Fluctuating AMMETER/VOLTMETER (MAD)

[OriginalDart]

Ok folks, here we go. I drive an all original 1969 Dart 273. I've replaced both the alternator and VR once before but one day while driving i noticed the Ammeter was fluctuating pretty rapidly between the middle and 'C' side(right) of the gauge and the headlights and dash lights strobed with the fluctuation. I kept driving this way for some time but one day it wouldnt start. I could jump start but it wasnt charging. Read some forums, did some multimeter tests and replaced the VR and alternator(it was under warranty still why not?) It worked but still had the fluctuation in the Ameter. After reading some more forums and finding the MAD electrical article about bypassing the Ammeter, I did so. I also bought a little setup from pepboys and wired in a volt meter, oil pressure, and water temp gauge. Everything worked GREAT for a few days. Lights were brighter and steady and the volt meter was reading a solid 14-16. Sometimes at high rpms it goes up to 18 on the volt meter...is that bad? Now even the volt meter is fluctuating and the headlights and dash lights are strobing again. I'm thinking to check grounds? Kinda stuck here... Has anyone encountered the same problem?

 

[BillGrissom]

Many have. The Vreg controls to its "sensed voltage". That routes thru the key switch on the dash. If you have voltage drops thru the firewall connector, Vreg will keep increasing the alt output until it is satisfied, by getting the desired 14.3 V at the IGN1 line. Problem is it might have to boost the alt output to 18 V to make that happen, and that will be driving most of your accessories like headlamps.

 

[67Dart273]

To expand on this further, the "ignition run" line (IGN1) coming through the bulkhead supplies ignition and the voltage regulator under the hood.

The factory wiring circuit path for this is

battery....starter relay stud....fuse link....bulkhead connector....ammeter connections...through the ammeter....to ignition switch connector....through the switch...back out the switch connector....through the bulkhead....to ignition and regulator

ANY poor connection(s) in this path causes a LOSS of voltage with respect to the battery.

The regulator IGN terminal "sees" this low voltage, and brings that point, that low point, up to it's regulator point. If the regulator is properly working at 14V, that is where it will be.

But because of the voltage drop, the alternator is actually outputting the difference between your IGN1 "run" point and the battery. That is, if there is 1 volt drop in that harness, then the battery voltage will be 14 + 1 at 15 volts.

Now there ALSO can be ground problems. If the regulator is poorly grounded, this will add MORE to the charging voltage because of the voltage difference between battery ground post, and the regulator mounting.

To measure these drops, get a decent multimeter. Turn the key to "run" with engine not running. Put one probe of the voltmeter on ground, and "try" both sides of the ballast resistor. You want the highest side. Now leave that probe there at the ballast high side.

Now move the ground probe over to the battery POSITIVE terminal, and if necessary, switch meter to a low DC volts position. What you are hoping to read is a VERY low voltage, the lower, the better. More than .3V (three tenths of one volt) indicates a harness drop.

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To check the ground side, run engine at a good fast idle. Stab one probe into the regulator mounting frame, the other probe into the top of the battery NEG post. Once more you hope to read very small voltage, the lower the better, and less than .3V.

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Other causes of high charging voltage is (rare) bad battery

There can be strange "ground loop" problems

And of course the regulator can be out of spec.

Make sure your dash mounted voltmeter is actually accurate!!!!!