on a late 502 mag mpi ... what is the sensor on the extreme back /top of the block, almost directly behind the distributor. it has a medium blue wire going to it. i know where the oil press and water temp senders and switches are and i didn't think this motor had an oil temp switch built in... anybody know what it is ? it is obviously a gage or lite switch of some sort.

Maybe the low oil pressure alarm switch, should have a tan wire with blue tracer.

how sure are you of that ?

Can you post a pic ?

Maybe the knock sensor. It is not oil temp or for a guage.

On Gen 6 Marine engines, there's apparently a low oil pressure alarm / warning sensor although the Mercruiser Manual up to 2001 make no mention of a sensor in your location, that would be my bet. Is it a pressure style or temp style sender? (a bigger brass dome look or a small thin probe look?) If it is plastic or similar, it may be some type of switch but until you know what kind of sensor (pres / temp) it's hard to tell.

if it has a tan and blue wire it is a low oil alarm

light blue is oil pressure gauge and wht/blue is oil pressure alarm orindicator hooked to EFI engine, tan/blue is warning horn. the knock sensor i believe was blue also if it is this wire then it should be longer to reach down to the lower part of the engine block some engines had too sensor one on each side and some had one.the fastes way to see what wire this gose to is to turn the key on and ground the wire out and watch the gauges and see what one will move.you will not hurt any thing by doing this test.hope this helps.

I see you posted a while back but related question. I have a horn warning; I show good oil pressure and normal temp on guages, but horn sounds. When I disconnect a temp sensor on therm housing it stops,,, if I ground it it sounds again. I have a tan/blue wire plugging into back of engine/into wiring harness. 500HP