(ET) Odd E-15 problem

[Steve Gaarder <gaarder ecovillage ithaca ny us>]

We've got an E-15, powered by a double string of BB600 NiCads, and it has 
developed an odd problem.  When I try to run it (speed 1, gearshift in 
neutral) the motor pulses at about 5 times a second and the current draw 
is very large - almost to the top of the yellow zone.  My first thought 
was no field current, but I get full battery voltage at the field relay 
contacts, and if I disconnect the power and measure the field resistance 
from there, I get 16 ohms, which seems reasonable.

Any ideas?

thanks,

Steve Gaarder