OK, but with all disclaimers about not being an authority.

The ECU is sending on/off signals to the frequency valve or EHA. These can be measured by duty cycle at Pin 3 as a %, or as DC voltage representing a % of battery voltage at Pin 3. Or as current at the EHA, I believe.

So, you measure battery voltage, V1, with the engine idling and say you get 13.5V. Then you measure Pin 3 to ground, V2, and get a reading bouncing around 6.25V. V2/V1 x 100 = 50%.

I am not making this up.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe