Severe voltmeter needle oscillations

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French demon

French demon

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Hi guys

I replaced my ammeter with a chinese voltmeter (plugged onto the fusebox).
When the engine isn't running, but the ignition switch "on", all is normal and the needle is immobile on 13 volts
I start the engine, the needle still reads 13V+, it's ok.

But after a very short time, the needle begins to oscillate rapidly between 8 and 16 volts.
When I engage the reverse gear (which turns the reverse lamps on) or put the turn signal on, the needle falls completely to "0", indicating (I think) a short-circuit.
And when the needle indicates "0", I have no turn signal, no windshield wipers (but the headlights, flasher, stop lights or horns work)
I tried to unplug the reverse gear switch but the problem remains.

Engine off, everything returns to normal, every function works properly... :rolleyes:
Bad voltage regulation? An alternator problem? A deficient voltmeter?
Any idea?

François
 
Did everything work properly before you installed this Chinese voltmeter? If so, I would suspect a problem with the voltmeter. Remove it and see if everything now works properly
 
French demon said:
But after a very short time, the needle begins to oscillate rapidly between 8 and 16 volts.
When I engage the reverse gear (which turns the reverse lamps on) or put the turn signal on, the needle falls completely to "0", indicating (I think) a short-circuit.
And when the needle indicates "0", I have no turn signal, no windshield wipers (but the headlights, flasher, stop lights or horns work)
I tried to unplug the reverse gear switch but the problem remains.
Click to expand...
What car and year are you working on?
What modifications to the electrical have you or a previous owner done?
as asked before did everything work correctly before?


a few things come to mind...

  1. The volt meter needs a positive source of power ( the fuse block assuming you hooked it to the right switched fuse)
  2. The volt meter needs a good ground (The AMMMETER wires are BOTH positive, one comes from the alternator and one comes from the battery, some stops along the way for both) co a good body ground is what is needed here
  3. when you engage reverse or turn signals, the volt meter goes to 0 makes no sense

JUST A WILD *** GUESS HERE, At this moment I'm betting when you bypassed your Ammeter you disconnected the 2 wires from the ammeter and did not attach the two together (typically one red and one black, both are 12V feeds, assuming this is 62 through 70 something before they went to the external shunt type Ammeter) That means that the car is running on battery or alternator only and some feeds might not be getting power correctly.

LASTLY go to MYMOPAR.COM download your cars factory service manual and wiring diagram and maybe go to CLASSIC CAR WIRING and buiy a laminated colored wiring diagram for your car. THEN learn how it all works and start tracing wires from the battery to the lightbulb for instance, and from the alternator to the same light bulb.
 
There are several things that might cause this, but the very first thing I'd do is round up a second meter, even a digital multimeter, and hook it right where the one with the problem is getting power, and see if the problem "is there."

Again...we need to know about your system. Year model of car, and what exactly you using for an alternator and regulator, and ESPECIALLY "is it" an electromechanical (relay) regulator?
 
Dana67Dart said:
What car and year are you working on?
What modifications to the electrical have you or a previous owner done?
as asked before did everything work correctly before?
Click to expand...


It's my 71 Demon, /6 198ci, 3 speed.
My dashboard harness is unmolested and strictly stock.
Everything works correctly as long as the engine is shut off

About the ammeter/voltmeter conversion, I carefully followed the MAD mod (Catalog)

Schéma3.jpg

20220426_142137.jpg
I modified the bulkhead connector with big wires passing through.

20220426_115721.jpg
Then soldered them together

20220426_115850.jpg

20220426_120122.jpg

@67Dart273 : Just know that my alternator is a "square back", so not original for a 1971 Chrysler car. I know nothing about my regulator.


Thanks for your help
François
 
Today, I disconnected the voltmeter.
The turn signals and windshield wipers work.
Ok

I decided to go for a little ride and after some meters, no turn signals anymore
I tried the wipers... nothing. :BangHead:
So, I stopped, turned the engine off and re-started it. At idle, the turn signals and wipers are back
And, once again, after some meters, no more turn signals.
This Demon is really devil-haunted. It turns me mad.... :realcrazy:

François
 
Last edited:
Here, generally is what the problem might be, in no order:

1....As I mentioned, a poor designed voltmeter
2....Voltage drop in the harness, which can include bad connections of any kind, in the bulkhead connector, the ignition switch connector, the switch contacts themselves, or partial failure of the big master splice in the black ammeter wire
3....Poor grounding especially the VR
4....In rare cases a bad VR
5....In rare cases something with the battery, perhaps sulfation.


I assume you have a mostly factory harness, and still have the bulkhead connector. Voltage drop from several sources, and sometimes a combination of them, can cause this. With what you describe, if it is NOT a bad voltmeter, you should be able to "see" this in the headlights/ or dash lights with the parking lamps on

One thing I recommend, because of voltage drop problems, is to add a relay to operate ignition "run" buss loads which then gives the VR a stable voltage.

Electrically cut the ignition 1 "run" wire coming into the engine bay out of the bulkhead. This is NOT fused originally, and feeds the ignition system, VR, and alternator field. On some cars it feeds electric choke and maybe some smog doo dads.

Take the bulkhead end of the wire and use it to trigger a bosch relay. Feed the relay contacts with a fairly large, IE no 12 ga through a fuse / breaker from the start relay "big stud." Connect the load contact to the engine bay harness end of the "run" wire you cut.

Also make REALLY certain the VR is actually grounded. I like to run a big jumper from the engine to the body, and in original form, this is pretty minimal.
 
