The ground is transferred from section to section by the mechanical connections (screws) which hold the sections together. Check for bad/loose/dirty ground connections and also check to see you don't have a burned up trace on one of the boards.

Also, make sure the big plug's indexing pin didn't break off and you don't have it connected wrong. I have seen that happen many times...the center pin stays in the hole and then you don't have it aligned right when it goes back together (it is one or two pins off). Check those items and you will likely fix it.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz