I think the ohms current source is a compliance voltage of +20V REF through range switches approx. 10MEG, 1MEG, 200k, 20k, 2k going to the +CURRENT output. Short-circuit currents I would estimate are:
2uA x10,000 range (10MEG)
20uA x1,000 range (1MEG)
0.1mA x100 range (200k)
1mA x10 range (20k)
10mA x1 range (2k)
To check if the ohms current source and range switching relays are working, connect another DMM (in mA or uA function) to the 5500 in ohms mode, and see what currents are seen for each range. If you have another meter, I would also check the voltages at the ohms jacks, as I am suspecting that ohms current is being drawn or pulled via the sense lines which might confuse us. Open-circuit (on ohms) I would expect almost 20V out, certainly over 10V.
Strange the ohms function seems to take 0-10VDC in and that gets inverted as a negative polarity, then to the A/D.