Have a ‘99 Formula 400ss that originally had a pair of 502 mag’s. Repowering now with a pair of complete take out 500 EFI’s from a 2002 36 Baja. I also installed 2 new Bravo Magnum transom assemblies that have the trim gauge and trim limit senders on the outside of the housings, just like original. Both engines have the 10 pin Merc engine harnesses. The original 502 mags have the two pig tail wires with bullet connectors hanging out for the trim gauge on the engine side of the harness. The 500 EFI’s do not seem to have these wires. I’m not at the boat to see it for myself but “my guy” is telling me the 10 pin engine harness on the 500 EFI’s is different than the harness on the 502’s. My local Merc dealer is telling me that a 10 pin is a 10 pin is a 10 pin. My question is……..if a 500 EFI boat originally came with HP transom assemblies and mechanical / cable style trim gauges would the 10 pin engine harness be different and not have the brown with white tracer trim gauge send wire and black ground wire with bullet connectors? Are there different 10 pin engine harnesses? Thanks.

I don’t have a 500 EFI service manual but the other HP manuals that I have, don’t show anything for trim related components ( probably due to them being attached to HP transom Assys which have no provisions for senders, of this era) they use wire position 10 for the oil temp sender on these, that is the same position that is used for trim gauge on a std transom assy behind a black engine

Ayuh,...... I've worked on alotta Mercruisers, but not yers, so take this with a grain of salt,.....
Every Merc I've worked on has a motor wiring harness, 'n the trim system has it's own, separate wiring harness,....

That said,.... Look over yer trim wiring harness for the 2 missing wires you need,.....

Thanks Jaay Teee, this was the answer. The 502 engine harness has the trim send on pin 10 and the 500EFI has oil temp there. Other than that the rest of the harness is the same. We just wired the trim send to the correct wire on the boat side of the harness. Thanks.

Pat the 500’s use the brown with white (trim sender) wire for oil temp. If your not running oil temp gauges take the wire off that and connect it to the trim wire.