may want to check the 24" prox sensor
if its bad or not adjusted correctly it will put the truck in 'slomode'

To test the sensor do the following:
PROXIMITY SWITCHES
These are noncontact devices which sense the presence
of steel. They have three wires going to them:
battery negative, 15 volts (approximately), and an output
signal. They also have a built-in indicator light that
lights the collar where the wires come out. These devices
can be checked by looking at the indicator light
and by testing the output with a voltmeter to see that it
changes with and without the steel in front of it.


the height encoder on top of the mast channel is a suspect also but generally when they malfunction they disable the truck, or at least in my experience thats what happens. There would also be a code displayed on the BDI indicator during this malfunction.
To test the encoder do the following:

ENCODER INTEGRITY
Steering and height encoders are tested continuously
for current draw. If the total current draw from the two
encoders and the Hall effect sensor is too high or too
low, a fault will be generated and truck operation will
be inhibited. Additional software continuously checks
the output bit sequence activity of the steering input encoder
to ensure that both bits are operating.

TEST ENCODERS
Conventional means can be used to diagnose the
encoders as follows. Using a voltmeter, determine if
5 volts and battery negative are going to each encoder
by back-probing the connector. If good, then connect to
one output channel and battery negative, and rotate the
encoder very slowly to see if the input goes on/off/on
(0 then 5 volts) when the encoder is rotated slowly.
Check the other bit also. If they are both outputting, the
encoder is OK.

Viper does this unit have an encoder on mast? should be located on top left chain pulley if i am remembering right. Also it may have 2 controllers. One controls drive and the other is like a VCM. If the VCM was changed out then you will have to program the encoder height read and all parameters unless it is an OEM unit ( which is a pain and done through joystick and deadman switch). If it is the encoder thats "wigged out" then a replacement encoder has to be wired correct as a couple wires will need to be reversed on the encoder or it will count reverse. Been several years since i have dealt with one of these. Perhaps someone else can add more info or be more help..