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trouble shooting TBolt V ignition issue

Old 08-06-2013, 04:47 PM
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Default trouble shooting TBolt V ignition issue

Cruiser has twin 350 mags. T-Bolt V w/ spark knock sensors.

1 motor lacks the same power as the other motor when getting on plain, also feels less responsive when cruising and cracking the throttles for a feel of power.
Motor has 150 psi compression, good vacuum, plug wires ohm good, all plugs look the same.
I've swapped carbs, ignition modules, and spark knock modules. No change.

When setting initial timing, the timing lead (purple/white stripped wire) is grounded to lock out advance, and timing is se at 10 BTDCt. I can run the rpms up till no more advance, (35 BTDC, still locked out) and motor sounds good. However, when I unground the timing lead and rev the motor up till the timing stops advancing, it'll hold for a second at 35, then drop down to 25 and the rpms will drop. The other motor does not do this, the advance and rpm both stay up there.

Also, the timing lead is supposed to have 8-10 volts at approx 1500 rpm. The strong motor is good here, the weak motor only has 2-3 volts then. I can't figure out where this lead gets the voltage, hoping this may be the problem.


Any ideas? Thanks
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Old 08-06-2013, 05:29 PM
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I was working on a tbolt 5 a while back and came across this thread. Post four has a link to pretty good detail on the tbolt 4 and 5. Including wiring diagrams. Reading it seems like you may be getting into the rev limiter.


http://www.offshoreonly.com/forums/g...derbolt-v.html
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Old 08-06-2013, 10:42 PM
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I have the Merc manual that shows the same. I doubt I'm getting into the rev limiter since Im around 3500 when the timing backs down, along with the rpms. It acts more like the knock sensor is kicking in, but I've unplugged it and had same results.
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Old 08-07-2013, 12:05 AM
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Not saying this your problem but make sure when you have tbolt V ign. set up that you keep the spark plug wires away from the knock sensor itself. The interference from the spark plug wires can trigger the knock sensor to retard your timing. Also make sure the wire that plugs in to the KS is not shorted nor the spark plug wires are touching that wire. I have had 6 marine motors in 24 years do this at some extent of lacking rpms and somewhat lack of power condition so just passing this on.

I had an 5.7 EFI Indmar engine with an E-controls ECM that would not go over 3500 rpms once - it was the connection to the Knock sensor broken off. You couldn't even stick your hand down there to feel it. Had to take half the interior out just to fix 1 damn wire connection.

Since you swapped out the knock modules itself wonder maybe the sensor under the distributor cap or the distributor. Did you swap out distributors to see if it follows or not ? Also wonder about your timing chain and or gear.

Last edited by BUP; 08-07-2013 at 12:21 AM.
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Old 08-07-2013, 06:01 AM
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Didn't swap distribs. Caps, plugs and rotors were replaced 2 yrs ago and don't have many hrs. Going to check the pickup sensor this weekend, But I think they were replaced with the newer design. I tend to recall the presence of the ground wire there. Also will double check KS wire lead and routing.
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Old 08-08-2013, 01:12 PM
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Thought about some other things here but had to go back many years. looked at my Mercruiser recertifation update notes. here is what was stated back then about the T Bolt ign V.

Rev limiter is designed to have the ign module take out spark pulses. These spark pulses drive the ignition driven tach, if the pulses are taken out the tach will read low.

1996 and older T bolt V ign. the idle stabilizer will not function until the knock control model receives 13.5 volts. When starting these models the engine rpm should exceed 800 rpms to allow the stabilizer to function.

Next I remember that on earlier T bolt V ign the purple / white timing wire if it rests or is too close to spark wires it can send RFI signals back to the module. This can retard your timing and or cause a miss engine running condition. On later T bolt V ign set ups the calibrations were change so that the purple / white timing wire would not cause this.

Always make sure you do not have any spark plugs wire(s) too close and or touching the knock sensor or its connection nor any shorts. I still would not rule out the sensor under the dist. cap and the new ones need a very good clean ground connection. Good luck and report back what u find.

Last edited by BUP; 08-08-2013 at 01:23 PM.
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Old 08-08-2013, 02:22 PM
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I'm going to check all aspects of the spark knock/components. Everything I'm hearing/reading sure points that direction. I can swap out the ign sensor no problem too if need be. Great thing about twins, I use the good one as reference during all the checks.
Have timing light, DVMM and testing printouts, going to spend tomorrow chasing this down!
Thanks
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Old 08-11-2013, 10:42 AM
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Update

After finding out I wasn't getting a full 12v to both ign and knock modules, coil, and alt excite lead, started tracing where the purple wire gets its 12v.
The main power wire from motor to the dash goes to a fuse block under dash, where a fused lead goes to ign switch where the purple wire is powered with key on. At that fuse block, by accident touched the incoming main power wire connection point and it was warm. The lock nuts were tight on the wire terminal, but not to the bussbar itself, creating resistance and loss of voltage to all accessories and the purple wire. tightened it all up and now have full voltage to the motor and the dash voltage gauge now no longer fluctuates and reads good now. I always just thought the gauge was bad since it read low and bounced all the time for last 4 yrs.
I can only assume that the knock module needs full steady voltage to operate correctly. Anything less and it would send the ign module a bad signal, in turn all timing advance would be removed, down to the initial base timing of 10 BTDC, even at WOT making the motor fell like a slug, which is why I started looking I to this.
US1 is a happy camper now!
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Last edited by US1 Fountain; 08-11-2013 at 01:13 PM.
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