Diagnostic tachometer question

[bjhc100]

I have a 1986 Force125 hp engine I would like to have an accurate look at engine speeds while tuning the engine. I don't trust the onboard tach but have an ancient automotive type meter which has a tach scale and a switch that allows for setting up to chech 6 or eight cylinder engines. Now, can this be used to check my engine rpm or do I need another type. If anyone knows -How is the signal picked up and sent to the tach? I know Force uses a peculiar tach, and the signal must also match this output. Thanks for anyhelp. Harley

 

[steelespike]

Re: Diagnostic tachometer question

I believe tiny tach has a universal model that covers a number of applications.
A relatively inexpensive easily used tach.Even if possible to use your ancient tach I don't know that I would trust it any more than your on board one.

 

[Frank Acampora]

Re: Diagnostic tachometer question

The Force tach uses AC pulses from the alternator. Simplisticly put, it counts pulses then divides and sends to the meter to give an RPM reading.

Your automotive tach uses DC pulses from the coil at 12 volts and divides by the number of cylinders. Even if you hooked it up to one coil on the Force, Input to the coil is around 200 volts so you would need a dropping resistor in series with the signal line.

Most tachs are guaranteed to within 2 or 5% and also have an adjustment hole in the back of the case. I have never seen tachs drift with age; internals are solid state (doesn't mean they don't though). Usually the meter just jams up and stops working. Unless proven to be way out or damaged, I would trust the existing tach.

 

[bjhc100]

Re: Diagnostic tachometer question

Frank, Thanks for all the good info, it really helps. Now about the tach, it is not original, it is digital and has a LCD readout. Only markings on it are OMC on the face. Thats it. Also it reads about double the RPM that the engine is running. Idle speed reads 1350 to 1450, or so, RPM. Top speed, around 42 mph reads near 9000 RPM, so I am sure this is not correct, but I do get a general idea whats going on underway. Be nice to confirm it for sure, So I guess it may be necessary to replace the digital tach with the right one.
Another weird thing with this boat is the fuel gage. It is eratic whenever the engine is running it tattles faster than a sheeps tail. So I checked all connections, sender gage, where ever I could see. No change, changed gauge, no change, still gitters all over the scale on gauge. It is acccurate though, wiht the key on and the engine shut down. So now I shut it down to read fuel level. This boat has had a lot of stuff done wrong on it when I got it and I have been slowly working out the kinks, when I can. Harley

 

[Frank Acampora]

Re: Diagnostic tachometer question

Some tachs were made that could service multiple engines. This MIGHT be one of them. Check the setting on the back of the tach. If it has 12 pole and 20 pole, set it to 20 pole. If it has 1234, set it to 4 If this doesn't correct it, then it is the wrong tach for the Force.

 

[bjhc100]

Re: Diagnostic tachometer question

Frank, I looked at the back side of the tach and found nothing, only the terminal attached to the purple wire. No switches or anything to set. Thank you fro replying, it is good stuff to know for when I get a new tach.