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1992 Volkswagen Cabriolet Carat / Digifant 2 / Automatic / Triple White
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Discussion Starter · #1 ·
The voltage gauge on the console bounces at idle speed but not while driving. Any ideas on why?

Me at a stop light:

Gauge Measuring instrument Motor vehicle Gas Electronic device
 

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Most of the VDO gauges of that vintage are faulty, ie, just worn out.

I would make certain that the wiring is ok, that means the ground and power wire. You could put a volt meter on that connector and see if the value fluctuates as bad as the gauge shows, but I bet it will not be that drastic.
 

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The voltage gauge on the console bounces at idle speed but not while driving...
Welcome to the club (as in my voltage gauge does the same thing). As Butcher notes, this is not unusual on 30+ year old gauges.

Do pay attention over time to small declines in the voltage level when driving as this will often serve as an early warning sign of a failing alternator or battery.
 

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'81 Trek 614, '95 Ranger
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My 1st guess is worn brushes on the voltage regulator assembly.

For a short term fix try this;
Engine off, loosen the two screws that secure the voltage regulator to the alternator, push the regulator housing down towards the center line of the alternator, tighten the screws while holding pressure on the regulator.

If you're just a little more adventurous, remove the regulator to inspect the brushes and the slip ring/commutator. From there you'll have clues whether brushes or a regulator or a complete reman alternator should be on your shopping list as a long term fix.

If the gauge itself was at fault, I'd expect the same malfunction at every engine speed.
What does your DVOM tell you when connected to the battery terminals? To the alternator's B+ and alternator housing? To the power supply and ground to the gauges?

Even when these cars were much younger, console gauges were notoriously inaccurate but through no fault of the gauges themselves but due to voltage and current losses across the multiple connectors in the Terminal 15 circuit on the way to the gauges.
I think the power supply circuit to these gauges is a good candidate for a relayed bypass.

As always, the first thought when there's electrical weirdness should be grounds and voltage drop under load tests.

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I would start by measuring the Battery Voltage.
Do you have a Multimeter?

Open the hood, start the car, put the Meter Leads on the two Battery Terminals.
You should see a relatively constant voltage, like 13.8 Vdc.

My Scientific Wild Ass Guess is the nervous meter reading you are seeing is a result of resistance in the fuse panel. Just don't ask why I think that.

fat biker
 

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I would start by measuring the Battery Voltage.
Do you have a Multimeter?

Open the hood, start the car, put the Meter Leads on the two Battery Terminals.
You should see a relatively constant voltage, like 13.8 Vdc.

My Scientific Wild Ass Guess is the nervous meter reading you are seeing is a result of resistance in the fuse panel. Just don't ask why I think that.

fat biker
Or you could get one of these which would be a better meter than the one in the dash. There is one negative thing about digital meters though, they do not respond as fast as analog to changes.

Amazon.com

I had charging issues on the 81 and wanted to be able to monitor the battery voltage (plus could use a USB port) so I hard wired one of these to the battery.

https://smile.amazon.com/gp/product/B071SCY3YQ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
 

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I would suggest that you CLEAN all Your Grounds, and replace them if wonky, or that have a voltage drop of .05 or greater.

The Voltmeter on the car tells you the Voltage on the system as in OHMS law, E = I x R
Cabby-info.com has a DIY on the early 80's and grounds, on the 90's there is one with pictures.

On your Alternator there is a Tune-up, for Cleaning the Regulator, to improve the thing, it deals with Cleaning things a wee bit. It could also be a bad Regulator as one side of the bridge is bad and instead of getting a true Sine-Wave of Voltage you are but getting one half of the wave which is why it is bouncing. One way to Verify that is to hang a Gauge directly on the Battery to see (as stated the Digital VOM is less iffy to the Hi-Low of the thing, where the Analog will stop and start or go hi-to-low quicker), and will show a iffy thing quicker.

90's

General Bosch tune your alternator up.

The OLD gauge is telling you something, It isn't because it is old that may be the issue it may be with your electrical system in General.

I also found that if I changed the source of power to the meter to a different location on the food chain of 12V I had a more accurate reading, just be sure that it is a switched item, as if you put it on the unswitched side you will drain your battery.
 

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Part of the problem is that the Voltmeter in the car reads the voltage in the fuse panel. It does not connect directly to the battery. I saw the discrepancy between the gauge in the car and the actual battery voltage when troubleshooting one of the many electrical gremlins in my Rabbit.

I have a simple scheme to reconfigure the Voltmeter for more accuracy. I will describe it here
Battery Positive Terminal > 1 Amp Fuse in Holder > Relay (single pole, single throw) > New Wire Runs to Voltmeter.
The relay would switch in Engine Run.

fat biker
 

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Or you could get one of these which would be a better meter than the one in the dash. There is one negative thing about digital meters though, they do not respond as fast as analog to changes.

