5.7L TB IV Intermittent Ignition Issue

[edsonfarm]

I have a 2001 Cruisers 3075 Express with twin Mercruiser 5.7L engines, Thunderbolt IV, Bravo III drives. Just replaced one of the two motors at the beginning of the season with a long block GM OEM Marine crate engine. It ran fine on an engine stand prior to the install. The other motor was replaced last year. For reference purposes original engine SN from one of the original motors was OL358841.
Since launching this year the stbd (new) motor has been intermittently dropping RPMs and does this much more frequently at higher RPMs. It appears that the ignition system shuts off for a split second and then the engine recovers, runs great, then randomly does it again. The engine never dies, just stutters. A test light on the negative side of the coil goes out when the RPM drop occurs. So far I've replaced the ignition sensor in the distributor, swapped the ignition module/knock sensor from the other engine, eliminated the boat wiring at the cannon connector with a knock out box, supplied positive directly from the battery to the positive coil terminal and positive supply to the ignition module, replaced the coil, replaced the cap and rotor, and installed new grounds with no change. Down to it being a short somewhere in the engine wiring harness.

Does anyone else have any suggestions? I have an important trip tomorrow that I've been planning for months and after a whole week of working on this I haven't gained any ground.

Thanks,
Matthew Edson

 

[R055]

Nevermind

 

[achris]

Check grounds to the distributor and from the sensor. Make sure the distributor has a ground wire connected to it.

 

[edsonfarm]

There's a ground wire from the ignition sensor to one of the main ground studs on the engine block. I don't see where there'd be a ground on the distributor itself?

 

[Bondo]

There's a ground wire from the ignition sensor to one of the main ground studs on the engine block. I don't see where there'd be a ground on the distributor itself?

Ayuh,.... Welcome Aboard,.... Read this,...

 

[edsonfarm]

I don't see a place to attach one, but I put on a jumper wire to ground it and no change.

 

[Fun Times]

Either try swapping or start ohms checking the wiring harness to/between the ICM, distributor sensor and coil. It should resemble like what is seen on the last page (3) of the following link, http://marinemechanic.com/merc/dist...lder/thnbolt-IV-module-replacements_EN_14.pdf

Also check for wire pin connectors backing out of the connectors and wiggle all the wires both while ohms checking and the engine running.

 

[edsonfarm]

Swapped all the wires from the ignition system with new wiring and still no change. I'm at a loss of what else it could possibly be at this point. Even installed a brand new distributor tonight to be certain it wasn't something in there.

 

[dingdongs]

Shift interuptor micro switch faulty perhaps

 

[achris]

Shift interuptor micro switch faulty perhaps

Bravo drive. No shift interrupt.

 

[Fun Times]

So you're loosing spark and a flashing test light when connected to negative side of the coil is consistent or intermediate?

You've tried a new,
1, Coil,
2, Distributor sensor,
3, Different Ignition control module,
4, Different ICM wiring harness, that connects between the coil, ICM and Distributor housing/sensor ?,
5, Different distributor housing assembly. Was it complete with a new sensor already installed???
6, Added ground wire to distributor housing with good contact using the longer screw from the ignition sensor? The dizzy hold down bolt is tight and seated correctly?
7, By passed cannon plug by jumping the 3 wires 5, 6 & 7 like seen in this video, https://www.youtube.com/watch?v=NUoXFIo2Ab8#t=25
8, Put direct 12v power to the positive side of the coil
9, Put additional 12v positive power to the TB5 ignition module,
10, Replaced the cap and rotor,
11, Installed new grounds... To the engine block I assume??? One of the ICM mounting bolts should have a ground wire too?
12, Anything else I missed that you've tried?

Carefully try swapping and looking for leaking spark at the high tension coil wire boot... Also turn/crank the engine over in the dark looking for a blue flashing light show at all spark plug wire connections.

