Problem with a Mercruiser 5.7 V8 LX

mikoman

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Oct 2, 2022
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6
Hay hay people,
I have a problem with a Mercruiser 5.7 V8 LX. The engine suddenly stopped after a quick decrease in speed, and before I've came to start figuring out what went wrong with it, the owner tried with several different mechanics to resolve the issue but with no luck. Prior to my arrival the ignition distributor was swapped along with the coil and there was no luck with starting the engine. I've discovered a large drop in voltage while cranking the engine (from 12 volts to meager 6-7 volts) which causes a weak yellow spark. So i've decided to bring 12 volts straight from another battery to the purple wires that lead to the ignition module, coil and knock sensor.
To my surprise the engine cranks but now i get no spark at all. Does anyone have a clue what might cause this issue or rather whats between the ignition switch and the main connector for the engine?
(P.S. i've already changed the spark plugs (2 were shorted), i've removed the shift switch, i've set the correct position for the ignition distributor and the knock sensor probably works too. Also the engine only sparks 2-3 times before stopping with the sparking sequence but continues to crank)
 

Scott06

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Apr 20, 2014
Messages
3,906

try to troubleshoot the ignition system. i think it lists the voltages required in there.
 

mikoman

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Oct 2, 2022
Messages
6

try to troubleshoot the ignition system. i think it lists the voltages required in there.
A very informative post that is much easier to read rather the actual manual. But sadly that dosen't help me much in figuring out the problem of why i get no spark when i bring a direct 12 volt cable to one of the purple wire or all of them together (since they all split from 1 purple wire after the main engine connector). (the purple wires work with 12 volts)
 

mikoman

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Oct 2, 2022
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There's a troubleshooting guide posted in the linked thunderbolt write up
Thank you for your reply as well,
as stated previously I am well aware of the troubleshooting method and the link. And I truly doubt that the engine will turn on with a weak spark that is caused by the voltage drop. Looking at the scheme that is provided in the link nothing suggests what would disrupt the ignition if a direct 12 volt supply is hooked onto the purple cable (I want to bypass the key switch and everything that comes with it but it should be just a simple switch)
 

stresspoint

Petty Officer 2nd Class
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Sep 19, 2022
Messages
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guessing you have tripped a sensor , the control module is the cause of your no spark when 12v is applied to the coil .

don't ask me how 2 plugs got zapped ????.most likely the reason for weak sparks. module failure but that rarely happens.
 

dubs283

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Jul 27, 2005
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Looking at the scheme that is provided in the link nothing suggests what would disrupt the ignition if a direct 12 volt supply is hooked onto the purple cable (I want to bypass the key switch and everything that comes with it but it should be just a simple switch)
Third step from the top suggests checking voltage, in your case it's not 0 volts but low voltage. Treat it the same as 0 volts and inspect wiring harness, instruments and wiring, battery and connections, etc...

My WAG is your issue is a weak battery/poor connections coupled with a possible faulty ignition sensor in the distributor.

Your first post mentions a new distributor, did you replace the entire distributor assy or just components of it?
 

mikoman

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Oct 2, 2022
Messages
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Third step from the top suggests checking voltage, in your case it's not 0 volts but low voltage. Treat it the same as 0 volts and inspect wiring harness, instruments and wiring, battery and connections, etc...

My WAG is your issue is a weak battery/poor connections coupled with a possible faulty ignition sensor in the distributor.

Your first post mentions a new distributor, did you replace the entire distributor assy or just components of it?
changing the distributor with the pickup now would be jumping the gun, in my first post the mechanics tried a different one and returned the old one back. I did managed to turn on the engine once after a few days but now again its dead. The batteries might be faulty since it lost all of its power in one or two days of time but that could also be the cause of a appliance that is draining power 24h. I suspect the cables or batteries to be faulty but in order to be 100% sure and to avoid disassembling half of the ship i went with the route of directly connecting the purple cable with a fresh battery to insure a normal voltage to the distributor, while an other battery will crank the engine. But to me it does not make any sense of why would't the spark park spark when it was connected directly to the key switch and not when its directly connected with a fresh battery.
ssing you have tripped a sensor , the control module is the cause of
This does shed some light and an idea. Maybe the module wont work well with low voltage and won't react to tripped sensors while with 12 volts it works as it is supposed to and reacts to them. (an guess with some light doubt). I guess my next step would be to connect fake signals to the module.
 

