If it is a "B" generation (LDC panel) the curtis BDI circuitry is different than units that are installed on trucks with contact hydraulics. Top cover of the Curtis will have Cat part number. Output from Curtis goes to a pin on the logic unit to allow hydraulics or if battery is dead, you will have extremely reduced hydraulics. I will have to dig out the sea scrolls to see if the logic input is High or Low at which situation.
On Curtis controller, terminal #2 (old large box) terminal #3 (later smaller box) if you read appox. 12 volts with respect to B- then the truck will be in lift lock out (reduced). With properly charged battery the voltage at terminal 2 or 3 should be zero. The wire that connects to the terminal should be PU/B (purple/black tracer) and inputs to the logic unit at card edge connector J2-R. Also check for voltage drop on the B+ wire to the curtis and on the B- wire to the curtis. Try disconnecting the PU/B wire from the curtis to see if hydraulics come back.

on the curtis box, there should be a wire that goes directly to battery neg, a wire that goes directly to battery positive (maybe from the key switch), a wire that comes from the switches in the hydraulic control valve, and a wire that goes to the hydraulic contactor (if it is a contactor and not chopper controlled, if it is chopper controlled, that wire is the input to the chopper controller. there may also be wires to the BDI gage itself, but they don't count _here_.
To -test- to see if the curtis box is the problem*, you only need to jumper the 2 wires, the one from the hydraulic control valve and the one to the hydraulic contactor.
* the safety reason for a lift cutout has to do with preventing a -traction- controller "run away" condition that can happen with too low a battery, and not life of the battery, or some other reason, therefor it is never wise to completely remove BDI-lift cutout.
As good as the battery readings may seem, differences between -any- 2 cell voltages and spec grav should not be more than 10% of the voltage or spec grav.

Hi edward_t
Have got battery voltages from engineer: -
Standing 77.3v, Under load 74.5v, SG's over 1250. (16 months old & is charged without fail).
No obvious frame faults.
Thanks
Steve.

yes Steve_h, but first you absolutly owe it to yourself to insure the battery is NOT the problem, there are safety reasons the truck goes into lockout that you really don't want to experience.
To check the battery, you must take readings from the battery itself and the frame of the truck (post them here, if there is NOT a problem with your battery that needs to be fixed first, we will tell you further how to adjust or bypass the Curtis BDI box)
The first reading is "battery voltage at rest", then battery voltage under load. (run the lift hydraulics to max and stall 3 seconds and read voltage at battery cables) then each cable of the battery to frame of truck in ohms, then each cell's voltage, then each cell's specific gravity (hydrometer reading).

we have a similar problem on a cat m60b, lock out flashes after minimal use of the truck, battery is well up on charge. does any one know how we can overide or short out curtis unit to allow full use without lock out kicking in?

yes a poor battary will be the root cause every time, good to hear you got it running.

the light flashes to tell you it,s battery is flat.

Thanks for everyone's help! It ended up being a very low battery charge. Not sure why that illuminated the "lockout" light, but a good, strong charge brought it back to life with no problems. Problem solved!

If you pull back on the lift and tilt levers you should get it to lift. Check the BDI box under the dash. Check the wires that are hook to meter - and meter +

I had this problem today. ended up being a broken wire.

Has the battery been charged? Does the battery gauge have the name "Curtis"?

this is not an older m50B truck is it?