What is the purpose of this stall test? I've never heard of that before?
When a torque converter is working normally (at low speed), the input is turning faster than the output, but by the magic of the internal hydraulic design, the ouput has more torque than the input, and the torque multiplication is nearly proportional to the difference between input and output speeds.
If you stall the convertor (in gear with the brakes on hard), with the engine at wide open throttle, the engine is turning the input of the torque convertor, but the output is not going anywhere, so you know that the speed difference is the same as the engine RPM, and this is also (usually the condition where the TC is producing maximum torque.
The usefulness is:
1. If the sprag clutch in the TC is slipping, you will know (and probably hear) all about it, or if the TC stator is seized, the stall RPM will be much too low.
2. If the wrong model TC is fitted, the stall speed will be wrong (as Jim just discovered).
3. If the TC is known to be correct, but the stall speed is a bit low, it can be a symptom of a sick engine.
BTW, when conducting this test, the entire engine power is being dissipated as heat in the hydraulic fluid, so don't do often or for long. I have seen an idiot do this until there was smoke coming out of the transmission on his V8 American lump.