dubs283

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changing the distributor with the pickup now would be jumping the gun,
Can say with absolute certainty the number one likely cause if no/weak/inconsistent spark on a thunderbolt is the ignition sensor in the distributor. Not saying that is your issue or to just arbitrarily replace it however there is a step in the troubleshooting guide for testing the sensor

You need to first figure out why/where the voltage drop is. Looking at the ignition prior to rectifying the voltage issue won't help
 

Bondo

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Messages
69,451

Can say with absolute certainty the number one likely cause if no/weak/inconsistent spark on a thunderbolt is the ignition sensor in the distributor. Not saying that is your issue or to just arbitrarily replace it however there is a step in the troubleshooting guide for testing the sensor

You need to first figure out why/where the voltage drop is. Looking at the ignition prior to rectifying the voltage issue won't help
Ayuh,.... A good place to start is cleanin' to shiny metal. both ends of both battery cables,.....
 

QBhoy

Admiral
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Mar 10, 2016
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6,969
Ignition sensor or…hate to say it…your safety lanyard has activated. This (on mine at least) will allow cranking but no ignition
 

poconojoe

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Sep 10, 2010
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That ignition troubleshooting chart is a very good step by step diagnostic tool.
I would return everything to stock and go through that chart. If it's an ignition problem, you will find it with that chart.

My first thought was a weak battery or corroded terminals/wires, but there could be other things at play.
 

mikoman

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Oct 2, 2022
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I am starting to believe that my ignition module might have become faulty.

What I did now was very simple, I've disconnected the coil from the distributor , disconnected the "kill switch" a.k.a shift switch, checked the knock sensor (it puts out steady 8 volt), disconnected the tachometer and disconnected the ignition sensor. After that i've connected the battery to the coil, ignition module and the knock module, plugged a spark plug at the end of the coil and used the green cable of the pick up and acted the part of the ignition sensor. What i've discovered is that the coil works fine, the spark plug dishes out a mighty bright white light but it only does so for 2-3 seconds and than sparks stop happening, only when i would quickly disconnect the module from the battery and reconnect it the same thing would happen, good spark but for 2-3 seconds.

And yes, i did follow the chart but it didn't help much
and no the ship dose not have a safety lanyard

So my next step is to test this module on another engine, or find another one to see if the module was really the cause.
As well i ll try to turn on the engine with a more simple coil from another engine for the time being.
 

Scott06

Captain
Joined
Apr 20, 2014
Messages
3,906
I am starting to believe that my ignition module might have become faulty.

What I did now was very simple, I've disconnected the coil from the distributor , disconnected the "kill switch" a.k.a shift switch, checked the knock sensor (it puts out steady 8 volt), disconnected the tachometer and disconnected the ignition sensor. After that i've connected the battery to the coil, ignition module and the knock module, plugged a spark plug at the end of the coil and used the green cable of the pick up and acted the part of the ignition sensor. What i've discovered is that the coil works fine, the spark plug dishes out a mighty bright white light but it only does so for 2-3 seconds and than sparks stop happening, only when i would quickly disconnect the module from the battery and reconnect it the same thing would happen, good spark but for 2-3 seconds.

And yes, i did follow the chart but it didn't help much
and no the ship dose not have a safety lanyard

So my next step is to test this module on another engine, or find another one to see if the module was really the cause.
As well i ll try to turn on the engine with a more simple coil from another engine for the time being.
I think when u get to that point in the chart it says to try another coil, if that doesn’t work replace module . Issue is TBV while a great ignition system, the module is pricey and now for some getting tough to find. You may find some on ebay, i got two that way when i repowered…otherwise folks replace the entire distributor and module with a delco est set up as it is generally cheaper then the TB module.
 

Scott Danforth

Grumpy old guy who plays with boats
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Jul 23, 2011
Messages
41,876
did you troubleshoot?

most of the time, its the trigger assembly. especially if you have the old-design 2-wire ignition trigger (labeled as "sensor" in the diagrams)

its a $60 part (was $40 2 years ago)
 

poconojoe

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Sep 10, 2010
Messages
1,954
did you troubleshoot?

most of the time, its the trigger assembly. especially if you have the old-design 2-wire ignition trigger (labeled as "sensor" in the diagrams)

its a $60 part (was $40 2 years ago)
$60!!
I think I paid $42 maybe 3 years back.

Mine was the original (2003) exposed printed circuit board. It was all corroded.
The new one is nice. Completely sealed.

Fixed the problem for me.
That troubleshooting flow chart was a big help.
 
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