Broken porcelain on the spark plugs could sort out the coil. Might want to try new spark plugs.
Plug wires could ultimately create a short at the coil as well if leaking/damaged, etc..

Put your test light to the white/green wire at the dissy sensor and see if it flashes without loosing it.
Ensure the white/red wire has constant 12v at the dizzy using a multimeter.

Can you post an good overall photo of the ignition related parts towards the rear of the engine? Also I'd like to see all the wiring on the starboard side right around the shift plate assembly if possible.

Being that you have a bravo drive, this engine doesn't utilize the Mercruiser shift interrupter switch so you should to may have a jumper plug that may be used as a bypass and if it's missing or failing/damaged it could affect the overall ignition system.

Ensure no ground wire at the rear of the engine are pinched, chaffed, cut, loose, damaged, etc. etc..

If all this seems/turns out good, you're back to ohms checking wires.

 

[edsonfarm]

So you're loosing spark and a flashing test light when connected to negative side of the coil is consistent or intermediate? THE DIMMING OF THE TEST LIGHT OCCURS WHEN THE MOTOR DROPS RPMS WHICH IS INTERMITTENT

You've tried a new,
1, Coil, YES
2, Distributor sensor, YES
3, Different Ignition control module, YES, TWO
4, Different ICM wiring harness, that connects between the coil, ICM and Distributor housing/sensor ?,
I CUT IN NEW WIRES FOR ALL OF THESE TO GET AROUND THE ENGINE HARNESS
5, Different distributor housing assembly. Was it complete with a new sensor already installed???
YES IT WAS NEW WITH A NEW SENSOR.
6, Added ground wire to distributor housing with good contact using the longer screw from the ignition sensor? The dizzy hold down bolt is tight and seated correctly?
ITS THE NEWER SENSOR WITH NO SCREW TERMINALS SO THE GROUND IS BUILT IN AND GOES RIGHT TO A GROUND STUD ON THE ENGINE BLOCK. THERE ISNT ANOTHER SPOT FOR A SECOND GROUND AND NONE IS MENTIONED IN THE WIRING DIAGRAM, BUT I RAN ANOTHER WITH NO CHANGE.
7, By passed cannon plug by jumping the 3 wires 5, 6 & 7 like seen in this video, https://www.youtube.com/watch?v=NUoXFIo2Ab8#t=25
I DID THIS WITH A BREAKOUT BOX TO ELIMINATE THE BOAT WIRING.
8, Put direct 12v power to the positive side of the coil. YES
9, Put additional 12v positive power to the TB5 ignition module, YES BUT ITS A TB IV
10, Replaced the cap and rotor, YES
11, Installed new grounds... To the engine block I assume??? One of the ICM mounting bolts should have a ground wire too? YES, NEW GROUNDS TO THE BLOCK. THERE ISNT A ICM MOUNTING BOLT GROUND. THESE ARE THE NEWER PLASTIC MODULES.
12, Anything else I missed that you've tried?
NOT THAT I CAN REMEMBER.

Carefully try swapping and looking for leaking spark at the high tension coil wire boot... Also turn/crank the engine over in the dark looking for a blue flashing light show at all spark plug wire connections.
DID THIS ALREADY.

Broken porcelain on the spark plugs could sort out the coil. Might want to try new spark plugs.
Plug wires could ultimately create a short at the coil as well if leaking/damaged, etc..
PLUGS LOOK FINE OUTSIDE BUT I HAVENT PULLED THEM ALL.

Put your test light to the white/green wire at the dissy sensor and see if it flashes without loosing it.
Ensure the white/red wire has constant 12v at the dizzy using a multimeter. I THINK I DID THIS THIS EARLIER THIS WEEK AND IT CHECKED OUT FINE.

