72 50 HP Evinrude Diode and Lead assembly

[Badger2008]

Recently purchased a 72 Electric shift evinrude. Trying to put togeather a nightmare of wiring. Where is the Diode and lead assy?
And could somone please explain what it does? Sorry my books show it in the diagram but no picture and no info.

Thanks Chuck

 

[jay_merrill]

Re: 72 50 HP Evinrude Diode and Lead assembly

The Shift Diode and Lead Assembly (shift diode) is a component in the hydro-electric shifting system of your motor. It is located on the starboard side of your powerhead, near the electrical termination block, and looks like a small brown ball with wires coming out of it. The purpose of this component is to hold the gearbox in neutral when the key switch is turned off.

As you may know, your motor is shifted via hydraulic pressure created by a pump in the gearbox and is controlled by two electrical solenoids, which position a pair of poppet valves in the hydraulic circuitry. When the engine is running hydraulic pressure is created, but the engine will default to forward gear if no electrical power is applied to either solenoid. This is true because there is a piston in the system, which is moved forward by the fluid pressure, but is spring load to move fully backward (forward gear position), if there is no hydraulic pressure applied to it. When one of the solenoids is energized, the engine shifts to neutral, and when the second one is energized (in addition to the first), the motor will shift into reverse.

The 12 VDC power that the solenoids receive to work, comes from three sources - the battery, the 12 volt charge system's rectifier and the shift diode. When you start your engine, you must have the shift handle in neutral, because a starter cutout circuit will prevent the starter solenoid from engaging, if it isn't. As soon as you turn the key to "on" position, the neutral solenoid is energized from the battery. When the engine is running, the power comes from the rectifier.

When you shut the motor down, however, power from the battery and/or rectifier is interrupted as soon as you turn the ignition key to the "off" position. If not for the shift diode, the solenoid that causes the gearbox to remain in neutral would lose electrical power, and the spring loaded piston would move into the forward gear position. At this point, the engine would still be "winding down" and the prop would rotate, driving the boat forward. Obviously, this is an undesireable situation, so the OMC engineer's provided a means to prevent it.

The reason why the shift diode is able to continue providing electrical power to the neutral solenoid, is because it coverts the 300 VAC coming from the engine's stator directly - it is independant of the charge system, which includes the rectifier. As long as the flywheel is rotating, the shift diode receives the AC voltage to convert, and power is available to the solenoid.

 

[F_R]

Re: 72 50 HP Evinrude Diode and Lead assembly

Pretty good explanation, except it isn't 300VAC. It is only battery voltage at the time, but AC.

To shorten the explanation, the shift switch is fed by two sources, the ignition switch with a blocking diode, and the shift diodes. Without the shift diodes, the shift switch loses it's source of power as soon as the key is turned off and the motor jumps into gear. The shift diodes prevent that by providing the second source of power--the alternator--till the alternator quits spinning. Then it loses that source of power also and slips quietly into forward gear.

The blocking diode: It prevents the shift diodes from supplying power to the ignition system after the key is turned off. If it were not blocked, the ignition system would continue to receive power and would not stop running.

 

[Badger2008]

Re: 72 50 HP Evinrude Diode and Lead assembly

OK one more question

First let me say thanks for some very detailed info.

It appears my Diode and lead assy is gone. And a new one costs more then what I paid for the whole motor.

Is there anything else that can be used in place of it?

Do I really need one?

 

[jay_merrill]

Re: 72 50 HP Evinrude Diode and Lead assembly

Actually, I misspoke slightly. The shift diode is not completely independant of the charge system, but I don't see how it could possibly be battery voltage supplying current to the shift diode or the shift circuitry, once the key switch is turned off.

The stator produces 300 VAC, which feeds both the charge system and the shift diode. The shift diode rectifies the alternating current via two positive diodes, but the rectifier also converts the AC via two negative diodes. That is why the shift diode has three wires coming out of it - one comes directly from the stator, one comes from the rectifier and the remaining wire supplies 12 VDC to the shift circuitry. If you trace the wiring, however, it can be seen that the power source for both is the stator. It can also be seen that the red, 12 VDC wire that comes out of the rectifier goes directly to the battery side of the starter solenoid, where it then feeds the battery for charging and the key switch for power, when that switch is in the "on" position.

One thing that I haven't yet figured out however, is what steps the voltage down from 300 to 12 volts. If anyone knows that, i would be interested.

 

[jay_merrill]

Re: 72 50 HP Evinrude Diode and Lead assembly

It appears my Diode and lead assy is gone. And a new one costs more then what I paid for the whole motor.

Is there anything else that can be used in place of it?

Do I really need one?

If your motor is not an electric start model, it didn't have a shift diode to begin with. I have actually never seen the manual start version of this motor, so I am not sure why that is true. It may be because there isn't a charge system in it, and there is some other means of supplying 12 VDC to the neutral solenoid, until the motor has completely stopped.

If you have an electric start model, you probably should replace the missing shift diode. The shop.evinrude.com site says that they are still available at $95.50. You might also find one off of a dealership "boneyard motor," or one of the auction sites. The cost of the used part should be a bit less.

In lieu of replacing the shift diode, I suppose you could try shutting it down by making sure you are at a low idle and then choking it. If that works, you can turn the key switch off after the motor quits.