Bouncing temp gauge...

[jclaus98]

Gentlemen, long time viewer, first time poster; have gotten a ton of valuable info from the discussions held here and while the situation that brings me here is unfortunate, I am happy to finally be apart of what goes on here.  Having said that, hopefully the wealth of knowledge represented here can help me with my situation.  I will try to be as thorough as possible with explaining what's going on: I have a 1968 Charger, SE with rally gauges and my Dad and I did the restoration ourselves.  Started with everything necessary to make it functional and then did the paint and body work.  Important to note is that the bulkhead connector and the wiring under the dash are all original and had nary a problem with until recently.  The first year we had it running and driving, my dad completely restored the original gauges and instrument cluster to a factory repro level.  He bench tested all the gauges prior to reassembly and reused the factory voltage limiter behind the cluster(which I'm only now finding out was, shall we say, unreliable?).  That first year we took it to Hot August Nights and on the way home, the oil pressure gauge quit.  Plenty of oil in the crankcase, no noises of any kind, no power loss, but the gauge went completely dead.  When I got home, I bought an aftermarket gauge and with the included sending unit, ran a separate wire to the gauge and was able to once again read my 383's oil pressure.  Fast forward about 3 years later when I started having charging issues due to the lack of amperage output by the factory alternator trying to power my dual electric fans, I ended up bypassing the ammeter and drilled through the bulkhead connector and used a heavy gauge wire with 150 amp circuit breaker to remedy the amp draw at the spade connector in the bulkhead.  Now I'm running a 150 amp, internally regulated alternator wired directly to the battery and have had no more charging issues.  However, during that time period, I still needed a way to see what the charging system was doing so I got a summit analog, electric, 3 gauge set with oil, water and voltage and wired those three up.  I wanted a redundant water temp gauge so I had a more accurate representation of the actual water temperature than the stock 170-230 sweep of the factory gauge. Since I had previously discarded the factory oil temp gauge wire and since it happen to run almost to the water pump housing, I repurposed that sending unit wire as my backup water temp gauge.  I tapped into it under the dash and it worked flawlessly.  Fast forward about 6 months and I was driving the car one day and the summit water temp gauge started randomly bouncing around, almost as if there was a bad ground.  I pulled over, checked my grounds, checked the wiring to the sender and found no issues.  I got back on the road and didn't have another issue until almost a month later and it did it again, only this time, it wouldn't quit, it just kept bouncing around the whole time I was running.  Irritating, but I at least had the stock gauge in the dash to tell me if there was a life or death situation brewing.  Another month later, I'm driving along, gauge still bouncing when it quit completely.  It wasn't buried like a short to ground, just kinda sat at the lowest hash mark on the gauge face.  Weird, but again, I still had the other gauge operational so I didn't worry about it.  In the offseason this year, I was determined to figure out what the root cause of this issue was, so I replaced the sending unit.  No change.  I replaced the gauge.  No change.  So I start grabbing wires under the dash and the bouncing came back.  WTF???  Ok, so I grab my DVOM(I know, that should have been the first thing I did...) and I still had continuity between the sender and the gauge, so I flip it to DC volts and what I found was something I'd never encountered before.  Voltage was fluctuating between 3 and 9 volts, never the same and never constant.  I later read that this is how the old voltage limiter regulated voltage down to 5 volts for reference, but at the time, all I could think was, I've got an intermittent short to ground somewhere and I have to find it before I burn the car to the ground!  So, I started with the bulkhead connecter.  I found the oil pressure wire had the insulation rubbed off and was making contact with the large black alternator wire.  So, with the dash still in the car, I fixed that and plugged everything back in looking for pay dirt.  No such luck; still the same fluctuating voltage as before.  Stumped, I started tugging on wiring again.  With my test light hooked up to the oil pressure wire, I was able to get the light to stop blinking in two different places, repeatedly.  So, against my better judgment, I started removing the near 50 year old wire tape and looking for more shorts to ground.  Nothing, I got all the way up to right behind the dash and still couldn't find anything.  So, I tore the dash out and much to my surprise, I found the strange voltage coming from the dash itself.  So, after the book of war and peace above, here are my questions: does anyone know why I went at least 6 months without any voltage at that wire to suddenly voltage there one day, all the while the original water and fuel gauges still functioned?  Is it possible, or more likely probable, that the original voltage limiter was the cause of the oil pressure gauge failure and if so, why were the other gauges still working if that was the case?  And finally, for those of you who have modified theirs to use the solid state, modern voltage limiter instead of the old "morse code" voltage limiter, did your water and fuel gauges work better after that modification?  The reason I ask is, while my fuel gauge works, it's kinda "special".  When full, it will read about a 1/4 of a tank past full and when empty, its at about a 1/4 of a tank.  While I can use that scale effectively, it would be nice to have an accurate gauge, so as to reduce my OCD.  Any thoughts would be greatly appreciated.  Hopefully I wasn't too terribly long winded. 

[Pete in NH]

Hi and welcome to the group,

Well, that was quite a lot to read through and after reading it all I think what you are asking are some basic questions about the gauge systems and 50 year old wiring.

As you may now know, the bulk head connectors can be a major source of multiple problems due to corroded and burnt pins. Also, cracked, broken, hardened plastic insulation on old wiring can also cause many intermittent issues. The factory gauges are mounted on a printed circuit board and these boards can also have age related problems. As you know the old electro-mechanical voltage regulators for the gauges are often the cause of gauge issues.

You can replace the regulator with an electronic type. You have two options, one is a RTE ready made unit and the second is a 7805 type regulator chip and if you do a search on Mopar 7805 voltage regulator replacement you will find  a lot of information about it.

One thing to keep in mind is that all Mopar gauge systems of the period used a  single sending unit resistance range. A 10 ohm resistance equaled full gauge reading, 24 ohms equaled mid scale and 73 ohms equaled the bottom of the scale. This makes it easy to check gauge operation with a few resistors from your local Radio Shack store.

That's kind of a brief overview, so , feel free to ask specific questions.

[fy469rtse]

nah , that's just too complicated ,
when your dad restored the gauges , his bent accidentally bent the needle to gauge or set the ohms to factory , after market fuel senders are off ,
you will have to calibrate the two ,
I did after reading like yours , works fine
as for temp ,
you have a loose connection , I suspect sender it self