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Doug,
Bentley's tiny reproduction of the Volvo diagram can be hard to decipher. As I see it, the #85 terminal is + voltage only to the Fuel relay coil. The Main relay coil + is from #30 (also feeds the armature + for both relays).
The ignition switch "wakes up" the ECU at pin #35. The ECU then puts a ground on Main #86/1 to energize the relay. It does the same for the Fuel relay at #86/2 when it gets timing pulses at pin 1 from Ignition CU pin 17 during cranking and running.
Yes, the ignition switch is a likely suspect for the Main relay, as it enables the ECU to make the coil ground at #86/1. That assumes the coil also has the constant + voltage at #30 from the battery and 25A fuse.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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