specstang

Hey everyone, looking for any trouble shooting ideas to find out why my amp meter fluctuates (bouncy) even though I have checked my connections, have a new alternator, voltage reg and pit on a second ground from the engine to the frame and my battery is charged and sytems checks out okay, my lightsalsogo dim on low idle, but below a1,000rpm the needle really shakes. Oh yeah, when the emergency flashers or blinkers are on the needle moves in harmony with them from 0- a few +
Thanks for your ideas...
67fastback
289 2v stock, auto,

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smittycm

It's common for a voltmeter to twitch back and forth at a low idle. Especially when the lights are on and you've got your blinkers going. At an idle, the alternator just isn't putting out the usual 14 volts. It's not a big deal, perfectly normal, especially with a really lopy idle. I mention voltmeter because in my experience with Mustangs, I never seen anammeter that moved more than a 1/16". Those things just don't give much of a reading, barely a twitch and you reallyhad to watch for it. Not that my word is the end all be all. Maybe you are a lucky one and got a good gauge. I swapped my ammeter out for a voltmeter first thing.

67 evil eleanor

I wouldget the alternator checked, maybe for a bad diode. Most auto stores will do it for free.

falconfixer

I would suspect a bad dashboard/gauge ground or a bad body-to-battery ground. The pulsing in time with a load (flashers) would support this. Have you put a a multi-meter on the system to verify the alternator current output? If so - i would suspect a bad recitifier in the alternator.

F14chuck

Did you put a stock alternator in?

Its common for the light to dim at idle because those 65 amp alts just don't put out enough at idle, especially considering the age of the wires, connectors and any other add ons we all like to do that draw more power. I've also had perfectly good looking (read intact casing) wires be bad. Broken or corroded internally but looks fine. Unfortunately the only way to be sure is checking each with a meter.

Also some aftermarket alts are just bad out of the box. Have the local parts store do their free check.

As for the meter fluctuating, mine displaces right just after start but settles down pretty quickly. Only other time I see it move is when the lights are on and my turn signal or emergs are going which should be normal. If the alt checks out good and you have a solid battery, but the gage still fluctuates wildly, you probably have a bad ground somewhere.

I've also pulled fuzes to try to isolate which system may be causing problems. Elec probs are frustrating.

109jb

The ammeters I'm familiar with don't have a ground except for the light so you can see it in the dark.The needle bouncing to the flasherdoesn't support a bad ground because if there is a bad ground it's bad whether the flasher is on or not. The ammeter basically senses whether the load is being supported by the battery or the alternator. The needle bounces because the load is bouncing and the regulator/alternator can't respond fast enough or doesn't have enough umph at that RPM. A negative indication means the battery is draining. As long as it doesn't go negative you are probably fine. I'd throw a voltmeter on the battery and check it. Not running you should see about 12.6V and running at idle with everything turned on you should see at about 14V. If you don't have that, and the voltage goes up with a slight rpm increase you may have a bad diode, but you said you checked the alternator. Don't get it checked at a chain store like Autozone because I've had them check stuff I knew was bad and they said it was OK.Go somewhere where they knowWTH they are doing. I go to a shop near me that re-builds alternators. If all else fails, and the bounce goes away at higher RPM, you can consider a slightly smaller alternator pully to bump its RPMs up at idle. I agree with Smittycm that a voltmeter is better than aammeterIMHO.

specstang

smittycm- yeah my gauge has been really good, it would show movement but not jumpy, now it does the jitters until I go above 1000rpm and as long as the blinkers or flashers are not on. but...... as others mentioned below, I will need to check the ground and wires becasuseI had the instrument dash cluster out to install, new bezels, mini tach, led lights and cleaning and maybe I'm not getting a good ground now.

67 evil eleanor - I will go back and have them check the alternator again, as the 1st checked recommended replacement of the voltage reg and when that didn't fix I replaced the alt.,

falconfixer- Thanks, I will check the dashboard ground, don't remember seeing a specific ground wire, is it in the harness or do the mounting screws make up the ground for the dashcluster to dash frame? Also what is the small silver voltage reg on the back of thecluster? could this be a problem, as I know it is still original?I need to go back to have it checked after putting on the new alt. yet, but when they checked it prior it was low and then jumped so being an orig. alt I just replaced it,

Oh, I diddisconnect the mini tach and checkto see if it stilljumps and it does, as my 1st rule of thumb is did I do it right,

F14chuck - I replaced with stock amp alt. yep, Will check the grounding and try the fuses.

Pic of the back of the cluster, the dash volt reg is above and right of the speedo hole

Thank you all for your replies!!


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RolandT3Speed

I replaced my mechanical voltage regulator with a solid state regulator and that helped "smooth out" the ammeter reading but there's still a fair amount of fluctuation.

Old wiring, old technology!

specstang

I'm thinking itdidn't like getting removed and replaced, even though it was with loving care, I know it didn't do it before and with disconnecting the mini tach not making a difference I thought maybe a ground somewhere which is why I added a secondary ground from engine to frame. Go figure...but will keep trying

falconfixer

Those tabs on the bottom of the unit look like grounds. Do they connect to something or a wire? Look to see if there is a ground strap or black wire not connected under the dassh when it is installed. Are the connectors shiny? They look kinda' cruddy in the picture...

'Exactly when did you notice the fluctuating meter?' After the dash work? After the alternator swap, or after the _____?