A fuse is protection. If you use one which doesn't blow BEFORE the part it's protecting than the part will probably burn up long before the fuse if there's a short or surge.... in which case why even bother putting a fuse in? You've already defeated the purpose.
Cyto, did you ever get this worked out? What was the issue? I'm pretty sure O/D requires an engine speed signal. If it gets that from the tach then that would explain the loss of that feature.
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