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New Voltmeter and its' Problems
#1
Hey all, good to be back - 

I looked thoroughly through this form and the internet, but was unable to find anything relevant. Apologies if it's somewhere on here and I missed it.

Earlier this month I finally bit the bullet and sent in my ammeter to have Rocketman give it the works. The gauge comes back, and I use the original plugs re-wired to the acc. stud on the fuse block (and a ground screw) to hook it in. With the car off and the key turned to power, the gauge reads fine. However, starting the car up and getting it to idle, the needle flies from side to side in a sporadic manner. With some sort of load applied (headlights on works most effectively), the needle steadies out fine. 

Thinking about it, I realize that adding some resistance in-line might  fix the problem, but I'm not 100% sure. I figured I'd add a small incandescent bulb either in-line with the positive or negative wire.

I'm hoping it isn't the gauge (why would it be?), and I'm hoping it isn't my alternator either - though if it is... maybe a blessing in disguise. 

If any one has experienced this before, or knows what I'm doing wrong, I'd greatly appreciate the feedback. 

All the best,
Tyler

Love is all you need.
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#2
Sounds like a voltage regulator problem to me. Do you have a digital volt/ohm meter you can connect to the battery, to see if it also fluctuates? The voltmeter is just reading the voltages it sees. Placing a load on it with the headlights and having it smooth out pretty much confirms that it's voltage regulator problem. A resistor in line with the voltmeter we won't help.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#3
(06-10-2019, 08:44 PM)Don C Wrote: Sounds like a voltage regulator problem to me. Do you have a digital volt/ohm meter you can connect to the battery, to see if it also fluctuates? The voltmeter is just reading the voltages it sees. Placing a load on it with the headlights and having it smooth out pretty much confirms that it's voltage regulator problem. A resistor in line with the voltmeter we won't help.

I was worried about that one, Don. So much so that, when I initially found this problem a day or two ago, I went out and replaced the cheap no-name regulator the previous owner had "installed". The wiring up to the plug thing it attaches to was and is just as bad, so tomorrow I'll go and work through that to see if there's any issues. In the meantime, do/does you/anyone think this could possibly be an alternator problem showing itself?

Tyler

Love is all you need.
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#4
I don't think it's the alternator, placing a load on the alternator, if it were bad, would probably make the problem worse, not better. Bad brushes and commutator rings could cause voltage fluctuations, but the problem would likely get worse under load. As has been stated many times on this forum, new does not mean good, especially when it comes to voltage regulators, and especially concourse correct ones from certain manufacturers.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#5
Often car power rails can throw some spurious back emf signals - or spiking from noisy alternators...
especially on meters that are sensitive with no electronic or mechanical damping . 

One can damp the input to a meter with the simple circuit below. 
A diode with the cathode on the positive so it is always in reverse bias mode 
this means any negative flowing current from back EMF pulses will not affect the meter so much 

the capacitor simply dulls the signal to the coil of the meter making it less sensitive to sharp changes 
Between 22μF to 47μF at 25volt should give you 250ms to 450ms of slug, depending on the coil resistance in the meter.

[Image: vmeter.jpg]

example of spurious signals and power spikes positive going and negative going 


[Image: grph.jpg]

hope this helps -- most expensive meters have some protection in place when polarized to a particular current direction Cool -- some cheaper jobs do not 

all the best 

Al

[Image: siggy.jpg]

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#6
Which original plug did you use.. was it the 8 pin connector used for the center gauges? Normally the ammeter wires are in there and are the ones disconnected when adding the voltmeter.

I also recently added a converted ammeter to voltmeter from Rocketman. I connected separate wires, the + to a radio power feed and the - to a good chassis ground. The reading is what my volt meter reads on the battery, around 14.2 or so when running and steady ( proper location of the needle in the ammeter range which is 3/4 or so).

So as Don mentions, the voltage regulator could still be an issue. Try an all electronic one from Virginia Mustang :

http://www.virginiaclassicmustang.com/Se...&ps=12&p=1

I doubt it would be the gauge.
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#7
Alright, after one day here's where I'm at:
Don, I tried my best to put my money where I felt it would be best spent and sprung for a NAPA Echlin electronic unit. The ones in the mustang parts catalogs sure look cool, but I have about zero confidence in how well they're built or where they're from. Echlin has done me well in the past, and around here it's either them or DuraLaugh.

I inspected the wiring and repaired what I could; the previous owner(s) did a terribly shoddy job down there. Looking through diagrams, and posts around here... I'm slowly starting to realize that this is turning into a wiring problem. Hoo.
Firstly, the wiring diagram I have states that there should be a junction block attached to the "A" terminal. That is no longer there, and instead the yellow wire goes straight to the starter solenoid. If this block controls anything, or if anyone has a picture of what it is, I'd find that very helpful.
Secondly, and curiously, my "I" terminal has a green and red wire going into the harness somewhere. I was under the impression that if you had a factory ammeter, this wire should be absent.
I've also read somewhere that the black and white wire coming out of my (70 amp) alternator and going into the regulator plug should be absent as well, but that was some time back and I could be more than wrong.

I appreciate all the help given from everyone this far; I know we're gonna get this thing fixed.

Regards,
Tyler

Love is all you need.
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#8
You are correct about the wiring. It appears that your car is wired like one without factory gauges. Have you confirmed that your car originally came with factory gauges?

This link has illustrations of the two different wiring schematics
https://www.7173mustangs.com/thread-alte...#pid320622



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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