5.7 MCM Crank but no start

[silverseal99]

2003 Mercruiser 5.7L Gen+ Alpha/Bravo 2 bbl Mercarb with Thunderbolt V ignition with knock sensor. Ser 0M634166 Mod 4312025RS mated to a Bravo 3 outdrive in a 25 ft sport fisherman.

Issue: Engine cranks, but no spark following extensive engine and hull maintenance.

Background: Bobtail package with less than 50 hours, sat on a trailer for 16 years. Engine ran before latest maintenance (pulled heads, new manifolds and risers, replaced coil and all oil/water/knock sensors). Pulled Ignition Control and Knock Sensor modules and set aside (dry, secure storage) while doing engine mechanicals. Reinstalled everything and attempted test run in driveway. Now it cranks well, but no spark.

Easy stuff: Battery is new and fully charged. Dead man switch is in “run” position, looking at 12.5 volts from the ignition switch with good continuity across the terminals. I’m seeing 12.5 volts at coil (+) terminal with ignition switched on. The gray wire from coil to tach is not grounded. All electrical connectors, wiring and terminals are in good condition and appear to be properly connected including the ground to the ignition control module bracket. No extraneous wires remain except one pre-existing black wire with a male spade connector in the vicinity of the carburetor (I suspect this may be merely a ready ground for setting the timing). I’ve tried a new distributor cap and rotor as well as a known good cap and rotor.

Harder stuff: I’ve run the Thunderbolt V troubleshooting regimen (page 1C-9, manual 24) multiple times with a new coil, a known good used coil, and two different Ignition Control Modules (ICM). I have 12.5 volts to the purple wires on both the knock control module and the ICM. My spark gap tester is known good and I have a good quality VOM. Sensor wheel is not touching the sensor in the distributor. Spec for resistance across the sensor is “not less than 100 ohms” and measured near infinite which bothers me somewhat. Knock sensor, oil pressure switch, water temperature sensor, etc are all new and connected.

What am I missing? I’m looking for a single component (other than the dead man switch) that would kill power to the ignition but am not seeing any. Either something simple that I overlooked, or I have two bad ICMs (or a bad sensor in the distributor). Grandkids are coming on the 20[SUP]th[/SUP] and want to go out in the boat…

Thanks in advance!

 

[alldodge]

Looks like you covered the bases.
Remove the Dist cap and crank to see if the rotor is rotating

When the motor is cranking do you see around 300 rpm on the tach?

When you run thru the TB5 troubleshooter, what does it point to as the failure?

 

[silverseal99]

Alldodge, thanks for your quick response:
1. Rotor turns fine.
2. Tach bumps down (sub-zero rpm) when I crank.
3. TB5 Troubleshooter says "Replace Ignition Module."

The "new-in-the-box" ignition module I bought off ebay was not new, not in a box so I learned nothing from swapping it out with mine.

I'm shifting focus to the tach which was replaced in 2003. Will re-check end-to-end continuity of the gray wire with a VOM to verify "not grounded" and clean up terminals and begin troubleshooting the tach. It seems that would be a likely source of a single point failure for the ignition. Any advice on what to look for (resistance values, etc) would be appreciated.

 

[alldodge]

Remove the gray tach wire from the negative side of the coil and see if it fires. It doesn't need to run and if the tach is grounding out the coil it won't fire

 

[silverseal99]

1. Removed gray wire from neg side coil, cranked engine, no fire.
2. Tested spark tester to ensure function--checks good.
2. Isolated and checked continuity of gray wire end-to-end - checks open?!?, not grounded.
3. Bypassed gray wire (Jumpered new wire from neg coil to tach) and cranked, tach the same as with gray wire (LCD displays but needle moves to sub-zero vice 300-ish rpm).

The gray wire checking open was a surprise as was bypassing the gray wire with no change in how the tach responded. Perplexing.

 

[alldodge]

Guess your back to the module if your sure its not the pickup
The TB5 is a great system, but once it goes bad most go to Delco EST kit

 

[silverseal99]

I would be interested in a bolt-on total replacement as the Mercruiser Ignition Control Module alone new is over $700 retail. I see a Sierra Marine Delco EST Conversion Kit 18-5514 with a new style coil and a 18-5520 with a traditional style coil. Both are listed for V8 applications. Which would you recommend?

 

[alldodge]

I would recommend a Real AC Delco EST and not Sierra, but if your set on Sierra then the 18-5514

Best part, is if the Sierra craps out some day, you can install real GM parts later

There is also a United Ignition

 

[silverseal99]

On it, thanks.

 

[silverseal99]

How do the EST kits deal with the knock sensor? Incorporate it or abandon it? If the former, do you know how? I don't see any discussion of knock sensor at any of the sites so far. Thanks again.

 

[alldodge]

The EST does not come with knock sensor module. All the motors which went to EST in later years also went to EFI/MPI and the knock module became part of the ECM

 

[silverseal99]

Awesome, thanks. Will get the AC DELCO EST on order today and update when I get it installed and checked out.

 

[silverseal99]

Alldodge,

Thanks for the interest and sage advice. I got the fire back.

Issue was broken tachometer wire (gray wire) in the engine side of the main cable loom.

Three things happened with this fault:
1. Crank, but no start, no fire at the coil or to the plugs.
2. The Ignition Control Module (ICM) failed the last test of the protocol on page 1C-9 of Service Manual 24 indicating a false requirement to replace the ICM.
3. My Faria 54704 tach failed the Faria troubleshooting routine indicating a false requirement to replace the tach.

The gray wire runs from the neg terminal of the coil to pin 2 of the harness plug but also feeds a back-spliced gray wire to the ICM (splice was about 2 inches engine side of the plug--I had to dissect the engine harness and trace the gray wires from both the ICM and the coil back to the plug). The gray wire was broken at the splice in a fashion such that all three gray wires were disconnected, one from the other (which is why I couldn't trace one leg or the other back to the plug).

Once I identified the fault and spliced the wires back together, I passed the troubleshooter, she fired right up, and the Faria tach operated per usual.

I appreciate your quick assistance and cogent recommendations. My hunch and search for a single point of failure proved correct. With your assistance, I avoided thousands of $$ chasing this 10 cent solution. This web site is value added for us shade tree mechanics.

I might also add that if anyone wants to convert to an EST ignition, good luck. There aren't any kits available that I could find (and I checked or called about 8 vendors)--something about "China..." Advice to anyone wanting to buy one: call the vendor and have them lay their hands upon it and swear to the Almighty that they have it in stock, ready to ship before you put your credit card on the line. If they're back-ordering, you won't get one anytime soon.

Cheers!