I do not know about 91 300E but I would go to the JY and pull on the belt of the same model year. Some belts work on 'jerking' movement of belt, some works on car's movement. There is a pendulum inside and if the car decelerates then it will trigger and lockup the belt. I think you can try to brake HARD and pull on the belt at the same time and see whether it locks. I suspect it would.

info here

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.