bouncing needle in voltage meter

saylor

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O wise and venerable forum !

why is my voltage meter bouncing around?

it seems like it is when accelerating that causes it.

if I am just sitting at a light, it is rock steady.

is a new alternator in my future?

thanks for the help!

- saylor
 
What is the range of the "bouncing" and before you do anything, check it with another meter.
 
What is the range of the "bouncing" and before you do anything, check it with another meter.


If your vehicle is an older mopar with a mechanical voltage regulator, it is probably just the regulator kicking in and out because the battery is fully charged. You can put a voltmeter on the battery with the engine off. It should read about 12.5 volts. Start the car, the voltmeter should now read about 13 volts. Have a helper engage the throttle until the needle in the car starts to bounce. If it is the regulator kicking in and out, you will get a momentary voltage spike at the battery on the second voltmeter. Probably means you are ok. I assume by this that the needle bouncing was over a relatively small range. Big movements, say peg to peg is usually indicative of a short someplace.

Dave
 
As was said, if you have a mechanical regulator, that could be the reason.

You should also check the connections at the battery, regulator and alternator as loose or dirty connections can cause a similar issue.
 
well, i just redid a bunch of wiring, but did amp gauge removal mod and now everything is fed off the starter relay post, so i'm not totally sure how this meter is supposed to look, or act.

trying not to die -

- saylor
 
As it was said check it with a meter, but does it have a mechanical regulator?
 
at this moment i am uneducated and do not know what you speak of. i must study the FSM. most likely it has whatever was stock in 68, so probably yes, right?
 
Yes on the mechanical regulator. I had to google it though. If you check it with a meter you will be able to watch what it's doing. Sounds like their right though. Someone makes a solid state one that looks like the old one.
 
check your ground strap, passenger side as it attaches the engine to the body. not attached, and you can get the fluctuation.
 
3 easy tests.

Do all with the car running, turn on every thing electrical you can.

1- DMM on A/C volts. Test at the battery terminals... if you have more than .5 volts you have a diode issue.

2- DMM on D/C volts. Test with one lead on battery + and one lead on alternator B+... this is your voltage drop for the alternator to battery circuit... .5 volts or less is good.

3- DMM on D/C volts. Test with one lead on battery - and one on engine block and then the alternator case... this is your voltage drop for the ground... no more than >5 volts and lower is better. The difference between the drop to the engine block and the drop to the alternator will show if you are getting resistance from paint or corrosion where those parts bolt together (alt is case grounded to the block).

If you like, you can redo #3 while cranking... that would be the heaviest current on the ground... still no more than .5 volts and smaller is better.

If these tests all work fine, I would be inclined to go with the rest of the replies here. My test suggestions should give you some piece of mind in about 2-3 minutes if everything is in good shape.

"Try not to die" :thumbsup:
 
fkn awesome test procedure outline. im on it as soon as noahs flood stops out there sheesh where did all this rain come from all of a sudden?
 
3 easy tests.

Do all with the car running, turn on every thing electrical you can.

1- DMM on A/C volts. Test at the battery terminals... if you have more than .5 volts you have a diode issue.

2- DMM on D/C volts. Test with one lead on battery + and one lead on alternator B+... this is your voltage drop for the alternator to battery circuit... .5 volts or less is good.

3- DMM on D/C volts. Test with one lead on battery - and one on engine block and then the alternator case... this is your voltage drop for the ground... no more than >5 volts and lower is better. The difference between the drop to the engine block and the drop to the alternator will show if you are getting resistance from paint or corrosion where those parts bolt together (alt is case grounded to the block).

If you like, you can redo #3 while cranking... that would be the heaviest current on the ground... still no more than .5 volts and smaller is better.

If these tests all work fine, I would be inclined to go with the rest of the replies here. My test suggestions should give you some piece of mind in about 2-3 minutes if everything is in good shape.

"Try not to die" :thumbsup:
This is some great advice here. I never thought of doing a diode test like that. Makes perfect sense!!

The voltage drop tests (2 and 3) are something that is easy to do and anybody with one of these old cars needs to understand. If you read the FSM, the field circuit resistance test is a voltage drop test!

This is the best video on the subject I've found.



This is a little "drier" and there's more white board explanations, but if you watch the first one, it makes sense.

 
The mechanical voltage regulator will look like this.

vr101-1.jpg


The mechanical regulator has a coil and contacts that make or break to turn the field in the alternator on and off to maintain the voltage. It's pretty simple and what you are seeing is the regulator doing its job.

The electronic versions do the same thing, only much faster.

Depending on how sensitive your voltmeter is and where you are taking the reading, you may see the fluctuations in voltage.
 
thanks forum for the replies. this site is the bestest!

i have not studied it yet im into some other crap (laundry, dishes, and sweeping) right now but I bet it is regulating like warren G - I had the amp meter in the dash still, and some iffy wiring, that I have since cleaned up (hopefully) and made safer (really morely hopefully) and maybe im now getting full juice from the ALT now and my batty is full. i hope thats whats going on.
 
Saylor, I don't think what you are experiencing is abnormal, especially with a mechanical voltage regulator based on what I saw in the short video.

What amperage alternator are you running and are you running electronic ignition or any other higher draw electrical items?

I sell the following item, which may help your situation:

Mopar Voltage Regulator - Electronic Solid State with Correct Restoration Look | eBay

Mechanical voltage regulators are not compatible with electronic ignition systems. The stock non A/C alternator only put out 37 amps which is not enough to keep the voltage from dropping at idle with the headlamps on so if you have something drawing a fair amount of voltage, when you accelerate it could be the charging system is trying to catch up and get the voltage back up to 13.8 volts.

Let me know if I can be of any further help.

Bill
 
thanks everyone for the advice, and to bill for the link to the part. its all stock 1968 charging - no electronic anything - all points and condensers and standard ALT andcoil and whatnot.

i like how everything is running, and no lighting issues, no hot wires, etc., so im gonna call it good.

thanks again!

- saylor
 
Great thread! Replaced both the alternator and voltage regulator, after charging system showed 11, and much less once the lights came on. Wasn't the alternator- that didn't solve it, but the voltage regular did. However- Voltage regulator is slamming my meter on the dash and dancing on the under-dash added gauge. Got it from O'Reilly auto parts.
 
Great thread! Replaced both the alternator and voltage regulator, after charging system showed 11, and much less once the lights came on. Wasn't the alternator- that didn't solve it, but the voltage regular did. However- Voltage regulator is slamming my meter on the dash and dancing on the under-dash added gauge. Got it from O'Reilly auto parts.

Take the VR back for a refund and buy a solid state/electronic one from me.
 
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