Tachometers Reading Too High

[Fireman431]

Here we go with another tachometer issue for the electrical masses...

Boat: 2001 Carver 374 Voyager
Engines: 2001 Mercruiser 7.4L MPI (x2). 425 hrs each.

Original issue: Port tach would read (intermittantly) anywhere from 200-400 RPM too high. Occasionally tapping the glass face would allow it to correct. Cleaned all contacts and problem continued. Starboard tach never had an issue. These were 0-6000 RPM gauges.

Decided it was time for new tachs. Couln't find original models, so I replaced them with 0-7000 RPM Faria tachs. All new terminals and good connections. Now both engines read 900 RPM at an idle when I know there are idling approx 650-700.

Tried adjustments on the back of the tachs. There are positions from 1-6. This is what they read at each position at an idle:

1) 0 RPM
2) 1200 RPM
3) 1100 RPM
4) 1000 RPM
5) 900 RPM :redface:
6) 0 RPM

Any idea what the issue might be? I get the same readings from both engines so I know the switches are secure in their position.

Any help is appreciated.

 

[Silvertip]

Re: Tachometers Reading Too High

You are using the wrong tach. Settings 1 - 6 are typically for the number of pulses from the stator on an OUTBOARD. An I/O tach is either 4 cycle (4, 6, or 8 cylinder) specific or the tach can accommodate both engine designs. Read the instructions and set your tachs for 8 cylinders.

 

[Fireman431]

Re: Tachometers Reading Too High

Thanks for the reply. I went through the instructions and set them up accordingly. Is there a way to set these tachs for the correct pulses or am I back to replacing these with different models?

 

[Silvertip]

Re: Tachometers Reading Too High

You have an I/O (a four cycle auto-based engine) hence there are no pulses. You set those tachs for the number of cylinders. Outboards use a stator type of charging system and the tachs are therefore not cylinder dependent. You either have an incorrect tach or or mistaking how to set these these tachs. You have an 8 cylinder engine. The tach needs +12 volts, ground, a light feed, and the tach signal coming from the ignition. Beyond that the only adjustment is the # of cylinders.

 

[PitchFork]

Re: Tachometers Reading Too High

On the old Tach does it have number of cyclinders on the dial on the back? If does try turning the dial around a couples of times back and forth to clean the contacts. I have the same problem sometimes and I take a flat head screw driver and turn the dial around and the flickering tach settles right on 650 RPM at idle.

 

[Barnacle_Bill]

Re: Tachometers Reading Too High

You have the wrong tach. If you look at Faria web site you will see that their tachs that read 0-6000 RPMs are for inboard and I/O motors. Tachs that read 0-7000 RPMs are for outboards.

 

[Fireman431]

Re: Tachometers Reading Too High

Thanks for the info. I checked back at the boating store and indeed they sold me the wrong tachs. I have since gotten the correct ones. Luckily, they said I can have a full refund even without the packaging.

I did try moving the selector switch on the old one. It would flutter around and settle in, but the misreading would come back in a day or two. Sometimes, things just break.

I appreciate all of the good info.