I am helping a friend with a 1991 300SEL that is not running because of fuel delivery issues. He had purchased a second hand fuel pump which he did not install. I installed it for him, but there was no voltage coming to it. A little sleuthing on the boards here led me to the Mass Air Sensor, which incorporates the fuel pump relay on this car. One of the users here suggested jumpering pins 1 and 2 on the MAS loom as a test. Doing this produced voltage, so it looked like the MAS was the culprit and not the pump.

However... in my haste to get the car running (my friend was two hours late to show as I worked on it in cold dark conditions with his mom watching, loads of fun), I left the ignition key on while connecting the wires to the pump. At this point the MAS was still jumpered, and as soon as I connected the wires to the pump it started whirring away. However, as I tightened up the connections (which took a while considering the working conditions) it just stopped and it won't do anything now. Did I fry something? The pump perhaps? Should I have not left the MAS wiring jumpered that long? Or is there a fuse or another relay involved?

If I can get an MAS at a wrecking yard and that's all it takes to get her going, he will be stoked.