motorola syntor X DES problem ENCODES DES doesnot DECODE des

i have searched the groups with no answers to this question, i have
been given to repair a motorola syntor X VHF mobile trunk mounted
radio for repair. the problem is, the DES system. the customer loads a
key with a keyloader, the unit will transmit in secure mode, other
units will decode the DES, but this particular syntor x mobile, will
not decode other units. it just opens its squelch to hear digital
static. it should either DECODE incomming DES signals with propper
key, or it should switch to clear mode automatically if it detects a
clear signal, it basically is always in clear mode when recieving,
even if it is set in secure mode. any ideas as to whats wrong? i have
tried to swap the secure head with a good one, but the same problem
happens with this unit with the other working secure head.
hmmmmmmmmmmm.......