Many posts are in the archives.
To look at EHA you will look at current (ma). To do this one must be in series with the circuit, This means the circuit passes through the meter. Proper EHA current should swing up and down about 2-3ma around zero ma when warm and in closed loop.
BTW there is a lot of junk out there and prices off the wall. This is not a part to buy in the aftermarket as Bosch's prices are outrageous. Anything else is a pig ina poke. Get the Bosch rebuilt from MB for less than 700 (I think it was 676 list for a 300E last look).
Bosch Master, ASE Master, L1
33 years MB technician