when van gets hot, the speedometer quits working and the engine light comes on.

The Sienna use the ABS brake sensor to monitor vehicle speed not a speed sensor on the transmission like most other cars. You need a wiring diagram to trace the circuit and look for a ac voltage that increases with vehicle speed as you drive faster. Does the ABS light come on? Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info. There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online