Points ignition with no balast resistor tach problem

[blaylock6502]

I have a 74 cj5 with a 258. I am still in the process of doing a frame off restoration but its on the road again but I'm trying to work out a few bugs. The radio would cut off and on when first started and after about ten minutes of running it clears up. As the radio is cutting out the autometer tach I installed registers very high (5000 rpm) and gradually comes down as the engine runs. About the time the radio starts to work the tach comes down to close to correct rpm but is very jumpy. I was checking voltages yesterday and found all components were getting around 8-9 volts. I tested the ignition switch and found that there was around 12 ohms resistance between b plus and ignition so I know I need a switch. I hard wired the ignition and the radio problem seems to be solved. That being said my tach acts much better but is still jumpy. I got online and downloaded installation instructions for painless harness I installed. I guess when I was at the ballast resistor part I had thrown back one too many and missed it. I saw it is not a necessity on all coils but as I was looking to make sure I had my tach feed on the correct side of the coil I found the side of the coil says "requires external resistor" so I know I need to hook it up. That being said what does it do? Is it possible It is causing the problem with my tach? Thanks for the input!!!!

 

[69jeepcj]

The ballast resistor reduces voltage to the points from 12 down to 9ish. If you do not run one your points will get constant 12 volts and burn them up in no time. Yes it could be your tach problem as well I think. Make sure your coil is for points ignition and not electronic, it make a differance. Most points coils say on them requires external resistor though so your probably ok there.

 

[PoliceMonkey161]

The way I understand the “points ignition” system is similar to what 69jeepcj is saying. The ballast resistor is used to regulate (down) your voltage to the ignition system after start up. After cranking the starter during start, the alternator is trying to recharge the battery at a high rate. The ballast is used during this period to regulation the voltage supplied to the ignition system. The magnetic field caused by the coil to increase the voltage to the points is not designed to handle the increased and/or varying voltage being supplied during this period. Voltage being supplied after the coil is multiplied expedationally without the ballast.
So, I would think (best guess), without the ballast your tack is sensing the massive increase of voltage shortly after start up and eventually levels out as the battery reaches a full charge.

 

[blaylock6502]

I put in ballast and had the same thing! Called autometer and they said to put a 10k ohm resistor in series with signal wire. Tried that no dice! So I called autometer back and now they say I need their 30 dollar tach filter. Go figure! At any rate it's on it's way so we shall see!

 

[69jeepcj]

Thats weird, I have put a tach on literally every jeep I have ever owned and never had to add a resistor or tach filter. You ran your tach green wire to the post on the coil that your dist wire is hooked up to (+side of coil) red wire on tach to ignition source, black ground, white is the light. Make sure your ground is a good connection.

 

[blaylock6502]

Yeah ground was autometers first idea they had me ground it straight to the block

 

[69jeepcj]

For your radio noise are you using non resistor or resistor plugs. resistor plugs are used to reduce radio noise, not so important on newer cars but the older ones used resistor.

 

[carnuck]

Make sure the body to battery ground isn't missing or broken (not the battery to motor ground but that's neccessary too). That will cause an issue.