Could be either. "Will" expresses volition or "willingness", ie agreeing or refusing to do something, often combining this with prediction - eg I'm sure John will help us /If John will help us, all our problems are over.
- ie If John is willing to help us...
Oddly, in English, machines are often seen as having volition, just lke humans. So just as we might say David won't turn the music down
(ie David refuses to turn the music down
), we can also say Oh no - the car won't start
or, as here, ...if the tuner won't turn on
(ie The car "refuses" to start / .. if the tuner "refuses" to turn on
However, if you don't want to suggest volition, after conjunctions (when/after/before/as soon as/if
etc), present verbs are used to describe "straight" future or habitual events : Call me when David arrives / If David doesn't arrive soon, we'll start without him / As soon as the meeting finishes, let's go for coffee.
As ever, grammar expresses meaning, and the grammatical form we choose to use depends on what we want to "mean" - ie our angle on the event. Thus, if I want to "humanise" my car and see it as having a will of its own, then I might say If the car won't start, then I'll have to take the train
, whereas if I just see it as an objective future possibility, then I'd choose If the car doesn't start, I'll have to take the train.
Whoever wrote the manual obviously doesn't subscribe to the view that machines have a will of their own. Personally, I'm not so sure