Starting a new thread in response to @emeryGT 350 comment in another thread as follow up

Here’s the background:
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I have two gauges, tested both at a variety of voltages

Both do have some “sticktion” in the needle due to damping that results in a minor delay between the time voltage hits the gauge and when the needle moves.

This is more pronounced the lower the voltage. At 8v it is very noticeable. At 11v it is hardly noticeable. At anything 12v or above, I’d say the needle movement is almost instantaneous with initial needle over shoot and then the needle quickly damps out to the indicated voltage.

Both gauges read a bit on the high side vs actual voltage. Approx .5v high. This diminishes as the voltage approaches full scale.

Pictures to follow

Gauge #1
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Very interesting! Could you do some readings around 13.4-13.7? I was chatting with another teener at the northeast gathering and his gauge was looking quite a bit like mine, reading low when at the battery it was fine. Not sure if his is wired direct like mine, but when we were looking at it, the turn signals and other lights were not on...

I need to pull mine and verify what I see at the battery is what it is seeing at the posts.

Yup will do

Here’s 14v. Click to view attachment

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Gauge #1 at 13.7v

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Gauge #2 at full scale - was basically same indicated display as gauge #1 as I ran up through the voltages.

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Will post video of the damping behavior if it is needed.

Gauge #2 12v nominal

What's the problem? How exactly do you expect your meters to agree? Your meters aren't made by HP. Are they NIST calibrated? Nope.

Wiggle your cables in the sockets to see if there'e better agreement. Corrosion will result in voltage drops. You're splitting hairs here.

This whole exercise is only providing a sanity check to EmeryGT’s gauge

No issues here at all on my part

Copy that. The power supply reads out to one place & your DMM reads out to two places.

The DMM is more precise & only more more accurate if it's calibrated.

Precision & accuracy are not the same.

Yup

Former USAF electronics tech. Fully familiar with using a $3000 (in 1980s dollars) HP bench DMM with resolution to the 4th decimal place with fully traceable calibration to measure a stupid 9v battery.

No need for that type of accuracy or precision for automotive hobby work.

Even working on complicated electronics it was very rare to ever need that level of accuracy and precision. Flukes were disposed of like toilet paper

The sad joke was that if you were going to break or lose a piece of test equipment, make sure it was something big and expensive like a $12k O-scope or $30k spectrum analyzer. They can withhold a Fluke out of your check via GPLD if they felt so inclined (Government property Lost or Damaged) but when you made $12k a year; they were never going to recoup something expensive.

I was interested in the behavior of the voltage meter as much as the variation in accuracy across units.

Good data. Unfortunately mine is right on.

60's-70's are what I call ball park gauges. They were designed to be cost effective. Most VDO stuff sold for $25.

I think of it more as a dummy light or perhaps a 'general suggestion' about the charging system. However, I have found mine does some disconcerting things that don't allow me to even use it that way. If it's sitting on 12 that scares me, but then I check the voltage and its 13.4. another day it will be at almost 14 and the voltage will be 13.6. That's not helpful. The original lines to the gauge now operate a relay which has a direct (fused) connection to the battery. Hmmm.... I should double check my ground shouldn't i?

Yes! Especially since you’ve previously had issues with the starter & hot starts.

A bad negative battery cable from Batt negative to chassis will cause all kinds of grief as will a bad ground strap from chassis to transmission. The most reliable way to judge the state of these grounds is to measure the voltage drop across the cable while cranking. Visual inspection and appearance is meaningless. Anything more than about 0.2v of voltage drop while cranking is too much - replace the cable / ground strap.

Required reading:
https://www.fenderbender.com/running-a-shop...ircuits-part-11

[Edit] check battery positive cable the same way! Its condition is no less important than the negative cable.


Have you painted your fan shroud or any of the alternator mounting brackets? These are the in the ground path for the alternator.

I’ll get some video of the gauge damping behavior up later tonight. There is nothing in the damping behavior that would explain the variance your mentioning between voltage and/or any sort of extended delay in the voltmeter reporting proper voltage within +/- 0.5 volt.

Unless you want to rewire it directly to the battery, the stock voltmeter is just as accurate as the instrument cluster idiot light (binary good-bad).

I removed mine.

Thanks, Superhawk!

The dash meters are FAR more accurate than I would have thought...

I once took all of the thermometers in a shop and dropped them into a cooler filled with beer/ice/water.

