If the bypass is properly done, it won't cause this problem. Are you saying have disconnected the bypass? Your perception that the "car won't charge" is probably wrong. With the bypass in place the AMMETER will not work, but the system should still charge. You need a voltmeter to tell. Here's a thread on that:
READ THE ENTIRE THREAD!! There's a lot of info there
http://www.forabodiesonly.com/mopar/showthread.php?t=119480
HERE is some info on the how and why of bypassing these ammeters:
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
This is very likely one of three problem areas
1 A cyclic sort of oscillation caused by poor connections in the electrical system
2 Rare for the 70/ later regulator, but MAY be a bad regulator. I emphasize RARE
3 Brushes in the alternator going down to the last bit and not making good connection. This is usually more intermittent, not cyclic
You should be able to see this on your ammeter, IE as the lights brighten/ dim, you should see it on your ammeter some.
You have a shop manual? For this problem, the 72 manual is close enough, which you can download here: (Do not trust the wiring diagrams in the 72 for your 74, although much of it is the same)
http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual,+download
The 72 link:
[ame]http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf[/ame]
1 Pull your alternator off, carefully remove and inspect the brushes. I would suggest replacing them, but I had one HELL of a time getting the correct ones from NAPA.
2 Get yourself a multimeter if you don't have one. Most parts stores, Sears, and Radio Shack sell them. 30 bucks should get one good enough. Look around on Craigslist for a good used Fluke
Turn the key to "run" but with engine off. Put one probe of the meter on battery positive post, and the other probe on the blue field wire on the alternator. You will be reading a very low reading, the lower the better. Anything over .2--.3V (3 TENTHS of a volt) is too much and shows a bad connection between the battery and the ignition feed
Try removing the connector from the regulator, "feel" for how tight it is, inspect for corrosion, and "work" it in/ out several times to scrub the terminals clean.
You can also make up a nice long heavy clip lead with at least no14 wire, and jumper from the battery stud on the starter relay over to the IGN switch side of the ballast resistor. This will insure good solid 12V power to the regulator, see if that jumper causes the system to settle down. IF so, this again points to connections in the ignition harness
3 Run the engine at a good fast idle to simulate "low to medium" cruise RPM. Stick one probe of the meter on battery negative post, the other probe directly onto the mounting flange of the regulator. Stab through any rust, paint, chrome. Again, you want a low reading, zero is perfect, and anything over .2V shows a ground problem between the battery, the engine, and the body and regulator
4 With the engine still running, measure the battery voltage. Try this with all accessories off, then turn on headlights, heater blower, etc, and see what changes.
