24 volt vehicle alternator question

[CRS51]

Hello all, I am currently looking to fit a tachometer to my 1958 Leyland 6x6 recovery truck. It no longer has the original cable drive to the rev counter as that went with the original engine. It has a Cummins engine with a 24 volt alternator.
My question is does the alternator produce a signal to go to the tachometer that varies frequency rather than voltage ?
I have been looking and have found so far that some need a DC generator and some need a AC generator.
My alternator W terminal gives out 13 volts AC and the frequency alters with rpm changes.
Have I got this right ?

 

[johnik]

not an expert on truck lectrics but i think that is right, cant see why it would output 13 V when the rest is 24 V unless it is a voltage stabaliser sort of thing, like you have on fuel / temp guages otherwise the tacho would read hi / low ith the battery voltage ?

 

[Barking Mat]

Yep, that sounds right to me.

 

[tom2207]

yep , thats it ,,