Shift Interrupt Switch Issue - Maxum Boat Owners Club

Vukdaddy · 03-15-2021, 07:49 PM

I purchased a 2000 1900 SR2 with a newer 5.7 carb engine. I love the boat but it will not come out of gear when docking. After hours looking at wiring and the switch itself AND several visits to shops, I have been told that the Thunderbolt V ICM is programmed for Bravo outdrive, not Alpha. Mechanic tells me fix is new ICM (861253T02) which will allow proper working shift interrupt switch. My (layman) question: is there a more basic fix to overcome issue ? If the switch grounds out the ignition (via coil) then why would new ICM be needed ? I have basic mechanical knowledge, so bear with if I’m not fully understanding how ICM solves this issue.

Thanks in advance

Dan

mmwjr · 03-16-2021, 02:58 PM

First question what is the idle rpm? If too high it will make shifting hard. Should be around 650 rpm.

Under Documents of this site you will find Mercruiser manual #24 and on page 4E-4 is the wiring diagram for you engine. View A component 5 is the switch interrupt. The not on the following page just says not used on Bravo but no mention of a different ICM nor did I see anything under section 4B ignition system saying there is different ICMs. If you look at the wiring diagram you will see the switch is wired to the ICM.

Vukdaddy · 03-16-2021, 04:18 PM

Thanks for the quick reply. Yes, I had previously looked at this section of the manual. Still scratching my head as to “how” ICM plays role in switch interrupt - I mean switch interrupt is a electro-mechanical feature. Shouldn’t matter what ICM. At this point, I’ll have to trust the mechanic who has been in contact with Mercruiser. On your question about RPM - the boat was running a little higher than 650, closer to 800. Don’t know if this variance would be enough to manifest the issue. Thoughts ?

mmwjr · 03-16-2021, 11:46 PM

Try setting the idle to 650 rpm to see if it helps. The shift interrupt switch is wired to the ICM which has circuitry to cut the ignition when activated. If you ground the ICMs output at the distributor the amplifier would be overstessed and fail over time. The Bravo module does have this circuit.

jparsons121 · 03-16-2021, 11:48 PM

Your mechanic is partly correct. The ICM’s are engine specific in the timing curves that are programmed in. The shift interrupt is not managed by the ICM. It purely processes the 5v signal from the distributor sensor (pulses the voltage from 5v to 0v) and signals the coil to discharge. The shift interrupt grounds out the signal to the ICM, so there isn’t a pulse to cause a spark. This effectively ‘stalls’ the engine briefly to allow the shifting.

Edited to provide clarity.

jparsons121 · 03-16-2021, 11:59 PM

Quote:

Originally Posted by Vukdaddy

Thanks for the quick reply. Yes, I had previously looked at this section of the manual. Still scratching my head as to “how” ICM plays role in switch interrupt - I mean switch interrupt is a electro-mechanical feature. Shouldn’t matter what ICM. At this point, I’ll have to trust the mechanic who has been in contact with Mercruiser. On your question about RPM - the boat was running a little higher than 650, closer to 800. Don’t know if this variance would be enough to manifest the issue. Thoughts ?

And... Yes a few hundred RPM will make a difference even if the interrupt is working properly.

Vukdaddy · 03-17-2021, 04:34 PM

Thank you mmwjr for the detailed explaination/clarification. I feel confident now that this is the right fix. After a week of searching marine part cartels, I finally found a new, OEM ICM module (for this engine) in Canada. Once I receive it, I’ll have it installed and dial-in the RPMs to 650. Thanks again !

mmwjr · 03-17-2021, 05:18 PM

Let us know how it goes.

Vukdaddy · 04-13-2021, 05:03 PM

After months of searching, I finally purchased an ICM for 5.7l (with Alpha drive). Found a new one in Canada, otherwise there were none available . . . . anywhere. This particular ICM is for a newer 5.7 Mercruiser with older Alpha drive. It enables the shift switch interrupt function. The mechanic has installed it - will test it this week on the water. Hopefully this simple (but expensive) fix eliminates the problem/issue of not coming out of gear. Fingers crossed.

Vukdaddy · 04-22-2021, 05:35 AM

Test run today verified the correction - new ICM is properly engaging the shift interrupt switch. Problem solved, and a big relief. Should not be any more angst when docking now. Looking forward to getting out on the water very soon. Thanks to mmwjr and jpsrsons121 for your thoughts and recommendations.

mmwjr · 04-22-2021, 11:42 AM

Glad you were able to resolve the issue.