 1992 Johnson 28SPL tach weirdness
History:
My dad purchased this engine new in a package with a 13' Whaler and OMC System Matched gauges. The tach worked fine for ~5 years and then started reading incorrectly at high rpms.
The tach seems to works fine at < 2/3 throttle. After I hit about 4300 rpm, the tach drops to 3800 rpm and starts climbing again at what I'm guessing is about half the rate the engine speed is actually increasing. At WFO, the tach tops out at about 4300 rpm after resetting to 3800 and climing slowly with one person and gas tank. I believe before the tach started acting funny this was reporting ~5600rpm with one person and gas tank.
The ground wire on the tach was somewhat corroded. I sanded all connectors/nuts on the back of the tach and drowned them in dielectric grease. I also tried rotating the dial around through all of the various pulse settings a few times in case one of the contacts was slightly corroded on the inside of the tach. The only change I noticed from doing this is that the needle now rests at 2500rpm when the key is off instead of 1500 rpm when the key is off. The connections under the motor cover looked clean.
The original system matched tach was set to 4 from the dealer. The Faria PDF says to set their tach to 4, but the Teleflex pdf and forums I’ve read say it should be 5 for the medium sized OMC engines. I tried changing the setting to 5 and get a similar weird behavior. The motor just revs higher before indicating a slightly higher 4400rpm and doesn't drop back quite as far before increasing at the slow rate again.
The voltage reading is 12v at idle, 13.5v @ 2500rpm, 15v at 3600rpm (minimum planing speed where we spend a lot of time) and 16-16.5v @ 4000+rpm. The charging system is unregulated, so this has always been the behavior since new.
Questions:
1) Since the SPLs are oddball engines, I'm not sure who to believe about the tach pulses. There is nothing in the owners manual about the number of tach pulses per RPM. Do you have any inside BRP knowledge on the number of pulses per revolution of this particular motor (J28ESLENE) and what number the early 90s System Match gauges should be set to?
2) I've read several suggestions to add a 2000 ohm resistor on the 88 and 112spl motors between the pulse wire and ground since they are not voltage regulated. Does that same advice apply to the 28 model as well?
3) What part would you troubleshoot or replace first? Add resistor? Borrow a tach from another boat? Check stator? Replace rectifier? Other?
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