IBM 5155 - Power Supply
This power supply requires a sufficient load in order to start/operate.
In some cases of insufficient loading, the power supply starts (you see the fan turn) but the power supply stops after a second or so. In other cases, the fan won't even turn. Note however that both of those symptoms can also be produced by the power supply being overloaded by something.
Some loads that you think may be sufficient are not.
* An IDE drive (eighties/nineties vintage) is not enough of a load. My experiments with two 5155 power supplies show that approximately four to five IDE drives are required.
* Even some full height multi-platter MFM drives fail to enable this power supply to start.
* The 5155's CRT unit is not enough of a load.
* The 5155 motherboard is enough of a load to start/operate the power supply. This is the case even if P8 is disconnected (but note that the motherboard requires both P8/P9 to operate).
From the above, it appears that it is the +5V line that needs the start-up loading, with the other lines (-5V, +12V, -12V) either having no effect or having little effect.