I have similar problem with 99 ML430. Check Engine light came on and I got P0446 and P0455 codes. Charcoal Canister Shut-Off Valve and Evap System leak. The fuel cap and rubber seal look good. I have Snap-on scanner and followed Gilly's procedure to do a vacuum test. No vacuum. I activated shut-off valve with the scanner and measured voltage at the valve on the back of the connector - 0 VDC. Positive 12 is there on the blue wire, but negative doesn't come from the pink wire. When I measure voltage with the connector disconnected from the valve I did see some activity on it while switching the valve on/off with the scanner, but not full 12 VDC. The valve coil reads 12 Ohm. It looks like pink wire has bad connection somewhere or ME module defective? I checked connector behind the rear left wheel and it looked good, no water or corrosion on the pins. Are there any more connectors for that wire on the way to the ME module?
Thanks, Mike