Marshtown wrote:...What next please?
Your troubleshooting tests allude to Ruby having suffered a shift in its ability to accurately assess main brush running current. Measurement of motor current is accomplished on the main_PCA via certain circuits and the MCU + firmware.
Your easiest tactic to compare Ruby & Fido's main-brush-current is to follow the manual BiTs procedure
given in Appendix A of the Roomba 500 Service Manual. By doing that, you will see displayed on your monitor results such as in this example:
Service Manual wrote:SUMMARY: (main-brush-motor-stall?) PASS
SUMMARY: (main-brush-current-ok?) PASS mA -283 min -500 max -200 mV 15947degrees-C 37
SUMMARY: (main-brush-stall-current-ok?) PASS mA -1338 min -1400 max -1250 mV 15892 degrees-C 37
If you need help parsing those records, just say so. It is unfortunate that iRobot people made two engineering errors in they way those data are expressed.
In the example, 283mA is the measured running current, and it is well between MAX & MIN limits of 500mA & 200mA. Your Ruby may very well indicate current beyond 500mA and possibly approaching the MIN limit of 1250mA stall-current.
By doing the main-brush BiT on both Fido & Ruby, but using the same CHM in each, you can compare the reported running currents to determine whether Ruby is suffering a main_PCA fault. If such is the case, your only recourse (I am sensing you are not EE capable) will be to buy a harvested main_PCA for Ruby and switch new for old.