tacho generator disconnected

as soon as the machine is switched on "tacho generator disconnected"alarm appears.can someboby help fast

No output from the Spindle Tach on the end of the spindle shaft, what machine?
If you pull the spindle over manually, you should see a plus minus DC voltage out of the tach depending on direction, the level depending on speed, check the tach brushes.
Al.

it is a fanuc all electric molding machine pretty old may be 1994 make. this perticular servo is y axis

Substitute Yaxis servo for reference to spindle in my first post, they must be DC servo's with a tach on them.
Remove motor or disconnect drive coupling to test., spin it over by hand and see if dc output from the tach, check all the Main armature brushes while you are about it.
Al.

this particular machine the fanuc motors are black body with red caps for the encoder(if color of the two is of any relevance as to know which type a/c , d/c etc).when the motor shaft is rotated by hand, the reading on the monitor changes accordingly, does this feed back help?

Red Cap, then they are AC servo's.
Try swapping two motors over, if they are the same size to eliminate the encoder, I have never seen tach loss message on AC servo's?
Al.

I have, I suspect that this alarm is an early days hang over from the change from DC to AC, quite often a wire break on the motor thermal sensor

we checked the drive card, there was a problem with what the expert said "module", which after replacement , the alarm message is gone. But we have a new problem. now , i think its because the encoder cable was disconnected from the drive and hence it has lost its Y position value,the new alarm says, "return to original position - Y".We donot know if there is any homing to be done for Injection Molding machine like VMCs,CNC Lathes.But otherwise we dont 'home' the machine at every 'power-on'

It seems to be a absolute pulse coder. Normally we will made Zero return afer the Drive service or Backup Battery of the Encoder fail. Once did ZRN after that is not required to ZRN. Check up with your manual for procedure.

how to do ZRN for absolute encoder. can u pl help

What is the control for ex. 15M, 15 IM,21Im ...
we need that info to give you a procedure for it
Djack

it is a fanuc 10 tf control