>I have fuel when I manually jumper the fuel pump. None when I install a known good relay.
>I believe the Bosch computer is at fault.
More than likely, yes. There's one chip in the fuel injection computers
which seems to be what most commonly goes bad. That chip controls the fuel
pump by supplying a ground connection to the fuel pump relay, causing the
pump to run. Some Saab and Volvo cars use basically the same fuel injection
system and also commonly have the same failure of that chip in the fuel
injection computer. That chip determines when to run the fuel pump based on
an engine RPM signal it receives. The idea is to run the pump when the
engine is turning, but cut it off when it's not, so that fuel won't be
pumped in the event of an accident. The older 928s have an RPM sensing function built right in the fuel pump relay in the fuse panel that does the
same thing. With the later cars it's done by that chip in the fuel injection computer separately from the fuel pump relay in the fuse panel.
>Has anyone opened the unit in question and effected a satisfactory repair? And if so is this unit potted with epoxy or poly resin?
The fuel injection computer is not potted. That chip that controls the fuel
pump will be the large strange looking one, not one of the typical black
>Has anyone "reverse engineered" this part and willing to share info?
At least some of the rebuilders of them use a circuitry board in place of
the chip that was originally in there. The replacement circuit boards
*should* be more durable and longer lasting than the original chips were.
There's some information on repairing the problem from the Volvo guys at:
If my fuel injection computer ever has the problem, don't anybody be at all surprised if they see an retrofitted older 928 fuel pump relay in my car.
'88 928S4 Auto Black/Black "PORSCHE" cloth
Well, it is not exactly one chip. It is a hybrid pcb that is stacked on top of the main pcb of the LH2.3 ecu. And the LH2.4 used in Volvo is similar but the pcb so much different that there is no way to interchange them, even if internal software would be upgradable. I looked at this in detail.
And I don't think it is a good idea to change the fuel pump circuit when the LH failed. The LH is deliberately managing the fuel pump so taht in case of an accident the pump stops when the engine stops. This is a life saver. You don't want to disable it !!!
The problem with the LH's is the hybrid pcb that fails over time, more or less starting at the age of 10 years. The pcb takes care of fuel pump, lambda signals, idle signals, AC on signal, watchdog, and many more. But the LH has some more bad spots, one is particularly difficult. It is an ASIC chip that can not be purchased anymore. And we see LH's that had water in them and were badly corroded. And after 20 years... all components suffer from aging which is not so hard to imagine. So a complete overhaul is required. We do this as one of the few in the world I believe.