"Jumpered left side of fuse 4 to fuse 6 and car started (both fuel pumps running). Removed the jumper and car shuts off and won't start."

The jumper bypassed two known failure areas:

1) The 25 Amp blade fuse and it's wiring from Battery + terminal.

2) The pump relay and ECU. Either a problem with the relay itself, or with the ECU failing to energize the relay.

My approach would be to replace the pump relay with one I rigged to be always "on", so it powers the pumps as soon as it's plugged in (see photo). If pumps run and engine starts, the 25 Amp fuse circuit is OK.
Still no start? Take a closer look at the 25 Amp fuse & wiring — or try Relay/ECU test below.

Plug the original pump relay back in and use a 12V test light (or DVM lead) to "back-probe" the relay harness plug at the Blue-Green wire. Then try starting.
Is there 12V at Blue-Green wire with starter cranking?
Yes = The ECU is doing it's job, trying to energize the relay (relay problem)
No = ECU is failing to energize the relay

Pump relay forced ON with small piece of hose holding armature closed.





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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.