Voltmeters read a "difference of potential". You appear to have nothing between the leads of the volt meter across (in parallel) the fuse input and output so any time something draws power, it's going to change the difference of potential. When you hit the turn signal, that provides a path to ground, which causes the volt meter to see zero difference in potential. if you put one across the turn signal lamp, it would read nothing until the turn signal was turned on and a difference of potential was read across the filament (a resistor basically). When you measure across a bunch of parallel resistors, there will be a voltage drop, until/unless there exists a path to ground, which is what the turn signal is providing. yes it's going through the bulb and causing a difference of potential, but the way your meter is wired, you'll never see it.
 
This sort of illustrates what I am referring to. Imagine the open end points are where the battery is. The "V" is your meter with leads across the fuse box in parallel with other loads. The "switch" represents your turn signal. Once that switch/signal is closed, there is zero load across the v-meter leads and so zero voltage drop (difference of potential) and no reading.

-circuit-ammeter0voltmeter-readings-june2012p11q36.png
 
Cuda416 said:
This sort of illustrates what I am referring to. Imagine the open end points are where the battery is. The "V" is your meter with leads across the fuse box in parallel with other loads. The "switch" represents your turn signal. Once that switch/signal is closed, there is zero load across the v-meter leads and so zero voltage drop (difference of potential) and no reading.

View attachment 1716099525
Click to expand...
Somehow I got lost in all this. Are you suggesting he connected the neg side of the voltmeter to a LOAD instead of a GROUND?

That makes perfect sense. The neg side of the voltmeter MUST be PERMANENTLY grounded, and not plugged into something that can be live at times.

Also the voltmeter needs to go to a key switched source --otherwise it will drain the battery eventually
 
67Dart273 said:
Somehow I got lost in all this. Are you suggesting he connected the neg side of the voltmeter to a LOAD instead of a GROUND?

That makes perfect sense. The neg side of the voltmeter MUST be PERMANENTLY grounded, and not plugged into something that can be live at times.

Also the voltmeter needs to go to a key switched source --otherwise it will drain the battery eventually
Click to expand...
Sort of. The leads need to be across something that

1. Causes a voltage drop
2. Doesn't have any intermittent bypass.

If you look at the diagram, when the "switch" is open, then the leads measure the drop across the load. When the switch is closed, then the easiest path is through the switch an there will be NO drop across the load because of the short that will bypass it. In that diagram, when the switch is closed, the voltmeter will read zero. This might explain the extreme fluctuation of the meter he's seeing.
 
Thanks Sirs, for trying to help me.

No, the negative side of the voltmeter is connected to a good ground (the metallic frame of dashboard).
And the positive is plugged onto the key switched 'ACC' connector of the fusebox.

Today...
I disconnected and reconnected the voltmeter = no change
I replaced my VR with a brand new one = no change
I sprayed some electrical lubricant onto the bulkhead connectors = no change
I tried other turn signal flasher units =no change

The problem occurs only with running engine, and not immediately. Only after a few seconds.
With only the ignition key "on", no problem.
My battery is very recent, no sulfation at all.
I suspect my alternator. I will measure the big black 'BATT' wire voltage (ALT to battery) tomorrow with my digital multimeter.

François
 
Your drawing shows a connection to a ground (right side) and the other side connected to what looks like a common hot. It also "appears" to show that connection is parallel with other circuits, one being a turn signal. When the turn signal is "active" there is another path to ground with very little drop so the volt meter reading would drop as well.
 
Two little videos with engine running :


With inoperative turn signal and wipers


With turn signal and wipers working
 
French demon said:
Two little videos with engine running :


With turn signal inoperative


With turn signal working
Click to expand...

Sorry, I can't see them from here. I'll try later. All I can go by is you description, your drawing and how a volt-meter works.
 
Ah ok. They are probably blocked by your intranet firewall

Have a nice day at work, and please, tell me later if the videos work :) :thankyou:

François
 
I'm not an electrical expert but I would use a digital or analog voltmeter or even a test light and check the fuse box for power when the turn signals and wipers quit. Do the headlights work all the time? Does the engine die when turn signals quit? Finding where you have power and not when things quit and a wiring diagram gets you closer. Along jumper wire to the battery neg. to meter or light eliminates a ground problem there and you can check for bad grounds under load.
 
Hi Bob
Yes, headlights work properly and all the time. Every function and every bulb work as long as the engine is shut down.
When it's running, I loose turn signals and wipers, but the engine doesn't die when turn signal quit.
 
I'm assuming that when the wipers and T.S. quit the heater blower also quits. If so you are loosing power to the ignition controlled fuses in the fuse box. There are 3 fuses off of 1 buss that comes from the ignition switch. It looks like there are 6 fuses in the fuse box. 2 are battery power or on all the time. 3 are off the ignition switch on and one is for dash lights or off the headlight switch. Wiring diagrams as in post 4.
 
Here is a quick test you can do to see if the issue is the alternator.

Disconnect the or one of the field wires from the alternator.

Make sure it can not short to ground.

Drive the car ( you will be driving on battery alone so don't go too far)

If the issue no longer happens then you MIGHT have narrowed down the issue.

How long have you had the current alternator?

Would you post a photo of the voltage regulator and the back side of the alternator.
 
"Field wires" are the blue and green wires? Or the big black one going to the battery?

A pic of the "square back" alternator (it was in the car when I purchased it)
IMG_20201224_112728.jpg


No good picture of the VR. I'll take one tomorrow.
 
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