I have a question about the behavior of the wires leading to the main fuel pump (under driver's seat).

When the key is turned on for the first time in the morning (or any other time that it would sit long enough for the fuel pressure to bleed down), shouldn't it get 12V right away, or should it run pretty much constantly while the key is in the "run" position?

Right now I have a no-start situation that as far as I can tell is related to no fuel delivery. When I probe the power connectors on the pump with a voltmeter with a trained monkey in the driver's seat to turn the key on it just stays flat at 0, like the pump is getting no power.

Power for the fuel pump via Fuse 1 then The Fuel Injection relay
that you can easily check by looking for +12V at Fuse 11
If the FI relay is kaput NO volts at F11

Good luck, Bill

The fuel injection ECU has an input from the ignition system, and the fuel pumps will not run if the engine is not turning over. The purpose is to shut the fuel pumps off in case of an accident. You could have an accident with the engine stopped, a broken fuel line, and the fuel pumps would run until the tank was empty and the fire very hot.

To test the fuel pumps, use a jumper wire across terminals 30 (hot, +12 volts) and terminal 87/2.

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john

Don't forget to jumper terminals 30 and 87/1 on the relay board to engage the ECU. The pumps do run just prior to the engine turning over for about 5 seconds.

There's an entry in the FAQs for troubleshooting the relay. Basically attempt to start the car with the cover off the relay and your finger on the relay arm. If it moves the relay is OK. My guess is the main pump relay is shot.
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D '90 745T 229k, '88 744GLE 129k, '96 328i, '73 911T 100k+

"The pumps do run just prior to the engine turning over for about 5 seconds."

That may not be true for the '88 740, unless it's got the LH 2.4 FI.
The older LH 2.2 doesn't "buzz" the pumps at Key On.

In any case, as John says the FI system requires timing pulses from the Ignition CU before it will energize the Fuel relay during cranking and running.

"The older LH 2.2 doesn't "buzz" the pumps at Key On"

Correct. The residual pressure in the system provides prompt starting. If the fuel pump check valve is bad, the engine needs to be cranked for a longer period of time, and it is noticeable.

I also have a 1994 944T with LH 2.4. The car isn't here at the moment, but I do not recall the fuel pump running when the key is turned from off to on (run).
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john

Have you checked the relay for the pump?
Its behind the ashtray, make life easier and remove tray above the fuse panel.
Remove cover bezel over the ciggie lighter and find the 2 screws holding it.
I tested it by listening for a click when key is turned, pop the cover and check solder joints .

Mine died yesterday. Always have a spare. It's the big relay in the back on the left side. I literally drove 500 feet, shut the car off and couldn't start it again. It happens that suddenly.