Where can I find a diagram of how to install a belt tension pulley on my Kenmore dryer Series 70 ?

For that type of info you have to buy a SERVICE MANUAL.

Try any of the following Sears links :

Sears on-line website (http://www.managemyhome.com/mmh/rails)

http://www.sears.com/Service (http://www.sears.com/shc/s/home_10153_12605)

http://www.searspartsdirect.com/partsdirecta (http://www.searspartsdirect.com/partsdirect/index.action?sid=PSHx20080114x00001I)

http://www.searspartsdirect.com/partsdirectb (http://www.searspartsdirect.com/partsdirect/index.action?sid=PSHx20080204x00001a)

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