Amazon.com

I had charging issues on the 81 and wanted to be able to monitor the battery voltage (plus could use a USB port) so I hard wired one of these to the battery.

Amazon.com: MOTOPOWER 12-24V 4.2Amp Dual USB Charger with Voltmeter LED Digital Display Universal for Car Boat Motorcycle RV (Modern) : Automotive
Neat looking gadget. However it cannot get an accurate measurement of the actual battery voltage. Compare the reading it gives with a DMM connected across the battery terminals. You will see what I mean.

fat biker
 

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Neat looking gadget. However it cannot get an accurate measurement of the actual battery voltage. Compare the reading it gives with a DMM connected across the battery terminals. You will see what I mean.

fat biker
two options for the hard wired one I ran that directly to the battery (with an in line fuse). For the plug in one which I initially used, I compared the voltage it showed to what the DVM measured at the battery and then knew how much voltage was being dropped across the old wiring and had to be added to what ever voltage it displayed. . It was a quick way to see what the battery/charging system was doing in real time.
 

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Anything is possible which is why testing will limit the possibilities but when a gauge flickers that fast, I highly doubt if you have a voltage/ground supply problem. My money is the gauge. The newer VDO gauges are dampened so they cannot move that fast.

As mentioned, it's a seriously quick test of the supply wires to determine if you got wiring issues or gauge issues.

Fat biker is right about how the power gets to the gauge, there are so many potential voltage drops in the fuse box that if you really are concerned, bypass that entire mess.
 

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I take it the OP's turn signals are not activated when this happens, but is Karl_1340 onto something? It may just be the GIF, but the frequency of that bouncing needle looks exactly like mine when the blinker is on. Is there such a thing as a rogue blinker/flasher relay?
 

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I take it the OP's turn signals are not activated when this happens, but is Karl_1340 onto something? It may just be the GIF, but the frequency of that bouncing needle looks exactly like mine when the blinker is on. Is there such a thing as a rogue blinker/flasher relay?
mine did this when activating my turn signals, i have since upgraded to all LED lights and changed the flasher relay and it doesn't move at all anymore
 

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Alternator bouncing is the correct normal operation.
The alternator output is constantly adjusted by the amount of current the regulator puts into the rotor, in order to create the electro magnetic field.
The alternator output will range from 0 to 14.8 volts, depending on battery voltage and engine speed.
The way the regulator controls output is to bounce rotor input.
So at slow rpm, you will and should notice bounce in voltmeter reading.
At higher rpm it is too quick to notice.

But it can never hurt to clean and grease all electrical connections.
Especially the engine to chassis ground, which many people tend to ignore.
 

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I would suggest that you CLEAN all Your Grounds, and replace them if wonky, or that have a voltage drop of .05 or greater.

The Voltmeter on the car tells you the Voltage on the system as in OHMS law, E = I x R
Cabby-info.com has a DIY on the early 80's and grounds, on the 90's there is one with pictures.

On your Alternator there is a Tune-up, for Cleaning the Regulator, to improve the thing, it deals with Cleaning things a wee bit. It could also be a bad Regulator as one side of the bridge is bad and instead of getting a true Sine-Wave of Voltage you are but getting one half of the wave which is why it is bouncing. One way to Verify that is to hang a Gauge directly on the Battery to see (as stated the Digital VOM is less iffy to the Hi-Low of the thing, where the Analog will stop and start or go hi-to-low quicker), and will show a iffy thing quicker.

90's

General Bosch tune your alternator up.

The OLD gauge is telling you something, It isn't because it is old that may be the issue it may be with your electrical system in General.

I also found that if I changed the source of power to the meter to a different location on the food chain of 12V I had a more accurate reading, just be sure that it is a switched item, as if you put it on the unswitched side you will drain your battery.
Alternators never put out a pure sine wave.
Not only does the regulator control the electromagnetic field strength of the rotor in on/off pulses, but the rectifier bridge of the alternator superimposes the 3 different sine waves on top of each other, so the best any alternator is going to ever put out is very bumpy looking on an oscilloscope.
 

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That is true but the bulk of the battery is going to smooth out the pattern even more. As bad as I think the older VDO gauges are, I to not think the needle would quiver as fast as the alternator signal would be.

In the end, we can all agree that making certain your signal is as good as it can be will help. A simple volt meter at the gauge connector would save you in hours of cleaning/improving your signal and seeing absolutely no change at the gauge. Those gauges just act up and there is very little smoothing circuit in the older gauges. Certainly not like the newer VDO gauges.
 
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