We receive a call from a contractor who has a Skyjack SJ III-3219, and the description of the problem is "we were using it and it just shut off stuck up in the air". We respond to their location and manually lower the platform and release the brakes and travel motors so they can push it out of the way. The customer declines transport to the shop and further diagnosis.

A few days later the customer transports the unit to our shop for further diagnosis. We notice that the "ground" circuit breaker trips after about 15 seconds of "key on". Checking voltages at the circuit breaker and lift motor, we are getting "inverted" voltages; negative battery volts. Checked total battery volts at the batteries and found that the batteries were installed backwards. The 300amp inline fuse/cable was also broken but touching enough to pass voltage. Corrected the battery direction and substituted a jumper for the fuse and now the circuit breaker does not trip.

At this point the machine will lift from base and platform controls, but it will not lower unless the holding valve manual release is activated. We disconnect the holding valve harness and test the coil and wiring harness resistance; the harness is shorted. The diode in the connector for the holding valve is shorted. Removing the diode from the circuit restores normal operation of the lower function from both the base and platform.

Now we are down to "no steer/no travel". Taking a clue from the "no lower" episode I opened the connectors for steering and travel; sure enough the diodes are shorted. Clipped those and test - steering and travel work now.

Customers tell you the story they want you to hear. Just nod, say thank you, and do the investigating for yourself. Hope this helps.