Posted by: Mark Polk | 2014

71 Power Wagon – Mopar Charging System Wiring Headaches

Mopar wiring harnessIf you ever worked on old mopar wiring, especially the charging system, you are in for a treat. When I rewired the Power Wagon project truck I wanted to upgrade the wiring to handle modern day loads. This involved installing a new blade style 12 circuit fuse box, electronic ignition, new alternator and electronic regulator just for starters. The wiring upgrade was not too difficult and upon initial start-up of the rebuilt poly 318 everything seemed okay except for the charging system. It seemed like no matter what I tried the battery was not getting a charge from the alternator.

After some indepth research on the topic I learned that Mopars old school wiring for the charging system was just that, old school. It was probably one of the worst wiring configurations for long term dependability ever,  so you can imagine how it functioned on a 43 year-old truck.

Basically Chrysler took 12 VDC from the battery and routed it through the bulkhead (#1 connection) went up to an ammeter on the dash (#2 connection), out the other side of the ammeter (#3 connection), to three branch wires off the main wire (#4 to headlight switch, #5 to fuse box, #6 to ignition switch) and then  the main wire goes back through the bulkhead (#7 connection) and the circuit completes itself at the alternator battery post (#8 connection)

So, you can only imagine what could possibly go wrong on a 43 year-old truck’s wiring system when the wire carrying all of the current to the system and back to the battery goes through 8 separate connections. In addition to this problem when you install electronic ignition and an electronic regulator the metal casings need to have good grounds to the newly painted vehicle body for everything to work. My first step was to make absolutely sure there was a good ground at these 2 boxes and since it was a fresh rebuilt engine installation I also double checked for a good ground from the engine to the vehicle frame, and from the engine to the truck’s cab. I will tell you that before you waste any time troubleshooting any wiring problem on an old Mopar make sure all these grounds are good. If not you will never find the problem you are looking for in the first place.

Next I wanted to make the charging system wiring bulletproof so I don’t have problems with it down the road, so to speak. I researched books and the Internet for possible solutions and as usual found lots of different opinions on how to handle the problem. One thing that almost everybody agreed on was to bypass the old ammeter and as many of the old wiring connection points as possible.

The best solution I found was to route the wire from the battery post on the starter relay directly to the battery post on the alterator. This eliminates the old ammeter and all the other bulkhead connections. I plan to install a new set of gauges, including a voltmeter, so losing the in-dash ammeter is not a problem. Next I ran a second wire from the battery post on the starter relay through a grommet I installed on the firewall (eliminating the bulkhead connection) and connected it to the wire that came out of the other side of the old ammeter. This wire still supplied power to the headlight switch, the fusebox and the ignition switch. To complete the circuit I routed the wire I used to go to the battery post on the alternator back through the same grommet on the firewall and back to the starter relay. What you need to keep in mind is the wire size you use going to the alternator battery post and the one supplying power back to the wiring under the dash. I used 10 gauge wire, and coming off the starter relay on both 12 VDC wires I installed a 14 gauge fusible link in front of the wire. A fusible link will protect the rest of the wiring is the event of excess current running through the wire. You should use a wire gauge about 4 gauge sizes less than the wire you are protecting when selecting a fusible link.

Something else I have learned over the years, as it pertains to any wiring, is an electrical circuit is only as good as the connections in the circuit. In other words you might do a great job rewiring the vehicle, but if  just one of your electrical connections are bad none of the other wiring matters. I like to solder connections whenever I can rather than depend on the typical red, blue and yellow crimp style connectors.

I rewired the old charging system and it is working pefect, even at an idle. Hopefully this post will help prevent someone else from spending the time it took me to figure out the charging system problems.

Go to next post:  Fenders, Hood & Steering Wheel 

See the full build HERE: 

Mark Polk

Auto Education 101

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