update: I obtained a used MT2500 Snap On red brick scan tool, with cartridge, cables, keys, and manual on Ebay, from 2 separate auctions on Ebay. It does everything needed to diagnose and scan the M120 V12 engine in the '94 S600 coupe that I needed.
it will scan 5 modules, i.e. 2 ignition l/r, 2 fuel injection l/r, and diagnostic module.
reads codes, erases codes.
live data for engine sensors while running engine:
MAP, MAF, o2, TPS, coolant temp, short term/long term fuel trims, variable valve timing on/off, egr on/off, fuel pump on/off, barometric pressure, total ignition advance, engine rpm, vehicle speed, and more.
the transmission module also shows up but I can't get into it, perhaps I need another key.
the only keys needed to read all the above are the S-20 and S-21 which were supplied.
$250 total. well worth it. extra cable lets you drive car with scanner and get live data.
the experts who sell these vintage scanners told me if you get the 2003 cartridge, that's all you need. it has all the updates. newer cartridges beyond that will not provide any additional information.
it has shown a lot already. the throttle position angle at idle was slightly too far open. spraying the linkage fixed that.
both egr valves are opening at idle, they are no doubt stuck slightly open. I'm going to disable them.
car has well over 32 degrees of total timing at high rpm, that's good.
MAF and MAP sensors are working correctly according to voltage and airflow measured.
o2 sensors are DEAD. they sit down at .100-.250 volts and don't vary like they should from .100-.900
so I ordered 2 new ones.
the car is pig rich because the o2 sensors are giving false lean readings at idle.
so the computer is overcompensating with a rich mixture, the fuel trims are too rich.
new o2 sensors will fix that. they are on the way.
I pulled all 12 plugs, soaked them, and cleaned, and reinstalled. runs much better.
ordered 12 new plugs Bosch nonresistor, they do spark better, I tested them, because they only have 1.5 to 4 ohms of resistance. resistor plugs have 5k ohms of resistance and rob the spark ! I tested the plugs on a magneto and the nonresistor plugs have a huge spark, compared to a wimpy little spark for the resistor plugs.
more to follow.