View Full Version : Dead Tachometer or Alt

07-08-2012, 07:18 PM
I have an old boat with a 1986 PCM 351(its a dixie). I was out on the lake a few weeks ago and someone needed a jump so I figured why not. The jump did not work it was if his boat wasn't even pulling any current.

After motoring off (can't remember if tach was working 100% but I think so) we came to a stop and killed the boat. When I tried to start it nothing. It had tripped the red large resettable fuse on the back of the engine. After this the boat would start and run fine but no tachometer operation. The tachometer is stuck at 2k rpm.

I know the tach is getting the IGN and GND my question is what should the send signal look like? I want to to diagnose exactly which one is dead the alternator or tachometer. The alternator is producing good charging voltage. I am an electrical engineer and can look at the alt signal I will bring an oscope home one day this week. I just wanted any thoughts before I really dig into it.

Any comments are appreciated.



07-08-2012, 09:21 PM
You can verify alternator with a dvom across the battery posts and compare that to the alt post.(simple charging voltage) As for the tachometer it should scope out with 4 pulses per crankshaft rotation for V8, keep in mind that filters may be needed to isolate voltage bleed off or random spikes. Personally i would not scope it out. If your getting battery voltage on the ign post and the ground is good, load test the signal wire going to the neg post on coil to the tachometer sig post, if that wire checks good then i would think your tachometer has failed. a quicker way to check is if your dvom has rpm mode you can go straight to sig post on tachometer and if rpm is reading then same result tachometer is dead.