Common Electrical Problems and Solutions on 1975 - 1980 Dodge Trucks
A common problem on 75-80 Dodge trucks is amp gauge failure. The problem may be caused by loose nuts on the amp gauge studs or nuts that get loose as you push the dash instrument cluster into place with the stiff black and red wires trying to turn the amp gauge outer nuts. Sometimes the nuts are loose from the factory, which will cause heating of the stud, then the plastic melts around it, further restricting the transfer of heat away from the stud. There is a thick factory nut with serrated faces against the PC card under the electrical connector lugs. Maybe the PC card shrinks over the years and lets the "inner nut" loosen, allowing the connector to get loose on the stud.
The solution is to add a flat washer between the PC board and the inner nut. Tighten the inner nut fairly tight, then attach the lug, a lock washer, and the outer nut.
Hold the inner nut with a thin 3/8" open end wrench and torque the outer nut with a 6 point socket. Make sure it is tight enough it won't loosen up.
Editor's note: On the metal base cluster we sell, we replace the outer nut with another thick serrated nut like the inner one, eliminating the need for a lock washer.
Another common problem is the battery and alternator lead connections at the firewall bulkhead connector. They melt the surrounding plastic, because a ¼" flag connector won't carry 20 amps, let alone the 40 plus amps the truck requires. The symptoms are; the truck dies suddenly one day, or all the lights flicker and the amp gauge needle flutters.
The solution is to solder and connect a #10 or #8 THHN stranded building wire at both the alternator (black) and battery (red) connections under the hood, cutting the original two heavy wires free at the back of the bulkhead connector, and running the new #10 or #8 wires through a rubber grommet in the firewall (you don't want them to short to the sharp metal edge of the hole) next to the bulkhead connector and straight to the amp gauge. Dress them against the left end of the dash cavity with a little extra wire, maybe in a split plastic 3/8" or ½" sleeve, so it will fold nicely when the cluster assembly is installed. Installed that way the wires won't try to loosen the outer nut. Having the extra length of wire also makes it easier to install the dash cluster.
After stripping the insulation from the end of the wire spread the strands apart then twist them tightly together with no sharp ends protruding. Solder the wire into the lug with rosin core solder, either using a very small propane flame or big (100W or more) soldering iron. Put two 5/16" or 3/8" shrink sleeves on the new wire first, and slide them far enough away from the work area (12") so you do not accidentally shrink them in the wrong place when soldering. Slide the shrink wrap over the joint after it cools and shrink it with a heat gun; check for any sharp points first, if you find any, fold them in with pliers (sharp points will penetrate shrink wrap). It is a good idea to put on a double layer, (two shrink sleeves). At the amp gauge, you will not need the red wire coming from the battery/wire harness at all now, but leave old black one connected, it powers the truck. The new red wire is alone, the new black wire is on top of the old black wire. Fold back or cut off the old red wire. Note the old colors first, the amp gauge will read backwards if the wires are connected incorrectly. You can use tape to identify your new wire if both wires are the same color. It is ok to leave the stock wires dead ended inside the firewall in the bulkhead connector. On the 78, these colors are black and red, you will need to check other years. The red (battery) wire can also go along the harness to the starter terminal via a 12 gauge fusible link (Ford) if the wire is in rough shape under the hood (sometimes these are very stiff or damaged at the starter relay).
Amp gauge lugs, even if crimp type, should be soldered too. There is a heavy current load here, and manual crimps only cause more heating, bad connections, and eventual corrosion. You will need a lug set for #8 or #10 wire on a #10 stud. Cut any plastic insulating sleeves off the lug, solder and put a piece of shrink sleeve over the soldered end for a professional look.
#10 wire is the stock wire size, good for about 40 amps in THHN, which is big enough. #8 is good for 55 amps, but the truck will rarely have more than a 30 amp current load on the wire even with a larger alternator, and then only for a few minutes. #10 is easier to work with as well as being the stock size.
This same general idea, soldered lugs (¼" female stake-ons) and sleeves works well on the heater feed wire, with a new fuse under the fuse box, to the 'ign feed' terminal.
I have also found '194' lamps are available in amber at Rock Auto, if your blue light covers have fallen apart. They are not original, but they look pretty good; better than no cover, or half and half! There are 3 blue lights in dash light section, leave the rest clear.
Written by John Grady and Joe Leonard
Joe Leonard 2010-02-25