The APU starter requires an enormous amount of current during the initial start of the auxiliary power unit.
The 767 APU is similar in size to the A300. An example of inrush current to the A300 starter can be seen here.
On this particular 767, we had no starter engagement. Voltage was not present when checked directly at the starter lugs.
When using the aircraft frame as a ground reference, we had 28VDC.
We could wiggle the firewall plug (upper one in this image) and have the starter ground go from 10K+ ohms to 2 ohms.
The ground lugs are located through a access door just forward of the firewall.
Initially, we could physically move the lugs by hand . A small amount of arcing soot could be seen below the lower lug.
The full extent of the problem wasn't known until a mirror was used.
If you notice, the nuts were not tightened down (look at the threads on the lower lug for the APU generator, it's tight). Loose connections equate to high resistances that cause heat. Especially with current well above 300 amps. This ground lug plate is at least a 1/4" thick.