Re: ’93 70hp wiring harnass diode?

[Don]

9/1 AM: I have rewritten this post and corrected misinformation….
That diode in that harness is the "SLOW" blocking diode….It is there because the only alarm we want to trigger SLOW is an overheat. If it were not there, SLOW would be triggered by low oil in the tank, along with the VRO pulse alarm….
So, what are the symptoms if the diode is shorted you ask…The engine would probably just be stuck in SLOW, even though no alarms are going off and no faults are present….This could be caused by the self test horn, or the older style low oil tank alarm circuitry causing a very slight short to ground, enough to trigger SLOW, but not enough to light off the horn. Unplugging the temp sender would not relieve this condition. Or, perhaps the engine is not stuck in SLOW, then SLOW would be triggered by the low oil and VRO pulse alarms.
Symptoms of a diode blown "open": you wouldn’t notice anything unusual, all other alarms would work as designed. But, if the engine did overheat, it would engage SLOW, but the warning horn would not operate.
So, you have checked the tan lead with an ohm meter, current flows in one direction, but not the other….So, seems to be working as designed…But, we do not know if the diode polarity is correct. Someone could have replaced the diode but installed it backwards. I am doubting this condition exists only because those connections look pretty factory stock. But, like I said, this theory is easily checked out by simply using a jumper lead across the ends of the diode, then rechecking horn operation.
There is one more thing to be wary of when it comes to troubleshooting SLOW operation….Once SLOW is engaged, it will not disengage until the engine is shut off (key switch off, not just engine stall out). So, let’s say you wanted to manually engage SLOW by grounding the tan harness temp lead while the engine was running above 2500RPM, and you didn’t fall out of the back of the boat while doing this, you would witness the horrible misfire/RPM reduction (and horn sounding) until RPM was lowered back down. Now, let’s say you reconnected the temp lead while the engine was idling, then hit the throttle again….The engine would go back into SLOW, but the warning horn would not sound…This condition would continue until you shut the engine off with the key switch.
PS: Yes, I know that some of the smaller SLOW equipped engines don’t have to be shut off to disengage SLOW, just returned to idle, but it is safer to just shut the engine off to be sure SLOW is disengaged…