the coil+ (don't leave it on too long, the coil will get 'warm'). If you still see a serious drop, the coil may be shorted internally. I'm VERY reluctant to say 'change the coil', as that's what everybody does, and 99% of the time, there's nothing wrong with the coil. You may just be that 1%....

Chris.......

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

thanks for the help on this. As expected, the voltage at the + at the coil read 12.6 V. Once I the key was switched on, the reading was 1.0V. I ran a jumper from the + on the battery to the +on the coil and got a strange result. The voltage DROPPED to .5V. There has to be some stray ground somewhere.

I spent some time trying to find where the issue may lie. I ran substitute wires to replace the purple wire between the key and the engine and a substitute wire for the yellow/red between key switch and the slave solenoid. No change, still reading 1.0V at coil.

I pulled the harness plug to check there per the sticky guidelines. Couldn't figure out how to check the individual studs, but noticed the coil voltage was back at 12.0V when the boat harness was disconnected from the engine harness. I suspect the issue must be somewhere in the boat side of the harness but I am a loss as to where. I figured substituting the wires would at least reveal where the problem was.

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

mbasura said:

thanks for the help on this. As expected, the voltage at the + at the coil read 12.6 V. Once I the key was switched on, the reading was 1.0V. I ran a jumper from the + on the battery to the +on the coil and got a strange result. The voltage DROPPED to .5V. There has to be some stray ground somewhere.

I spent some time trying to find where the issue may lie. I ran substitute wires to replace the purple wire between the key and the engine and a substitute wire for the yellow/red between key switch and the slave solenoid. No change, still reading 1.0V at coil.

I pulled the harness plug to check there per the sticky guidelines. Couldn't figure out how to check the individual studs, but noticed the coil voltage was back at 12.0V when the boat harness was disconnected from the engine harness. I suspect the issue must be somewhere in the boat side of the harness but I am a loss as to where. I figured substituting the wires would at least reveal where the problem was.

Click to expand...

I assume this is with the wire still connected from the battery+ to the coil+... If so, then you have a short to ground somewhere on the 'switched 12v' wire in the harness or the instrument panel.

Chris.......

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

achris said:

I assume this is with the wire still connected from the battery+ to the coil+... If so, then you have a short to ground somewhere on the 'switched 12v' wire in the harness or the instrument panel.

Chris.......

Click to expand...

I have sunk many hours trying to find the stray ground to no avail. Out of curiosity, replaced the coil and disconnected the engine harness and tried to fire up the engine as you would on a test stand with just the minimum wiring, with no alternator, switch cutout, etc in the system. For the test, I had the + of the battery connected to the big post on the starter, ran test leads from + to the battery to + of the coil, grounded the battery and had my remote key wired between the + of the battery and the "S" stud on the starter. The - of the coil was connected to the distributor base. The starter battery was at 13.6V. When I hit the remote starter, I got a very weak spark, almost invisible, but it was not enough to fire up the engine. During the test I was getting 12.6 V at the + of the coil, but only .5V at the - of the coil.

This tells me it is not a stray ground and has to be something wrong with the basic system. The battery is good, the starter kicks strongly, the coil is new; I had previously replaced the points. The alternator, interrupt switch, slave solenoid, ignition key, were all bypassed and I got the same result. The only components that haven't been replaced are the condenser, primary wire and plug wires and distributor. I checked the primary wire and it was passing 12.0V along. The carb jets a decent amount of fuel, so I do not think that is the problem. When the problem first cropped up I checked the compression and had around 160 psi on all cylinders.

I am completely stumped - I once saw a video of a russian tank that got pulled out of a bog after being underwater 40 years and they fired it up using this method. If the test stand method didn't work what will?

Thanks in advance.

Re: 4.3 no spark voltage at coil drops from 12.6 to 2 volts or less while cranking

have you tried disconnecting the tachometer as if there faulty may be your problem.