Testing alternator and excessive AC ripple?

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cawcislo

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inhave an erratic tachometer and figured it was just bad, but I also have higher rpm miss and came across a thread on fabo regarding faulty alternators causing this.

a test was suggested to test dc volts then ac volts which I want to try, but it asks to test voltage at ignition feed, I’m assuming that’s the positive powering the ignition coil? Which on my 70 duster with points ignition would be the navy blue wire. I have a 3 wire alternator.

I’m attaching a pic of that post. Am I interpreting this correctly? And would this test also apply to points system as the post below related to someone with factory mopar electronic distributor.

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cawcislo said:
I’m assuming that’s the positive powering the ignition coil?
Click to expand...
At the coil the blue wire is on a side branch of the wiring and any current has passed through the ballast resistor. That resistance to flow drops voltage at the coil every time the points are closed.

upload_2020-4-5_9-28-18.png

J2C is the ignition feed with key in run.
Easiest places to clip on or back probe are at the ballast resistor, the field terminal and at the bulkhead connector, in that order.
 
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If the alternator output has bad ripple, it probably will also show as trouble providing power at low rpm.

For the higher rpm miss; have you looked at the spark plugs? Could be a clue there.
 
Also a bad noise capacitor on the coil can do it also.
 
My local auto parts store was able to diagnose a bad diode in an alternator. I assume it is looking for ac on the DC wire.
 
Okay I tested. At idle. Dc volts into ballast 13.5volts Exit ballast 11 volts. Ac volts into ballast 0.9 volts exit ballast 3.2 volts. So is the alternator working properly?
 
I too have a point ignition and I got almost the same readings as you.
All my tests were neg probe to neg battery terminal positave probe to test points

67 273 stock
 
Dana67Dart said:
I too have a point ignition and I got almost the same readings as you.
All my tests were neg probe to neg battery terminal positave probe to test points

67 273 stock
Click to expand...
So everything checkout fine? I also did the tests the same way you did.
 
I too have a missfire but I have a low compression cyl I attribute it to that OR the original Dist???
 
Seems reasonable to me, although .9 is somewhat high. Clip onto the alternator output stud and check at idle, at fast idle, and with headlights on and off

Reading at the ballast may be introducing coil noise as well. Do you even have the radio suppression cap? Should be from coil+ to ground. This actually is somewhat important and CAN AFFECT spark output to some degree. It proves a low impedance AC path for the spark pulse in the primary circuit. As the spark pulse forms, the primary current collapses (with the points open) and the pulse current must have a low impedance path. One end is the cap in the distributor, and the other end is the ignition battery supply and the supression cap.
 
I checked at the alt output too but the AC was so irratic my meter couldn't read it.

Off topic...
The voltage drop between neg and body was 10s of millivolts

Voltage drop between alt output and battery was 0.5v

Back to topic...

I'm guessing the mis is not alternator caused. Easy to test. Run engine with fan belt off, if it still missed not alt related.
 
I have the radio suppression cap at the coil wired in. I guess pulling the fan belt would be an option to test it. I’ll give it a shot. I’ll also check at alternator output for AC volt readings.
 
Dana67Dart said:
Voltage drop between alt output and battery was 0.5v

B .
Click to expand...

That is not bad, but DEPENDING on how hard the alternator was charging at the time. Drop is caused by current flow. If the battery was "up" and normal, and the alternator was merely providing basic ignition and no heater or lights, that might be "a lot." If the alternator is at full tilt, I would consider 1V about the limit maybe as much as 1.5 if no system upgrades have been done. And even that varies with alternator---------35--45---65A etc
 
Okay results testing at alternator output stud. At idle 14.7 volts DC, 106-115 varied AC. With lights in at idle 14.3 volts DC no noticed change in AC. At fast idle 15 volts DC, 135 to 150 AC, no noticeable change with headlights on. Is everything working correctly. Any other tests to do?
 
Looking back at the old post I cut above there’s a previous post stating the erratic tach and missing went away when he unplugged the voltage regulator both issues cleared themselves and it turns out it was a bad alternator. Should I unplug my voltage regulator and try it out or could I create more problems or mess something up by doing this as a trouble shooting exercise.
81080F98-AC75-41F5-BBA5-2376561977AE.png
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