I would say if you only measured 3.3 volts at the valve, then you need a new control unit. The most reasonable source of dependables units is the ProgRama rebuilts sold by a variety of online vendors.

The idle valve is a frequency valve that regulates the amount of air to the engine, and hence the idle speed, by closing more or less of the time. The control unit sends ground signals to the valve to close it. For a reason that escapes me this can be measured as voltage. The higher the voltage, the higher percentage of time the control unit is sending the ground signal. So at 1500 RPM, a good control unit would be close to 5 volts trying to slow the idle down.

You might also make sure that your throttle valve is closing completely. Unhook the throttle and cruise control linkages, and see if that improves the idle. It's not unheard of for the linkage to bind and give the engine just enough "false air" to raise the idle. I suggest this because 1500 RPM is quite high, even if the idle system is completely non-functional.

__________________
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