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NOTE: A 12volt Test Light is recommended for voltage tests below. A light will be dim if weak voltage is present, where a meter will still (misleadingly) show 12 volts.
1) Begin at the 25 Amp blade fuse in the coil/battery area. Could be corrosion problems in the fuse holder or wiring—especially its wire connection at the Battery + terminal.
2) If the 25 Amp fuse & wiring test OK, use test light to see if good 25A fuse current is reaching Fuel Relay terminal 30. "Back-probe" the big red wire in the relay harness plug. Should be a good +12V there all the time.
3) If Relay 30 has good +12V, back-probe relay plug 87-1 (red wire) and turn the Key to On. The Test light should come ON when key is On.
4) If Relay 87-1 voltage is OK, back-probe Relay 87-2 for +12v pump power (yellow-red wire) and try starting.
5) If no Relay 87-2 voltage when cranking, the relay might be bad (if it clicked when cranking). Or it could be a known ECU problem if the relay didn't click (see 6).
6) To test ECU for Fuel relay control, back-probe relay 86-2 (Blue-Green wire). The test light should come ON when the Key is turned to ON, then go OFF when cranking the starter (when ECU grounds B-G wire to energize Fuel relay).
If the test light stays ON, the ECU is probably failing to ground the B-G wire. But if the engine starts with the test light probing 86-2, the light itself may be making the needed relay ground.
See if it starts without the test light back-probing the 86-2 blue-green wire. If not, it could be a bad ECU* — or the ECU is not getting the "OK" signal from the Ignition CU (not likely if plugs are getting spark).
* A temporary fix for this known ECU problem is to "back-probe" a jumper from a good ground to the B-G wire at the harness plug.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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