Yes, at some point during 88 the FI wiring changed so that the power for
the ICU comes from the fuel pump relay instead of the ECU power relay.
Thus in those later models the ISV only hums when the fuel pump is
supposed to be running.

Mark

Neil N wrote:
> In Bentley wiring diagrams for '86, to '90 and up,
> a change in wire gauge aside, I see the same
> wiring scheme from ICU to the ISV.
> On my '88 (which tends to use an '87 wiring diagram),
> the ISV "hums" with key to ign. on, engine off.
> IIRC, on later model years the ISV "hums" only when engine running.
> Possibly ONLY due to a change somewhere else in the wiring harness??
>