bouncing volt and ammeter

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j.d.duggan

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my car has the factory ampmeter and a under dash volt meter.when you start it it shows about 14 volts,then slowly goes up to about 15,then it starts bouncing from 12 to 16 at idle.when rpms increased to1200 or so it usually stops the bounce. Evens out to about 14.4.thought the vr was bad so replaced that and added a ground wire from one of the mounting bolts to the firewall to make sure it was grounded.did not change anything.what do you electrical goo-roos think?:poke:
 
Sounds about right depending on what car it is, modifications, battery condition etc.
 
When mine did that the fault was in fact the regulator. A ground jumper to the firewall doesn't help if the ground path is degraded elsewhere. Try running the ground jumper all the way to battery neg' . If that cures it you'll know everything else is suffering weak ground. Could be neg' battery cable/terminal.
 
Maybe it would help to look at battery and alternator operations.

Batteries suck power in proportion to need if it can be supplied. Time lapse photo sequence of low battery on charger.
Alternator's maximum potential power output goes up with rpm. Output chart for an alternator

So it seems like you are seeing system voltage going from being supplied from the alternator to the being supplied by the battery. Then as rpms go up, the alternator produces enough power to run the ignition and recharge the battery and voltage rises from the battery's to control by the regulator.
 
Have your charging system checked at the auto parts store (free). you might have a bad diode or two in your alt.

BUT..

I had the same issue. The pulley on my Alt was ~3in dia and parts book called for smaller dia (2.5 to 2.75 if memory serves me) smaller Dia = faster Alt speed and more ability to charge at idle. I changed the Alt to one that had the smaller dia pulley, now my ammeter and voltage is rock solid at idle.

I can not say if it was the pulley or the alt but physics says the pulley did the trick.

I did some math... (feel free to correct me if i'm wrong) I arbatrailery chose 6 inch Dia for crank pulley and 3 and 2.5 for alt pulley just to illustrate the point.

upload_2019-3-17_9-17-30.png


upload_2019-3-17_9-17-57.png



from link above, notice the turn on alt speed. Also the amperages listed are what the alt CAN do not what it always does in terms of output.

output-20chart-jpg-jpg.jpg
 
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Have your charging system checked at the auto parts store (free). you might have a bad diode or two in your alt.

BUT..

I had the same issue. The pulley on my Alt was ~3in dia and parts book called for smaller dia (2.5 to 2.75 if memory serves me) smaller Dia = faster Alt speed and more ability to charge at idle. I changed the Alt to one that had the smaller dia pulley, now my ammeter and voltage is rock solid at idle.

I can not say if it was the pulley or the alt but physics says the pulley did the trick.
Yup. Or there could be additional (non-stock) loads on the system, or fast idle needs adjustment, or..? without knowing the details we can give generalized answers.
 
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had a check on charging system before replacing VR,showed the same thing that the volt meter shows.
 
had a check on charging system before replacing VR,showed the same thing that the volt meter shows.
I did too and my test said bad VR but after changing alt now its normal.

If you want measure your alt pulley dia and your crank pulley dia and post I'll plug it into my spreadsheet and see what you alt speed is at various rpms.

or do the math yourself

Engine rpm x crank pulley dia / alt pulley dia = alt rpm at a given engine rpm
 
alt.pulley is 2 3/4 overall=2"belt ride and crank pulley is6 1/2 overall with 6" belt ride
 
This almost has to be one of several things no particular order

1....What is commonly called a "ground loop" although that is not always the actual situation

Check voltage between battery actual negative post and the VR mounting frame. Do this with engine warm and running fast as in "low to medium cruise" RPM. Check first with all accessories off, and again with lights, heater, etc powered. You are hoping for a very low reading, the lower the better, and zero is perfect

The VR MUST be grounded and at the same voltage as battery NEG. Remove VR, scrape around bolt holes and rear of VR flange, and remount with star lock washers.

2....With key in "run" and with engine stopped. Check voltage between VR IGN terminal (or as close as you can get) and battery POS terminal. Again, you are hoping for a low reading. More than 3/10V you need to improve this

3....Might be bad VR

4...In rare cases might be battery. Best is to try a known good battery and see if the symptoms change

5....This could be bad connections/ terminals/ connectors in the charge wire path from alternator to battery. Make up a temporary jumper out of some BIG wire, at least no10, or maybe a couple of no12 in parallel.

6....If nothing else works run a made up temporary jumper direct from the battery (or starter relay junction stud) to the VR IGN terminal. Don't forget to unplug when shutting down

7.....Further ramblings. I like to add an additional ground cable from engine to body. Look at the front passenger side of a V8 head, these same bolt holes are on the rear of the driver's side head. Buy a 1 ft or 18" "starter" (eyelet to eyelet) cable and bolt to the head and a good ground on the firewall, either a through--bolt, or to the master cylinder mounting studs.
 
Have you checked the flasher/hazard, my aftermarket alt gauge pulses with the flasher even when not in use. Holding it you will feel the thermal moving. I unplug the suspect flasher and the gauge steady's
 
Any update? Your pulley diameters are in the ballpark of normal IMHO
 
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