That’s a diode, current only goes in one direction. Depending on which VOM lead you put on which side, you could get continuity or not.

Sorry I can’t help more, I have no idea what circuit the diode is installed in.

Tom

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…

Which lead are you grounding to the block? If you are grounding the temp lead to the block, the horn won’t sound because the diode is blown open, so the ground signal never makes it back to the horn. Again, I am assuming the diode is blow "open".

Before you do that you might want to check to see if you have a three wire horn? If it is a self test horn (when you turn the key on it sounds for just a second to show it works) then the blocking diode will have to be present or the motor will go into SLOW after each horn test after the ignition key is turned on. The diode is there to keep other horn functions from turning on SLOW. Only over heat is protected by SLOW. I believe only a three wire horn is availble for service now #585992. The horn has a purple, brown, and black lead. The momentary ground is what causes the horn to self test. A nice feature to prove the horn is operational at all times. Especially in salt water!

Dan in TN