Mercruiser 5.0L tachometer troubleshooting: request advice

mcginnal

Cadet
Joined
Jun 27, 2013
Messages
21
Hello all,

First, my boat:
2003 Monterey Montura LS 248
Engine - Mercruiser 350 (5.0L)

Recently purchased from previous owner with the tachometer not functioning. I have been paging through the engine owner's manual with not much luck on where to start troubleshooting. How would I determine whether the issue is the sending unit or the gauge?

Any help would be greatly appreciated; thanks.

-Alex
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: Mercruiser 5.0L tachometer troubleshooting: request advice

Hello all,

First, my boat:
2003 Monterey Montura LS 248
Engine - Mercruiser 350 (5.0L)

Recently purchased from previous owner with the tachometer not functioning. I have been paging through the engine owner's manual with not much luck on where to start troubleshooting. How would I determine whether the issue is the sending unit or the gauge?

Any help would be greatly appreciated; thanks.

-Alex

Ayuh,.... Welcome Aboard,.... There is No sendin' unit, it's driven by the pulses of the (-) terminal of the ignition coil,...
It'll be the grey wire from the coil,...
 
Last edited:

chimmike

Petty Officer 2nd Class
Joined
May 26, 2013
Messages
141
Re: Mercruiser 5.0L tachometer troubleshooting: request advice

Can't be a 350 if it's a 5.0L! It would be a 305. Not that the principle of tach wiring is any different.
 

mcginnal

Cadet
Joined
Jun 27, 2013
Messages
21
Re: Mercruiser 5.0L tachometer troubleshooting: request advice

Ayuh,.... Welcome Aboard,.... There is No sendin' unit, it's driven by the pulses of the (-) terminal of the ignition coil,...
It'll be the grey wire from the coil,...

Thanks, Shipwright. With this in mind, should I be able to attach a voltmeter either close to the engine or up at the dash and get some sort of reading?

P.S.: My bad, Mike, that must be my "wishful thinking" dyslexia. It's the 305 cu. in.
 

chimmike

Petty Officer 2nd Class
Joined
May 26, 2013
Messages
141
Re: Mercruiser 5.0L tachometer troubleshooting: request advice

Yeah, I share your dyslexia!

Regarding the tach reading, have you double checked all the wire connections between the tach and the distributor? I found connections all over my boat just needed to be cleaned or re-done in order to make things work.
 

mcginnal

Cadet
Joined
Jun 27, 2013
Messages
21
Re: Mercruiser 5.0L tachometer troubleshooting: request advice

I went under the dash today, and I didn't see any loose wires. I turned the cylinder selector knob to ensure it was on "8 cyl" just in case it may have vibrated off its setting. Started the engine, and it still read zero RPM. Side note: I did notice oil on the back of the tach and the oil pressure gauge, and I have no idea why there would be oil in that location. Power steering was my only guess (the column is relatively close), and it was dry as a bone. Any thoughts?

Also a follow-up question: Would it be worth checking continuity from the coils to the dash? I was thinking of checking voltage via multimeter at the signal line from the coil.. Only thing is, I'm not sure if the coils send a DC signal proportional to the engine speed, or an AC signal w/the frequency of whatever RPM the engine is at, etc. I did check the voltage from battery to tachometer; it had juice at the gauge (12V).

Any thoughts/ideas would be greatly appreciated!
 
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