Shift interrupter switch. How does the electronics work?

[Wolfgang]

I'm having trouble getting my engine to start. It seems there is no spark. I have a 1989 5.7 Mercruiser with a thunderbolt IV ignition system. Using the shop manual I've tracked the problem to the ignition sensor. However, before getting the manual I tried trouble shooting the problem my self. I was thinking the problem was in the shift interrupter switch because it was giving me some strange readings. Before I go order a new ignitions sensor I want to ask this question here. I know what the switch is and what it does, but how does the electronics work? During troubleshooting I put an ohm tester on the two bolts connecting the leads of the switch. The reading was about 50%. I disconnected the leads and tested the switch. The switch seems to be working fine. Engage the switch is 100% on the ohm meter, disengaged it's 0%. The strange thing is after disconnecting the switch the motor started once just fine. But not anymore. And it gets stranger. Today I checked the same to bolts connecting the switch leads and it reads 100%. To me it seams that if the there is connectivity between these two leads it will not let the engine start. But I don't know for sure. That is my question. Can anyone help me here. <br /><br />Thanks<br /><br />Wolfgang

 

[Haut Medoc]

Re: Shift interrupter switch. How does the electronics work?

The shift interupter is very close to where the main plug for the engine and the boat mate. I would see if the two are corroded. Pull 'em apart and see what's up.

 

[Haut Medoc]

Re: Shift interrupter switch. How does the electronics work?

A'int electricity wunnerful?

 

[newport dave]

Re: Shift interrupter switch. How does the electronics work?

The interrupt switch used with Thunderbolt IV ignition systems should be an open circuit when not activated (lever is up/engine runs), and there should be connectivity when activated (lever is depressed/kills engine).<br /><br />Dave

 

[Haut Medoc]

Re: Shift interrupter switch. How does the electronics work?

If the switch is stuck it should be obvious. The roller should be sitting in the detent. If it is not it is stuck. Bypass the switch with alligator clips, [neutral -out of gear ]. What -if anything -happens? The interrutper's funtion is to momentarily kill the engine by grounding the ignition system to ease the shifting of the gears. Hope this helps........J

 

[waterone1@aol.com]

Re: Shift interrupter switch. How does the electronics work?

Regardless of points or a solid state sensor, your coil gets +12Volts (or close to it) to it's positive terminal anytime the ignition switch is on. The distributer pulses the negative(ground) side of the coil (again, either points opening and closing, or "pick-up" doing the same thing) this is what causes the spark. The shift interupt provides a constant ground to the coil anytime the switch is engaged or closed....so, no spark, untill the interupt switch is open again. If you have any doubt about the switch stopping your engine from starting....disconnect it, DO NOT JUMPER ACROSS IT !! That will only make sure that your engine does not start(for troubleshooting only). Something to point out here: the two screw terminals on the switch mounting are not the same, one is connected to the engine ground through it's mounting, the other is isolated from ground. even though this is a single pole switch, if you get the wires backwards, the engine will not run, as the coil lead is grounded out.

 

[Wolfgang]

Re: Shift interrupter switch. How does the electronics work?

Waterone1, I did notice the two screw mounting, how one side is connected to the engine block. However, I didn't think about the grounding aspect. It is very possible I got the leads mixed up. I will have to check this again. I should make sure the lead from the coil is not on the ground side, right?<br /><br />Whats confusing to me is, my ohm test of the two leads coming from the engine (not the switch) show continuity, same as if the switch was closed. Can this be correct? Could there be a short in the wire harness?

 

[waterone1@aol.com]

Re: Shift interrupter switch. How does the electronics work?

I would start by dissconecting both leads from the switch (the wires comming from the engine) and try to start it. If still no luck, use a volt meter and make sure you have + 12V on the coil + terminal with the key switch on and your meter - connected to a good ground. If not, find out where it is lost. Next put you meter across the + and - of the coil, as you turn over the engine you should see the voltage pulsing. If you see a steady 12V then something is grounding out the neg. side of the coil, if you see no voltage, then the pickup, module or wiring is bad.

 

[Wolfgang]

Re: Shift interrupter switch. How does the electronics work?

No spark from the coil. 12v on the + side of the coil with the key on. I also got 12v on the red/white wire on the distributor. I took the red/white wire off and still got 12v. The troubleshooting manual says to replace the ignition sensor. <br /><br />I didn't get a chance to see if the voltage was pulsing from the + and - side of the coil. It started to rain. I'll try that later.<br /><br />I did have the coil grounded out at the interrupter switch terminal mount. And I did some more checking. After disconnecting both terminals I was getting 50% continuity again. I tested it using the voltage meter and it was giving me around 3 volts. Is there something wrong with these terminals?

 

[Wolfgang]

Re: Shift interrupter switch. How does the electronics work?

Thanks everyone for the help. It was the ignition sensor. Installed a new on and it works fine.<br /><br />Thanks again.<br /><br />Wolfgang