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Check my previous posts on electrical topics for details. Simply fitting a larger value fuse and incorporating that into your car's loom will provide the upstream part of the loom with little or no protection from your modifications, much like fitting a 50amp fuse for an "upgraded" fan on your radiator - the wiring will melt long before the fuse ever blows.
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You really DO need a relay and you also need to have a separate fuse for the overdrive solenoid additional circuit. The black box is the relay that handles the 20A peak current drawn by the O/D solenoid as well as changes the negative polarity supplied by the Volvo relay to a switched +12V. That's the one in conjunction with the oblong board with electronic bits that gives me the push button operation as well as ensures that the O/D remains disengaged when I leave 4th gear and re-engage 4th without touching the push button. My understanding: when I change out of 4th OD will automatically disengage BUT, when I change back to 4th, OD will automatically reengage, if I haven’t manually switched to the OUT position. Mine will be more basic, with 2 position IN/OUT switch. I’m guessing that the black block is a transistor that, together with the rest of the electrics, automatically disengages yr overdrive when you change out of 4th gear, as well as enabling that 1-position switch on yr knob, press it once to engage or disengage OD?