Starting Problem; E28/Motronics/Fuel Pump

[Tom Mathews <bimrone@xxxxxxxxxxxxx>]

I have a starting problem.  I can start this 535is in the morning and then drive to  work (12
miles).  Then, when I try to start the car at noon, it will not start (cranks normally).

I then checked the power to the fuel pump relay.  The fuel pump relay has power, but the
relay is not closing because there is no relay control signal from the motronics unit.  If
you "hot wire" the fuel pump (by connecting the negative side (motronic relay control) of the
fuel pump relay to ground), the relay will close, the fuel pump will operate, and the car
will start and operate normally.  If you then take off the "hot wire", the fuel pump remains
powered.

I suspect that the car starts when cold because the residual fuel pressure is injected
through the cold start valve.  At some later point, the fuel pump turns on.

My question:  Does the fuel pump operate at startup?  If so, then this motronic unit must be
bad because there is no relay control signal output at startup to close the fuel pump relay.

Tom Mathews
88 535is

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