Pulsing Voltage

[SeattleQQ1Fish]

I know this has been discussed before, but I thought I'd get some fresh perspective.

When I start the car, the voltmeter sits right on 13 for about 5 seconds. After that, the gauge flutters back and forth like a dog wagging it's tail. At idle, the voltage at the battery fluctuates between 13.9 and 14.6. In drive, giving it some throttle, the gauge still moves, but slower and fluctuates somewhat less.

I use the vr-128 voltage regulator and tried replacing it with no effect.

Alternator is a rebuilt 1966 unit, single field. Pulley is the same size as the 71 model I took off. Ignition is a MSD 6a box, msd distributor and Blaster 2 coil. Engine wiring harness is new from M&H, and I'm using Crackedback's wire around. Battery and alt. connections are all clean and tight. Grounds all look good. The battery is about a year old and measured 12.65 volts with the car off.

The engine is a 273 and runs good; I've never had any problems with it.

Could it be the alternator? Or maybe just ripple? I know rebuilt alternators sometimes have a bad reputation...

 

[67Dart273]

This is almost always a wiring/ voltage drop problem/ ground loop.

Try running a temporary or permanent ground from the VR mount to the battery. Try splicing into (the ballast maybe) to the IGN run line and run that direct to battery--just for test.

 

[SeattleQQ1Fish]

This is almost always a wiring/ voltage drop problem/ ground loop.

Try running a temporary or permanent ground from the VR mount to the battery. Try splicing into (the ballast maybe) to the IGN run line and run that direct to battery--just for test.

I tried grounding the VR mount to the battery with no effect. When I ran a wire from the ballast to the battery, the fluctuation was less, more like 13.95 to 14.24

 

[70dart340]

I'm no expert, but it sounds like a diode in the VR is intermittent or bad. Any chance of getting an O'scope on the line?

 

[MOPAROFFICIAL]

These guys got you in the right direction. Voltage regulator issue is it mechanical? Or the alternator itself

 

[Dave999]

I'm going to say it but i might be worng....

are you sure its not just the voltage regulator switching the alternator on (approx 14.v) and off approx 12 volt as and when it thinks the battery voltage has crossed the line of too low

charge the battery and see if it improves

My ammeter occasionaly fluctuates in the same way as the regulator decides to charge and not charge the battery when little else is consuming current. with stuff switched on you don't see this.

If switching on the fan and headlamps stops it i'd suggest its probably nothing to worry about.

you just have a comnination of ambient temperature, current load, battery voltage and hence charged state of battery, that sits bang on the fence between alternator ON and OFF

change to one or many of those things might see the problem go away.




Dave

 

[SeattleQQ1Fish]

I'm no expert, but it sounds like a diode in the VR is intermittent or bad. Any chance of getting an O'scope on the line?

No scope available, unfortunately. I did test with a known good VR and that didn't seem to help.

 

[67Dart273]

The thing is, both the old electro mechanical and the newer flat electronic VR pulse the field. The reason that does not show up is because of the inductance/ time constant of the field. It takes time for that to go up and down, so you don't really see it in the output. This could be a bad VR, I think I had it in mind you'd replaced it. I doubt it is in the alternator itself. You might examine the ammeter/ bulkhead connector connections if you have not done so. Also examine, with a light, etc, the terminals in the VR and harness connector, and work that connector in/ out several times to scrub the terminals. If you have not, I'd try a different VR. This is rarely however, caused by a bad Vr.

 

[SeattleQQ1Fish]

The thing is, both the old electro mechanical and the newer flat electronic VR pulse the field. The reason that does not show up is because of the inductance/ time constant of the field. It takes time for that to go up and down, so you don't really see it in the output. This could be a bad VR, I think I had it in mind you'd replaced it. I doubt it is in the alternator itself. You might examine the ammeter/ bulkhead connector connections if you have not done so. Also examine, with a light, etc, the terminals in the VR and harness connector, and work that connector in/ out several times to scrub the terminals. If you have not, I'd try a different VR. This is rarely however, caused by a bad Vr.

I’ve read a few things where the high voltage of the MSD ignition can cause similar issues . Any thoughts on this?

 

[67Dart273]

That is possible and is alleged to have happened. If you can pull the MSD temporarily that might be a plan. I would think that a short jumper, run from the battery along the fender apron metal (somewhat shielding) and jumpered to the key side of the ballast should bypass that effect.