1968 Dart gauge help

Since "you diddled" with it, likely something you did. Maybe you shorted the ammeter connections to ground. The ammeter is NOT grounded, so check each stud of the ammeter to the cluster ground. Should be open.

Wander over to MyMopar and download a FREE factory service manual, and there are also aftermarket wiring diagrams over there, which are sometimes easier to follow than factory, but sometimes not exactly complete

MyMopar - Mopar Forums & Information - MyMopar Tools/Reference

Also there's an article on 'curing' the bulhead connector problems, and even if you are not intending to do so, there is a simplified diagram there of primary power distribution

This is simplified, but shows the major main power. Notice that NOTHING is fused in this diagram, IT IS ONLY (somewhat poorly) protected by the fuse link

From the battery, through the fuse link, through the firewall / bulkhead connector on the BIG RED ammeter wire, and to the ammeter. On the other side of the ammeter the BIG BLACK comes off and goes to the WELDED SPLICE which is under the dash in that harness. That splice splits power off, feeds back out the firewall to the alternator, feeds power off to the fuse box "hot" buss, and feeds power off to the ignition switch. And it feeds power to the B1 terminal of the headlight switch. NONE of that is fused.

Catalog

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