The old school glass mercury variety were SUPER accurate. 33-34 degrees

Everything else including a $400 Snap-On pyrometer with "special liquid probe" was NOT!

Same shit with compression tester. None showed exactly the same readings on the same car done "lab" style.

I LIKE "lab" style comparison testing!

That was one of the first things I did. Thus the relay. All new ground strap and no silly paint. Resistance from ground to neg post was all good last year. Gonna poke around with the multimeter and see what I find this year.

Recognize anything?

Just checked the grounds engine to body engine to post, post to wall etc, , all good, less than 0.1 resistance everywhere. Didn't pull the gauge yet to check that. Gotta go shopping first.

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.1 ohm x 10amp draw = 1v drop. Even 0.05 ohm x 20 amp load = 1v drop. Unacceptable

Measure voltage drop across the cable while cranking to get an accurate assessment of resistance.

Not sure how you’re wired to battery. Separate independent wire or are you pulling though one of the smaller OEM leads that come off battery positive?

@windforfun

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Nice test bench!

gauge #1

Delay and damping behavior related to internal capacitance used for damping.

Sorry for shaky video, couldn't find my little mini tripod.


https://www.youtube.com/watch?v=Hq94EuI9_2g?si=bXc5csV9tLxkHtav


@emeryGT350

For what it is worth, I use the stock voltmeter as a visual brake light checker. If the brake lights come on, the gauge moves.

If you want to use it for something else, you will have to rewire it to have a connection directly to the battery.

emerygt350 might just have a bad gauge. Mine gave inconsistent readings, started reading ~5 volts under, then later 10 under, and then ultimately nothing. Tapping it once in a while would bring it back to life, but eventually it completely died.

Anyone have one opened up?

I’d be curious to verify what’s doing the damping. I’m assuming it’s just a little ceramic disc capacitor via RC circuit.

However based on how this one gauge will linger at 16v position while the other doesn’t (as long) I suspect there is some difference in capacitance and/or resistor values between my two gauges.

Mine is running straight to the battery post. The original wiring now operates a relay.

I suspect it may be failing, as mentioned above. Tapping it causes it to move and it's been slowly getting worse. I checked all the connections behind it when I noticed tapping made a difference (couple weeks ago).

I say less than .1 because on my lowest setting I get a 0 on the resistance.

That is really interesting. The way it sat at 16...

Would be totally consistent with an RC charging constant that takes time both to build up - which will happen more slowly at low voltage and will take longer to bleed off at high voltage

I’ll try to get another video of the other gauge up later so you can see the difference between the two.

The absolute worst case is that it gets mechanically “stuck” up there after being driven to full scale but I don’t think so based on how consistently it releases.

Gauge #2

Slightly different damping behavior. Perhaps a little slower to rise up to voltage when starting from 8 volts. Much quicker to release once it has been brought up to 16 volts.


https://www.youtube.com/watch?v=9ar5kOuEUAk


@emerygt350

It is interesting how in both cases they behaved best at 12 volts but one was slow lower and the other misbehaved higher.

The variance in damping isn’t terribly surprising. Assuming I’m right that they are using an RC circuit for damping, the tolerance of resistors would have been +/- 10% and capacitors could be 20% or more if there is an electrolytic capacitor in there. And that would be before 50 years of aging.

I think it’s a combination of what was said earlier - cheap gauge built to do a basic job at a an affordable price point.

However what has always been pretty clear to me is that the accuracy is pretty good. 0.5 volt +/- really is no big deal in the scheme of things.

Add in 50 year old aging wire harness with convoluted 12v+ sourced off the brake lamp circuit and you get the typical bounce as lights cycle on/off.

Let us know what you find with yours.

If I ever find one that is in-op, I’ll tear it down to verify what is being used for damping.

I finally dug into the console. I had compared the voltage on the dmm at the gauge to the battery, and they were off. The gauge was showing pretty close to what it was getting. The DACO has wired his Autometer AFR into the same relay controlled circuit as the gauge, so the gauge is not alone on that circuit (completely forgot). It's the damn O2 sensor heater. A little tiny 20 gauge or less wire takes a big hit while the sensor is heating. Disconnected the AFR and all was good.

Now those delays and variable voltage drops you were seeing make sense.

sigh. All I had to do was hook the o2 heater wire to the old lines that are now triggering the relay rather than the hot going to the gauge and all is good.