Cummins VT555 tachometer reading half RPM

New tachometers were installed by a previous mechanic but they indicate half the actual RPM. Tried adjusting the tachometer settings but won't go higher. Both engines are the same.
Anyone know what tachometer is compatible, or if something can be adjusted on the engine?

Were the alternator pulleys changed?

No idea, I put a scope on the tach signal and its a steady pulse, at idle about 10 hz if I remember right. So 10 a second, gives 600 rpm, but it shows 300 on both tachs.

Does the back of the tach have a selection switch “2,3,4,6” ? Alternator driven tachs usually are on “6”.
Do you have a synchronizer? The tach signal may be coming from there.
Out of ideas

Yes it has a DIP switch with the 4 positions, its on the max, all other settings give lower readings.
No idea where the tach signal comes from as haven't traced the cable to the sensor yet.
As both engines produce the same half RPM readings, I've assumed that the problem is that the replacement tachs are just not compatible.

It sounds like the tachs may be W signal tachs for the typical alternator signal. The signal may be from a flywheel or tach driver. The W signal I believe has a much higher frequency. You will need to figure out where the signal is originating and then buy the correct tachs or vice versa. Matching tachs to signals can be a bit of a PITA.

Or you can use the Dakota tach signal adapter to correct nearly any signal and then fine tune it. Its an easy solution for $99

On my V555 the tach signal is driven by a sender right next to and part of the PT pump drive in the Vee at the very back of the engine.
My tach is still the old AC unit but it still shows rpm reasonable accurately.
I have checked it several times with my digital handheld tach.
I have never bothered to check its signal as it has worked just fine for the many years I have had the boat/engine.

Barking sands may have a good fixup.
Otherwise I would suggest calling the tach mfgr. and asking them if they can help suggest anything.

Thanks, I guess I'll have to borrow a tach meter and see what it says, the scope signal is sort of W shaped, so maybe its off the alternator, though don't know if that's why it's called a w signal.