NO SPARK!!!! Thunderbolt IV headscratcher

[Libertychartersto]

I have a 1994 302, merc 5.0L with thunderbolt IV ignition.
-I have 12 V at the + on the coil
-I have 12 V at the red and white wire behind the distributor.
-I have disconnected the gray tac wire
-I have disconnected the green/white wire from the shift Int switch
-I have a gap light at the coil, no spark.
-If I take the green/white wire and tap it to a ground I get spark. This points to a bad ignition sensor.
-replaced the ignition sensor with no luck.
-I suspect a faulty ignition sensor, so I hooked up the red/white wire straight to the POS on battery. The black wire to the NEG post on the battery. I put the POS lead of my voltmeter on the green/white wire from sensor and NEG lead on the NEG post on the battery.
I then took a steel ruler and passed it though the sensor (should go from 0v to 5v) but it didn’t,
Can that mean faulty sensor??
-the boat has twin screws. So I have swapped the coil over with no luck, swapped the module over with no luck.

 

[dubs283]

What brand ignition sensor did you install?

Anything aside from oem would be suspect ime

 

[Scott Danforth]

first, welcome aboard

merc would have a 305 GM motor in 1994 and not a 302 Ford motor Volvo and OMC used 302 ford motors as well as 305 GM motors in 1994. please confirm this is a mercruiser

go thru the troubleshooting guide in the stickies https://forums.iboats.com/threads/must-have-technical-information-for-diyers.335378/
(link 21)

 

[stresspoint]

you have an oil pressure sensor or temp sensor cutting the ignition would be my guess .
? does your motor have the module on the left hand exhaust manifold , if so that is tripping and not allowing spark to the sensor in the distributor.

sorry for the vague explanation , i just know from when i hot wired my 4.3 Mercruiser to run an aftermarket HEI distributor i had to work that module into the system in order to get spark.

i think Scott Danforth will explain the workings of that module better than me.

 

[Scott06]

Follow troubleshooting guide scott posted. Thee are no temp or oil pressure ignition cutoffs on the tbolt. Only cutoff is man overboard switch. If this is tripped will start on starter then die when key goes to run.

use oem sensor in dist only. Lots of failures on aftermarket stuff. Achilles of TB is if the module fails. If that is the case unless u can find a used module on ebay swap in a complete delco est set up

 

[Libertychartersto]

I have gone through the trouble shooting guide multiple times with no luck, it keeps referring me to the distributor ignition sensor, I replaced it with an aftermarket one, same result. I ordered another one and still same result. I can’t get OEM where I am so I just ordered one from Amazon.
Is there a way to bench test the ignition sensor?

 

[Libertychartersto]

Also ran a new ground wire, 8 gauge from the neg post on the battery to the to the ground on the manifold riser where the module is screwed to. No change

 

[mrzooom]

Just try this even if doesn't sound right...I took black ground from manifold screw and mounted it with the other wire under the module mounting screw
All of a sudden I had spark!?%§@#$&. Did this today.

 

[Scott Danforth]

Just try this even if doesn't sound right...I took black ground from manifold screw and mounted it with the other wire under the module mounting screw
All of a sudden I had spark!?%§@#$&. Did this today.

You got a ground by moving the wire