Can you post an good overall photo of the ignition related parts towards the rear of the engine? Also I'd like to see all the wiring on the starboard side right around the shift plate assembly if possible.
REALLY TOUGH TO GET A DECENT PICTURE SINCE THERES NOT MUCH. SPACE IN THERE


Being that you have a bravo drive, this engine doesn't utilize the Mercruiser shift interrupter switch so you should to may have a jumper plug that may be used as a bypass and if it's missing or failing/damaged it could affect the overall ignition system.
THE WIRING DIAGRAM SAYS THATS A GREY/WHITE WIRE. I HAVE A BLACK/WHITE WIRE BUT NOT GREY/WHITE. IS THIS THAT LEAD OR DOES MY ICM HARNESS JUST NOT HAVE THE WIRE SINCE I DONT HAVE THE INTERRUPTOR?

Ensure no ground wire at the rear of the engine are pinched, chaffed, cut, loose, damaged, etc. etc..
ALL GROUNDS ARE GOOD

If all this seems/turns out good, you're back to ohms checking wires.
IM NOT SURE WHAT TO OHMS TEST SINCE IVE REPLACED THOSE WIRES TO TEST THEM FOR FAULTS AND NOTHING CHANGED.




[/QUOTE]

 

[Badfish95]

My recommendation will probably bring out the naysayers but here goes.... Replace the TB IV distributor with a Marine ready to run distributor from MSD, Mallory or Pertronix.

I fought the intermittent issue on my boat for nearly a year. Swapped it for the MSD 83606 and it's been running like a top ever since... Please don't buy a $400.00 ECM(like I did) just to have it go bad as well.

 

[Fun Times]

So you're loosing spark and a flashing test light when connected to negative side of the coil is consistent or intermediate? THE DIMMING OF THE TEST LIGHT OCCURS WHEN THE MOTOR DROPS RPMS WHICH IS INTERMITTENT

How often does it seem to do this?
Engine cold, hot or both?
It does this at idle RPM (650) or higher?
On the garden hose or in the water only?

4, Different ICM wiring harness, that connects between the coil, ICM and Distributor housing/sensor ?,
I CUT IN NEW WIRES FOR ALL OF THESE TO GET AROUND THE ENGINE HARNESS

Cutting/splicing in new wire to old wire still leaves the wire connectors terminals pins which could be defective, corroded, backing out/off the terminals of the unit they plug into.

9, Put additional 12v positive power to the TB5 ignition module, YES BUT ITS A TB IV

Mercruiser went to Thunderbolt V (5) starting in 1994 plus your description of a plastic box helps confirm you'll have the TB V. The Thunderbolt IV (4) systems where used before 1994. THUNDERBOLT V IGNITION SYSTEM Here are 2 TB 5's that would fit for your engine model, http://www.ebay.com/sch/i.html?_fro...C0.H0.X861254T02.TRS0&_nkw=861254T02&_sacat=0

11, Installed new grounds... To the engine block I assume??? One of the ICM mounting bolts should have a ground wire too? YES, NEW GROUNDS TO THE BLOCK. THERE ISNT A ICM MOUNTING BOLT GROUND. THESE ARE THE NEWER PLASTIC MODULES.

While I can't seem to locate the wire going to one of the ICM mounting bolts looking at many online photos, for some reason I could of sworn there is/was a black ground wire that goes to it on the bottom.. It's even mentioned in a Mercruiser service manual seen on page 4B-12 under Ignition control module as a case ground, http://www.boatinfo.no/lib/mercruiser/manuals/mercruiser24.html#/296
But I think they meant to say wire number 11 not 12. Also read the Note #1 covering the shift cut out switch between Alpha and Bravos, http://www.boatinfo.no/lib/mercruiser/manuals/mercruiser24.html#/298
You're wires would be White and the other Black. Are they jumped together using a cap on both engines? If so, try swapping them (the caps) just in case.

Broken porcelain on the spark plugs could sort out the coil. Might want to try new spark plugs.
Plug wires could ultimately create a short at the coil as well if leaking/damaged, etc..
PLUGS LOOK FINE OUTSIDE BUT I HAVENT PULLED THEM ALL.

