Fanuc 21T 701 alarm - Fan overheat

I've got a customer with a Hardinge lathe with a Fanuc 21T control. It sets a 701 alarm for fan motor/overheat within 8 second of booting up. They said before it was intermittent, but now it's all the time.

They've done the following things:

Checked all drive fans. All are working.

Swapped all of the drives from a sister machine to this machine.

Pulled monitor panel and replaced both fans above the circuit board cage.

Replaced several of the cabinet cooling fans installed by the machine builder.


I've checked the drive fans, both the fans inside the yellow cover and the fans on the heat sinks. All are spinning and moving air. The new fans above the monitor are also working.

These are all brushless DC fan motors. I'm assuming that the control monitors the commutation feedback from the motors to know if the fan is actually spinning.

What I'm worried about is some kind of cable or connector issue that is losing the signal from the motor to the control. I don't know how the system really works. Does the drive monitor the fan motor and give a yes/no signal to the control, or does the control monitor it directly?

Is there another fan somewhere that I don't know about?

I'm thinking the cabinet fans are not being monitored because they are two wire induction motor fans. The spindle is an integral motor with a separate chiller unit, so I don't think that's it.

I have a feeling you missed a fan somewhere.

Not sure if these fans are among those already changed or checked but here is an old thread:

https://www.practicalmachinist.com/vb/cnc-machining/fanuc-18-mb-701-fan-motor-overheat-alarm-214328/

If all fans are positively working could be a bad control board. The circuitry detecting the fan's RPM on the third wire may be bad.