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How Mercruiser Thunderbolt ignition systems work.

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  • How Mercruiser Thunderbolt ignition systems work.

    Mercruiser Thunderbolt ignition systems.

    Mercruiser introduced the Thunderbolt IV onto their engines in 1982, alongside the introduction of the 1-R drives. The TB-IV system was used until the introduction of TB-V mid-way through 1996. Thunderbolt V was an improvement on an already exceptionally reliable and well performing system. TB-IV modules were programmed with each specific engine’s own timing curve.

    The modules started out as a large box mounted to the (usually) Port exhaust elbow.

    Later the module housing was redesign. It became considerably smaller and was mounted to a plate that fitted into the place of the first generation modules. The flying leads became a socket for a plug and the interconnect wires became part of the engine harness.

    This module was later removed from the plate and mounted directly to the outside of the distributor body with a suitable heat transfer compound.

    All the different modules do the same job. They are supplied by a 12 volt wire, usually purple, via the positive terminal of the coil. They supply a current controlled 12 volts for the sensor (located inside the distributor) and receive a series of 5 volt pulses back from the sensor. More on the sensor later. When a negative going pulse (one going from 5 volts to ground) is received the module electronics calculates the time required to fire a spark, based on the engine speed and whether it’s a 6 cylinder or 8, and the timing curve programmed into the module. The coil is then fired via the grey wire on the coil negative terminal, just like a points based system. The coil negative terminal also carries another grey wire, this for the signal for the dash tachometer. The coil, on all engines, be they TB-IV or TB-V is 0.6Ω to 0.8Ω primary resistance and 9.4kΩ to 11.7kΩ secondary.

    The distributor internal sensor.

    All engines use the same distributor sensor, V6, V8, small block, big block. 2 versions were released. The first had just 2 screw terminals that sat on the outside of the distributor. One terminal was for the white/red wire from the module and the other for the white/green wire from the module. On engines designed for the 1- and Alpha series drives, another white/green wire went from the sensor terminal to the shift interrupt switch. This sensor design relies on one of the mounting screws for its ground.

    The sensor was later redesigned to remove the screw terminals and incorporate ‘flying leads’, and include a black ground wire. It also encaptulated the solder connections and the circuit board, making it more durable and less subject to corrosion.

    The original screw terminal sensor was made NLA, and a new sensor kit includes a set of adapter leads to allow easy installation and connection of the new sensor assembly.

    Below is a diagram showing how the various parts of the system connect together. (BTW. The shift interrupt works by grounding the output pulses from the sensor in the distributor. No pulses, no sparks.)

    TB-V introduced a ‘variable’ advance curve. The actual engine timing is based on a pre-programmed curve (much like the TB-IV curve), but the electronics is able to modify the curve based on engine speed, rate of change of engine speed (ie, if the engine is accelerating) and if fitted, a knock sensor output. The basis of the system remains the same as the TB-IV, and uses the same distributor sensor for the trigger pulses. The module also came in a new package. In addition to the programming, TB-V can also have a knock sensor connected, which also modifies the timing curve, and an extra wire to force the unit into 'base mode' for setting of the initial timing, and a feed from an overheat sensor and a connection to the audio alarm system. As TB-IV is now also NLA, this new module is the direct replacement for any TB-IV modules that fail, again based on each specific engine and its advance curve. Ordering a TB-V module as a replacement for a failed TB-IV will include any connectors that need to be changed too.


    Now we know how the system is connected and what each part does, fault-finding becomes quite simple.

    And as the only differences between TB-IV and TB-V are the program, a possible knock sensor and an extra wire (base mode), troubleshooting uses the same procedure and flowchart for both systems.

    For troubleshooting I alway remove the tacho feed wire and the white/green to the shift interrupt switch, eliminating them as a possible cause of a problem. I also remove the HT lead from the centre post of the distributor cap and put a spark plug into it, lay it on the engine where it is well grounded and I can see it. A good spark if a health blue one that you can hear go 'crack'. A yellow spark is weak and may not fire a fuel charge properly.

    The most obvious checks are, has the system got 12 volts? Are all components properly grounded? Are all the wires intact?

    Once we can answer YES to those questions we can move onto the next step, running through the troubleshooting flowchart. This chart has been published many times on these forums, but I have included it here anyway.

    The steps need to be followed in order and without missing a step.

    And remember, if the results are strange, or intermittent, start looking for bad grounds or dodgy wires.
    Last edited by achris; September 7th, 2017, 10:18 PM.
    The world takes on a whole new perspective when viewed from 100’ below.
    1972 Bertram ‘Bahia Mar’ 20
    2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
    (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)

  • #2
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    So, how exactly does the electronics in the module know when to fire the spark and how much advance to apply?

    First we need to understand a few simple principles. For this there will be maths involved, so if maths makes your brain hurt, stop reading now.

    Ok, for the couple of you left, here we go.

    Inside the module are a few key components. Firstly there an area of memory that stores a ‘map’ of the advance curve, like a lookup table. For any given RPM, there will be an advance value. So let’s use a V8-24 module as an example. It’s a good one to use as a lot of engines use it. It has a module advance of 24° at 3700rpm. Here’s what that curve looks like..

    Click image for larger version  Name:	Timing curve.png Views:	1 Size:	23.0 KB ID:	10843160

    Within the module is also a microcontroller which does the looking up of the advance curve and sets up timing sequences to fire the spark..

    The timing is initiated by the sensor in the distributor. The sensor is fed 12v (on the white/red wire), and when the windows on the rotor pass it, the white/green wire moves between (nominally) 0 and 5 volts. As the pulse goes from high to low the microcontroller starts a couple of timers. One timer is for the engine speed. With a V8 the program ‘knows’ that it will see 4 pulses for each revolution of the engine. When it sees the next negative going pulse it then has a time between pulses. These times are very quick. Let’s say the time between negative going pulses is 5 milliseconds (5 thousandths of a second!). That means the engine has turned 90° in that time. So the microcomputer can calculate the engine speed. That would mean a full revolution in 4 x 5 milliseconds = 20 milliseconds/revolution. That’s 1 revolution in 1/50th second (1/50=0.020 seconds, 20 milliseconds), 50 rps (times 60) is 3,000rpm.

    So, all that timing is done in 5 milliseconds at 3,000rpm. That 3mS at 5,000rpm..

    As well as the engine speed timer, there is also the spark advance timer. Once the microcontroller knows the engine speed, it can go to the look up table to find out what advance to apply for that particular speed. At 3,000rpm the advance is 20°, which is 20° degrees of 90° for the time between pulses. Remember we have 2 timers start at the pulse. That second timer fires the coil switch to create a spark after the amount of time between pulses for that speed (in this case 5mS) LESS the advance time for that speed. 2/9 of 5mS is 1.111mS, so after 3.889mS, the controller will fire the spark… and 1.111mS later, it gets the pulse that it calculates the engines speed from, looks up the advance for that speed and starts the timer for the next spark…

    Hopefully you can see what I mean by a pulse from the sensor in the distributor fires the spark for the NEXT cylinder…

    I hope this makes sense. It does in my head..


    Last edited by achris; March 24th, 2020, 11:42 PM.
    The world takes on a whole new perspective when viewed from 100’ below.
    1972 Bertram ‘Bahia Mar’ 20
    2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
    (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)