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Ignition sensor ohms test

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JasonSouliere

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Has anyone tested an ignition sensor a Thunderbolt IV?

What is the ohms reading for a good working sensor?

My specific boat is a 1988 Mercruiser 5.7 Thunderbolt IV.

My original sensor reads very high, and my ordered-from-Amazon sensor also reads very high. Maybe it’s doa, or my meter is not accurate enough.

Thank you.

Jason
 
IMG_6476.jpeg
 
Run a piece of steel thru the gap and see what or if there is a change.
Its a magnetic sensor with sends a high or low pulse tp the timing control module
 
a good search engine will get you the thunderbolt IV troubleshooting flowchart with a few key clicks...
 
kghost said:
Run a piece of steel thru the gap and see what or if there is a change.
Its a magnetic sensor with sends a high or low pulse tp the timing control module
Click to expand...
You sir, are smart. My engineer at work and I just talked about the same thing this aft. Will be trying it shortly.
 
makomark said:
a good search engine will get you the thunderbolt IV troubleshooting flowchart with a few key clicks...
Click to expand...
Yep. I can list all those addresses here. Seen most of them. AI says between 700 and 1200 ohms but I have 2 sensors here and neither achieve that reading. I’m thinking of just ordering a complete Thunderbolt IV replacement module and sensor. At my wits end.
 
before apending more money, whats your issue?

To test a thunderbolt ignition there is an easy to follow procedure.

I am on my phone so no documents, but i and others have the procedure available and can uplaod here tomorrow
 
before apending more money, whats you issue?

To test a thunderbolt ignition there is an easy to follow procedure.

I am on my phone so no documents, but i and others have the procedure available and can uplaod here to
 
Here
 

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kghost said:
before apending more money, whats your issue?

To test a thunderbolt ignition there is an easy to follow procedure.

I am on my phone so no documents, but i and others have the procedure available and can uplaod here tomorrow
Click to expand...
Hey there. Thanks so much for taking the time.
Long story short, took boat out of winter storage, connected to garden hose, fired right up. Idled smooth as silk and let it run until temp came up to normal. Oil psi normal. Revs to 3000 smoothly, back down to idle smoothly. Smoooooth as silk. Shifter tested perfectly, ready for the water.
Until…
I had just reached for the key to shut her down when it went into a fit. Ran roughly, could not keep it running.
Did the obvious. Drained fuel tank, emptied lines and carb, changed fuel filter/water separator, cleaned distributor terminals and rotor. I did not check each wire individually or check plugs. Dist solid in place, not loose. Base timing couldn’t have changed. Well, it didn’t.
Remember, it was running PERFECTLY, then out of the blue seconds later started running roughly - something has failed. My guess is electronic so hall effect sensor or ign module.
If I go outside right now it’ll start right up with a flick of my wrist but will run like a bag of crap.

That’s why I’m looking into ign module or ign sensor. I did replace the ign sensor with a cheap Amazon part but no change. Will return it for a genuine part. But before I do that I want to know the ohms value of the sensor. Is it a simple coil? Or is there a magnetic micro switch? How do they work?
 
First off, It starts and runs. That says ign parts are working.
Running crappy could be many things.

Disconnect shift interupter switch, white with green stripe from small terminal block and keep away/isolate from any ground while testing.

Disconnect Tach wire on - side of coil (grey wire). Tachometer issues can cause ignition to act stupidly.

Typically when pick up sensor goes bad it wont spark/start.
A new OEM are fully encapsulated and have three wires. Two wires have bullit connectors and the third, Black has a ring terminal to be connected to a ground lug.

Typically when timing module fails timing does not advance so you would loose high speed under load.

Cleaning the dist cap may not be enough, and carefully inspect rotor at the tang/rivit location. Could be weak contact. Make sure the rotor assembly is all the way down and look closely at the center tap of cap. If that is worn down you may need a new cap.
 
Thanks. I’ll go thru those items shortly, especially the tach - I remember last year the tachometer was acting “odd” and reading 3000 rpm when we were cruising smoothly at about 1800, but never caused engine to misfire. I just tapped the gauge and it came back down.
Also, years ago when I replaced shift lever I never reconnected the shift interrupt switch. Been running without it for 5 years now. I’ll eyeball those stray wires today as well.
 
How old is the disti cap.... It can cause an engine to go from O to crap in just mins. I can never get a third season out of one. REplace every other year or annually if I'm flush.
 
sandkicker said:
How old is the disti cap.... It can cause an engine to go from O to crap in just mins. I can never get a third season out of one. REplace every other year or annually if I'm flush.
Click to expand...
I’ve never changed it, and had the boat for about 8 years. I did remove it and clean the terminals and rotor+rotor terminal until they looked new. Fired right up. But i think you’re right, for the price I should replace the cap and rotor anyhow.
Thanks for taking the time and responding.
 
As an update to all who’ve chimed in to help with their advice - thank you.
I did order an all-in-one dist/ign mod/coil unit, dropped it in and engine fired right up straight away…..but still ran the exact same. So I abandoned troubleshooting it, brought it down to the marina and got her into the water. Fired right up, ran crappy but engine had lots of will to get us to the slip just off idle. About 3/4 of the way to the slip the engine started running smooth as silk like it did before all this **** happened! Instantly began running silky smooth, quiet, and I have no idea what’s going on other than it’s the power of Christ lol. Or more likely something worked its way thru the carb. 🤷‍♂️ Still stumped, but fingers crossed it keeps up.
 
JasonSouliere said:
As an update to all who’ve chimed in to help with their advice - thank you.
I did order an all-in-one dist/ign mod/coil unit, dropped it in and engine fired right up straight away…..but still ran the exact same. So I abandoned troubleshooting it, brought it down to the marina and got her into the water. Fired right up, ran crappy but engine had lots of will to get us to the slip just off idle. About 3/4 of the way to the slip the engine started running smooth as silk like it did before all this **** happened! Instantly began running silky smooth, quiet, and I have no idea what’s going on other than it’s the power of Christ lol. Or more likely something worked its way thru the carb. 🤷‍♂️ Still stumped, but fingers crossed it keeps up.
Click to expand...
check that you are not loosing 12 v supply to the positive side of coil - purple wire on engine harness. Wonder if a bad connection or corrosion has a voltage drop considering you have two completely different ignitions TB and I assume Delco EST. the only reason to pull the TB ignition is if the ignition module (NLA or $$$) goes, otherwise it is an excellent system.

I would do this before and after it acts up to rule out something squirrely and if ok then rebuild the carb. Is choke fully opening and what do the plugs look like. And yes makes sense to replace dizzy cap before 8 years.
 
I have no reason to pull off the TB system, and have kept the original distributor in a box in case I need it again. I consider the TB, original dist/cap/rotor not any part of the reason it was running poorly. There is however a coil (transformer?) looking thing on the port side just under the engine, mounted on the engine beam. What is that? I can send a pic in a couple days when I’m down there again.
 
if its like the attached image, its the starter's slave solenoid...
 

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