Thanks for all the advice everyone.
I read through the manual and could not find anything additional, so went back to it this morning. Had the battery fully charged, cleaned the power steer motor out, tested the truck off the floor without any load on the drive motor, which connects to the power steer motor through the chain mechanism.
I tried changing voltage settings on the Zapi EPS system.
Still no joy, the customer (being a forklift engineer himself) is adamant its nothing mechanical as he has already checked all other aspects.
I noticed when I repalced the tacho generator it had 24v sticker on it, could it be this tacho should be 48v? Im just ready to try anything now :)
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