:whip::encouragement:

Put your test light to the white/green wire at the dissy sensor and see if it flashes without loosing it.
Ensure the white/red wire has constant 12v at the dizzy using a multimeter. I THINK I DID THIS THIS EARLIER THIS WEEK AND IT CHECKED OUT FINE.

While pretty much confirmed with a new distributor system, It seems you're losing connection between the ICM and coil which may be a unseen wire connector issue.
The nuts on the coil aren't overly painted possibly preventing a bad connection.

Can you post an good overall photo of the ignition related parts towards the rear of the engine? Also I'd like to see all the wiring on the starboard side right around the shift plate assembly if possible.
REALLY TOUGH TO GET A DECENT PICTURE SINCE THERES NOT MUCH. SPACE IN THERE.

This must be pretty tough to get in there and do all the testing that you've been doing which is a lot. There doesn't seem to be much left to check. Guess I was just hoping to see if maybe we can see something out of place to suggest trying next.

Being that you have a bravo drive, this engine doesn't utilize the Mercruiser shift interrupter switch so you should to may have a jumper plug that may be used as a bypass and if it's missing or failing/damaged it could affect the overall ignition system.
THE WIRING DIAGRAM SAYS THATS A GREY/WHITE WIRE. I HAVE A BLACK/WHITE WIRE BUT NOT GREY/WHITE. IS THIS THAT LEAD OR DOES MY ICM HARNESS JUST NOT HAVE THE WIRE SINCE I DONT HAVE THE INTERRUPTOR?

Like mentioned above, See the service manual links for this one.

If all this seems/turns out good, you're back to ohms checking wires.
IM NOT SURE WHAT TO OHMS TEST SINCE IVE REPLACED THOSE WIRES TO TEST THEM FOR FAULTS AND NOTHING CHANGED.

Thinking any two points that you can find for the wiring you are working with. Such as the from the White/Red/Green Dissy sensor plugs to the ICM connector.
The ICM purple and black wire/s to the engine side cannon plug. Coil to ICM connector.

 

[edsonfarm]

How often does it seem to do this?
Engine cold, hot or both?
I HAVENT FOUND A CLEAR
DIFFERENCE AS TO WHEN IT HAPPENS. IT DOESNT SEEM TO BE TEMPERATURE DEPENDENT. ITS RANDOMLY INTERMITTANT BUT HAPPENS EVERY 5-10 SEC AT HIGHER IDLE.
It does this at idle RPM (650) or higher?
BOTH BUT MUCH MORE AT HIGHER RPMS.
On the garden hose or in the water only?
ONLY IN THE WATER- ITS A 33' BOAT AND STAYS IN THE SLIP ALL SUMMER.

Cutting/splicing in new wire to old wire still leaves the wire connectors terminals pins which could be defective, corroded, backing out/off the terminals of the unit they plug into.
TRUE. I DID WIGGLE ALL OF THESE TO SEE IF I COULD GET A RESPONSE AND COULDNT.

Mercruiser went to Thunderbolt V (5) starting in 1994 plus your description of a plastic box helps confirm you'll have the TB V. The Thunderbolt IV (4) systems where used before 1994. [url]http://www.boatfix.com/merc/bullet/94/94_02.PDFTHUNDERBOLT V IGNITION SYSTEM[/URL] Here are 2 TB 5's that would fit for your engine model, http://www.ebay.com/sch/i.html?_fro...C0.H0.X861254T02.TRS0&_nkw=861254T02&_sacat=0
THE PLASTIC ENGINE COVERS SAY TB IV (I KNOW THAT DOESNT MEAN A LOT. I THOUGHT THE TB Vs MOUNTED ON THE DISTRIBUTOR- MINE ARE ON THE RISERS. THAT IS MY MODULE, THOUGH.

