Tach troubleshooting on OMC 574APRGD 350 engine

[stancalame]

While on the boat last I noticed some thing strange. I turned on the Nav lights and noticed that the tach started reading between 1K and 2K regardless of throttle position. Anchor light was not installed but green/red lights were on. When I turned lights off, the tach returned to normal operation, so that is one issue. The second goal is to insure tach is working properly and as accurate as it can be. I'd like to confirm proper timing and idle however not sure I can trust the tach. Boat is 92 model and I assume tach is original. I find a reference to Tach at very end of Electrical system and all it says is check for tight connetions, trace leads for proper routing and insure its programmed for 8 cylinder engine.

Other sections of the service manual shows a gray wire that runs from a tach terminal labeled PUL (pulse I assume?) to main cable connector, then to 5 pin connector, and then to ESA module. I've read numerous posts on Tach troubleshooting and some mention a rectifier ( I assume that is on outboards and does not apply to me), some mention checking signal from a gray wire for signal.

So with all that said my goal is

1. To gain confidence that the value my tach is showing in in the ball park and within specified accuracyof the gauge
2. To get to the bottom of the erroneous reading when Nav lights are on and resolve.

Where do I start? I have a Fluke 87 that measures Hz.

• Can I use the Fluke as a tach to verify my dash tach?
• Will it read the raw signal (whatever and wherever ??) that is fed to the tach PUL terminal?

Having to mentally shift gears, I've been turning wrenches on the boat recently and now need to use my gray matter for the sparky stuff. Look forward to your helping hand!