Warm up valve only getting 9 to 10 volts WUR

Hi the wur on my mk1 golf 1.8 Gti is only getting about 9 to 10 volts but in the Haynes  manual is says it should be 11. Volts and higher if this has happened to anyone could they point me in the right direction of what to do or if this has been posted before I could have the link to page. 
Thanks
Ben