'69 Dash voltage limiter ??

Why did Chrysler reduce the voltage from 12 to 5 volts supplying the gauges in a 69 Plymouth? And, why does the 5 volts "pulse" and not stay steady when it's measured?

Not just Mopars but also Fords

Back in the days even the transistors were already invented transistorized systems weren't a standard electronic supply to make anything and were part of bigger setups ( valves were still somehow standard ). The alternator regulator was also a points system

Pulsing the voltage to those two instruments makes sure they don't overheat is what I was told years ago at one of the old Mopar drag race seminars: up:

The instrument voltage regulator is nothing more than a bimetallic strip that makes and breaks the connection between the 12v source and the gauges. It supplies an average 5v (approximately) to the gauges. That is what heats up and cools off, nothing to do with the gauges heating up.

Why did they use that? It's cheap, it works and it is sufficient unto the day. Nothing more. I am sure it was developed in the 50's when they converted to 12v and your other option was a vacuum tube, not optimal at all. remember transistors were barely 10 years from having been invented. By providing the gauges with a run voltage less than 12v you can eliminate the effect of the charging system voltage fluctuations on the gauge readings.

Mid 30s saw the advent of voltage regulators. As cars started adding electrical features, there was a need to be able to supply those loads and maintain a battery. Though the transistor was invented in 1947, it would be 20 years later before the solid state voltage regulator would be invented. Given the newness of the technology, I am sure it was much higher costs than the analog regulators used at the time.

We are not talking about the generator or alternator regulator. We are talking about the instrument voltage regulator. In the 30-40-50's the gauges were mechanical, except for the ammeter so there was no need for an IVR.

If it sticks closed those gauges will peg and eventually destroy their selves from the heated overload.

That's a failure mode, not how it normally operates. There are electronic equivalents these days, if that is a worry for you.

https://www.classicindustries.com/product/ivr607.html

Point is when they fail the gauge pegs and burns out from overheating. And the solid state electronic can fail also.

Point is irrelevant. The discussion what about how they work, not how they fail, everything fails.

just an observation, A few are debating the reliability of 50+ year old part. I would say the item was well designed and did it's job
most things will fail eventually. The usage, design and components usually dictate when

Irrelevant to you obviously. Might have some value to others that want to understand more.

I'm pretty sure when a solid state device fails, it fails open.

Kevin

No Sir, they too can fail either way

The electronic one made by RT engineering is designed that if it fails won't hurt the gauge.

It works that way because it was cheap and it works. Chrysler gauges also use a bi-metallic movement so response is relatively slow. Unlike GM cars of the era turning corners did not cause the gas gauge to fluctuate wildly.

Wondering if this might be my issue ??
I have a 71 Duster with the factory rally dash. The issue I have is two fold.... first the dash lights are very dim but the radio lights up good. Tried a new headlight switch thinking that might be the problem. Recently the oil pressure gauge reads high all the time. Starts out okay, especially at idle but after a while and the engine is at normal temperature it reads high. Needle is reading at the very extreme high point. Also the temperature gauge is reading higher than usual but not to the point that it's dangerous. Sometimes I think the Alt guage is off but it seems to be acting properly. Gas gauge always seems to go down quickly especially at these prices. LOL. Fuel gauge seems okay. Just wondering if the volt meter that your talking about is also what is giving me grief ?? It's never been replaced by me in the 25 or so years I have owned the car. Thanks for any help.
Bennoel

I suspect the ground to instrument panel is your culprit. I would start by verifying all connections and mounting screws on the circuit board(s) /film are tight. I am not overly familiar with this particular cluster but a quick look at the circuit films appear as though it gets grounding from multiple attachment points on the circuit film. Make sure the cluster itself is well grounded as well.

Amp gauge isn't in the limiter circuit. As far as the dash lights try new brighter bulbs.

I was told that it might be the printed circuit board and I got one to replace the original one. I'm gonna take the whole speedo cluster out to install a factory dash tach and at this point I would like to rectify all the issues as it's not what I call an easy job. Getting hard to work upside down and in tight quarters. Might just replace the volt limiter and be done with it.
Thanks for the suggestions.

I am reaching that same point. But based on your description I doubt the limiter is the issue. Pulling the front seat makes things so much easier for under the dash work, especially if they're buckets

That was the first thing I did remove the bench seat. Second thing was lower the steering column. Still a tight squeeze.

30 years ago I could get the cluster out of my 68 Charger in 10 minutes with the seats in. this was after repeated "practice" while calibrating gauges, (Long story)LOL

30 years ago iirc I didn't care to much about anything as long as the vehicle (car, truck, motorcycle) ran and got myself and friends to the party. LOL

Update
So I removed the instrument cluster and replaced the two printed circuit boards along with new LED bulbs and sockets for the dash lights. The fuel gauge was tested and found to be faulty. Replaced with another used one that tested good. Bench tested and everything seemed to work fine. Reinstalled in the car and my gauges all working properly. Not pegging to the extreme high reading. (Happy). Waited till dark to test the dash lights. Bench tested and they glowed bright as they still do now only problem is as I'm driving the headlights along with the dash lights flicker on and off rapidly. First guess is a ground issue. Any suggestions greatly appreciated. Really hoping not to have to take the instrument cluster out again. Ugh !!!

Are you saying the headlights AND dash lights are flickering? or just the dash lights

Yes the dash lights as well as the headlights are flickering at the same time.

Thought I'd start checking grounds and I just noticed that the ground strap from the block to the firewall is missing. By missing I mean it's not there and if I'm not mistaken they all had a ground strap from the factory. It's been like that for a very long time and I don't know why I haven't had an issue until lately.
Thanks again for any and all input.

please let us know what happens when you install the engine to firewall ground strap[s].

Well yesterday I checked the connections at the bulkhead and everything there seemed good, no funky green stuff or anything. Next I checked the ground cable at the battery and it had quite a bit of corrosion so I cleaned the terminal (post) as well as the connecter. Sandpaper and file. At this point I also ran a ground from the negative post on the battery to the rad cradle. Then I checked the lights with the engine running and couldn't see any flickering. Had to wait until dark to be sure. So I went out for a drive after dark and after a 25 mile test drive not a flicker could be found. Also my temp. and oil pressure gauge seems to be reading a slight bit lower . They're now where I believe they used to be. Still have to add a ground strap from the engine to the firewall. Hopefully today.

Thanks to all for your help.
Bennoel

Thanks for the follow up

always good to hear a problem has been solved !
thanks for letting us know.

Yes problem resolved and thanks again to all. Now on to the next. LOL