I have a service manual from the late late 80s before they stopped reverse rack production and moved to the DWU machines, it shows multiple versions of main wash heating, and named them different features like Sani Wash, Temp Boost, or Power Boost. All had different functions like a thermo hold to 140F, or just a timed section in the wash, or a heat on at all parts of the cycle. For the most part you had to spend the money and buy a machine that had a water heat option.
On my WU902 I jumpered the Power Boost switch as it used to extend the wash to 26 minutes AND do a thermo hold to 140F, which is too long to me as the main wash goes for almost 35+ mins. So by jumpering the switch it still does a thermo hold but doent extend the main wash to 26 minutes.