When that happened to our M59 last year it turned out to be the battery cables themselves. The ends that connected to the battery seemed good when I cleaned off the corrosion, but it turned out that the copper itself was completely gone inside the insulation for about an inch back from the battery terminal connectors. When I felt the cable, the corrosion was hard like wire and that fooled me.
I ended up buying some new terminal connectors on Amazon or Napa...forget which...., cutting the cable back to good wire, and clamping them on.
BTW, that OEM wet cell battery lasted 15 years but it did make enough fumes to require cleaning the cables every few years. Even the connection to the frame was corroded. I didn't realize the same fumes were reaching into the battery cables and traveling inside the insulation. The neg. cable was the most affected.
If that doesn't do it, open the fuse box and check the fuses - particularly the 5 amp fuse. It's the one for the safety switch under the seat.
I really ought to replace the cables and ends both some day. I've already replaced the battery with an AGM.
rScotty
