I suspect he his talking about an accessory option that heats the coolant in the engine on cold mornings. Information on this will not be in Haynes. Your best option is to contact your local Volvo dealer, with any luck, they ca supply you with a manual.
George
[link:www.ttsservices.co.uk/my%20volvos.htm|
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'95 940 Polar LPT estate (B230FK/AW71)
'87 740 GLE Turbo Diesel Intercooler estate (D24TIC/ZF22)
'79 262C (B27E/BW55)