I took the car back to the shop yesterday because they had a similar car, but an automatic, which means the ECU is different. We swapped the MAS modules with no difference. The MAS module receives tach signal from the ignition module and compressor on and passes them to the ECU. So there went that theory.

The we dug around in the cabinets and found the MAS testing harness and a breakout box. So I was able to test a couple of things with the engine running:

Voltage from ignition module (tach) to MAS: 6.4V, spec is 7.5-9V.
Voltage from MAS to ECU (tach): 6.6V, spec is 5-7V.
AC signal from MAS to ECU 9.5V, spec is 5-12V.

I also tested the vehicle speed input from the speedo. At stop it was .77V. It went to 3.3V when the car started to move, then gradually increased with speed, like 4V at 50 mph. Spec is >1V at 13 mph.

So it looks to me like the ECU is getting good input on tach and speed. I do wonder about the low reading from the ignition unit to the MAS, though.

Thoughts ???

__________________
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