Evinrude Big Twin 35 cutout switch

[hojo]

Help...I am confused about the electric start circuit on a 1957 Evinrude Big Twin 35 electric start, Model No. 25930, Serial No. 13759. I do not understand how the cutout switch is supposed to work in this starting circuit. If I jumper around the cutout switch, the starting circuit works properly. If I do not jumper the cutout switch out, I have an open circuit. I have traced the circuit according to the wiring diagram, and confirmed that the cutout switch is open, and requires a moderate vacuum to close the contacts. If I apply a vacuum, I hear the diaphragm in the switch move, and my continuity tester shows the contacts closing. How is this switch supposed to operate? What generates the vacuum to close the cutout switch contacts that complete the starter circuit?

 

[Paul Moir]

Re: Evinrude Big Twin 35 cutout switch

I thought it was odd when I first looked at it to. The vaccum cutout switch is used to kill ignition if the engine runs at high rpms under little load, ie a high manifold vaccum situation. It grounds an ignition coil (thereby killing spark) when it closes through the saftey switch. I guess on your engine this would be a mercury tilt switch, on mine it's a pushbutton that contacts the armature plate. That prevents the vaccum cutout from doing it's thing if the throttle is under ~25% open. That would prevent it from killing spark if your just idling. The solenoid is also grounded through the saftey switch so you can't crank the engine with the throttle open too far. <br />These engines are susceptible to "running away" where under no load the rpms can climb to the point where the engine is destroyed. It'll happen if you open the throttle too much in neutral, or otherwise under no load. The vaccum switch is there to protect you from that. <br />It sounds to me like your mercury tilt switch is somehow stuck open, or your trying to run the circuit with the throttle too far advanced, so when you jump the vaccum switch your grounding the solenoid through the magneto coil. It has very low resistance to ground and would probably allow your solenoid to operate. Be careful, doing that could toast the magneto coil pretty quick (assuming the points are open).<br />Hope this helps and welcome to iboats! (I wish I was in Fresno today!)

 

[hojo]

Re: Evinrude Big Twin 35 cutout switch

Paul, thank you very much for taking the time to explain this. I will take a look at the mercury switch and see how that is behaving. What a great explanation...I do appreciate it. I will post the results of my troubleshooting as reference.

 

[hojo]

Re: Evinrude Big Twin 35 cutout switch

Hello Paul,<br /><br />Your diagnosis was right on target. After a bit of searching, I found the Mercury switch on the speed adjustment shaft. Well, it turns out that this switch has no wires leading to it, so as you thought, it was this open condition that was the problem. I will replace the wires to this switch, and and see what happens. According to the wiring diagram, it looks like one lead goes to the base of the cutout switch, and the other goes to the ground on the engine frame. <br /><br />If I understand this circuit properly, at a low throttle position, the starter circuit is completed through the Mercury switch. Once the engine is running, as the throttle position is increased, the Mercury switch opens so the starter circuit is no longer complete. If the rpm of the engine goes too high, the vacuum becomes strong enough to close the contacts, grounding the ignition coil. <br /><br />With that in mind, if the coil was grounded when I jumper out the cutout switch, why did I still see a spark as I cranked the engine? I was checking compression and spark strength with the a jumper across the cutout switch to allow the engine to crank for these tests. I did not have the fuel supply connected at this point, so the engine never actually ran. But, I did get a good spark...hmmm.<br /><br />Please accept one grateful "Attaboy" for pointing me in the right direction. I really do appreciate it. I wish we could send you a bit of the warm weather from here as a thank you. It was 16 degrees Celcius today, and sunny. My wife and I were married in Saint John, New Brunswick about 23 years ago, and we remember how cold it can be in that part of the world at this time of the year. <br /><br />Thanks again for your help, you seem to know your way around these motors pretty well!<br /><br />Howard

 

[Paul Moir]

Re: Evinrude Big Twin 35 cutout switch

Glad to be of service.<br />You're right about how the switch should be wired. Why you saw a spark is a bit of a mystery to me. The way my engine was originally wired, grounding the ignition coil cutout lead killed the spark on only one cylinder. I presume in your tests you checked the spark on both leads though. If I remeber correctly, the signal on the primary of the ignition coil (which is attached to the cutout lead) was a *very* short jump from 0v to about -90v. Since the battery (through the solenoid key winding) would hold this at 12v you would not get any spark. Perhaps though, since the signal is so short, the inductance from the key winding on the solenoid plus the inductance from the lead wires prevented the primary from "seeing" the battery. That's definitely a longshot, since it seems to me even great resistance to ground from the primary killed the spark.<br />Thanks for sending the warm weather! It was -22C here last week, now we're seeing +. Since we're stuck out in the Atlantic it doesn't get as cold here as New Brunswick, but we seem to be getting it bad here this year...

 

[Paul Moir]

Re: Evinrude Big Twin 35 cutout switch

Hmm, I couldn't leave this alone so I mocked this up on my engine with the "key" side of the solenoid grounded. I got spark too! I guess that ignition system isn't as fragile as I first thought. I jumpered the solenoid just to make sure nothing weird was going on and the spark dissapeared as it should.