While I can't seem to locate the wire going to one of the ICM mounting bolts looking at many online photos, for some reason I could of sworn there is/was a black ground wire that goes to it on the bottom.. It's even mentioned in a Mercruiser service manual seen on page 4B-12 under Ignition control module as a case ground,
I HAVE SEEN THIS TOO, BUT IT THINK IT WAS FOR THE OLDER METAL-CASED MODULES. http://www.boatinfo.no/lib/mercruiser/manuals/mercruiser24.html#/296
But I think they meant to say wire number 11 not 12. Also read the Note #1 covering the shift cut out switch between Alpha and Bravos, http://www.boatinfo.no/lib/mercruiser/manuals/mercruiser24.html#/298
You're wires would be White and the other Black. Are they jumped together using a cap on both engines? If so, try swapping them (the caps) just in case.
I HAVE A WHITE WIRE WITH A BLACK STRIPE LEAVING THE ICM. I *think* THATS THE SHIFT CUT OUT WIRE EVEN THOUGH THE MANUAL SAYS ITS SUPPOSED TO BE WHITE/GREY. IT HEADS OVER TOWARD AND COMES OUT NEXT TO THE TIMING LEAD. IT PLUGS DIRECTLY INTO A BLACK (GROUND?) WIRE THERE. INTERESTINGLY IF I UNPLUG IT FROM THE BLACK IT STILL STARTS AND RUNS THE SAME. WHEN I CUT IT AT THE ICM THE MOTOR WOULD TURN OVER BUT NOT START. IT MUST GO SOMEWHERE ELSE THAT IM NOT FINDING.

WHAT DOES NOTE #1 SAY ABOUT THE SHIFT INTERRUPT SWITCH? I CANT OPEN THOSE LINKS ON MY PHONE.

:whip::encouragement:

While pretty much confirmed with a new distributor system, It seems you're losing connection between the ICM and coil which may be a unseen wire connector issue.
The nuts on the coil aren't overly painted possibly preventing a bad connection.
THEY ARENT- VERY CLEAN.

This must be pretty tough to get in there and do all the testing that you've been doing which is a lot. There doesn't seem to be much left to check. Guess I was just hoping to see if maybe we can see something out of place to suggest trying next.

Like mentioned above, See the service manual links for this one.

Thinking any two points that you can find for the wiring you are working with. Such as the from the White/Red/Green Dissy sensor plugs to the ICM connector.
The ICM purple and black wire/s to the engine side cannon plug. Coil to ICM connector.[/QUOTE]

 

[edsonfarm]

At this point I'm leaning toward either replacing the whole wiring harness or going to the ready-to-run distributor that was suggested to eliminate the old ignition system. I'm just not sure which makes more sense first.

 

[achris]

I know what I would do. I would make a simple test jig to test each compnent. To test the module, I'd make a simple relay board to simulate the sensor. To test the sensor, just a 'flag' to go passed the sensor 'window' and watch with an LED to make sure the sensor is operating properly. Coils are a simple replacement, so cheap. If the module performs, and the sensor performs and a the replacement coil didn't change anything, start looking at the harness. Simple enough to then test each comopnent through the harness with the test jigs already set up.....

But, that's just me, I hate 'throwing money' at problems in the hope of finding it early.....

Chris.......

 

[Badfish95]

You can buy two Pertronix complete ignition systems for the price of one ICM(Better hope it's good out if the box).

Some throw money and others throw time, I guess it depends on what you have more of...

 

[achris]

Have a good read of that ^. It's a mechanical advance.

 

[edsonfarm]

I ended up ordering one of these to try https://m.summitracing.com/parts/msd-83606
Unfortunately with the 4th being next weekend I had less "time" right now. I've been through just about everything with no success so far. I've checked and/or swapped every part in the ignition system but the wiring harness, but that distributor setup should eliminate the potential problem parts of that too. With time so short I can't risk just replacing the harness and then not having time to get a new distributor mailed if it doesn't work. We'll see what happens.