['85 GSL-SE]

I have gone though the test "checking signal for switching solenoid valve" on page 4B-32 of the shop manual. It seems that the signal is not quite right. The manual says the solenoid should suck air at all RPMs, however, mine only sucks at idle. I have tested the soleoid itself and it works fine, so it must be a signal problem. However, the manual does not explain what to do if the signal is bad, or even where the signal comes from. Does anyone know?

(I searched first but didn't find anything)