1986+ Mercury 6/8/9.9/10/15HP 2-stroke Factory Service Manual, Page 74Get this manual
CHARGING AND STARTING SYSTEM
Tachometer Module Static Test
1Disconnect GRAY, YELLOW, and YELLOWBLACK tachometer module leads2Set meter to
uses the batteryvoltage within the meter
3Connect the multi-meter black lead to engine ground or BLACK harness wireConnect the REDmulti-meter lead to the GRAY tachometer module engine bullet connector4Measure voltage91-854009A1
YEL BLK GRY YELBLK 59093