Barracuda Rallye dash Temp and Oil gauges underreporting

[Traxfish]

I'm having issues with my stock temperature and oil gauges underreporting in my 1968 Barracuda. They appear to report about half of the values that I'd expect. I paid good money to have all of the gauges refurbished and they still do it. Could something outside of the gauges be causing this?

About a year ago I removed the dash and replaced the boards because a pin was broken on the left side. I went ahead and replaced the right side too and the gauges overreported to peg and fried the voltage regulator built into the gas gauge so they all stopped working. I removed the dash again and sent the gauges to a guy recommended here to have them refurbished. After that they worked but they all under report by about half. I then decided to do the gas gauge voltage regulator delete and installed an external regulator, and now JUST the oil and temp gauge underreport. The gas gauge seems to be working fine. Could this be due to an issue outside of the gauges like poor grounding, a problem with the newly installed board, or somehow both sending units coincidentally having issues?

 

[Super-Cuda]

Couple of simple things to check if you have the cluster out.

1. ground the metal gauge housing to a battery using a test lead. Then, hook another test lead to the positive side of the battery and touch it off the post of the gauge that goes to the round black connector. Do not leave it connected, just touch it to the post and the gauge needle should deflect to the right. Do this to both gauges to test them.

2. If cluster is installed, make sure its grounded . There should be a black ground wire running from the metal dash frame to the body on the passenger side just above the kick panel.

3. If all else fails , try another sending unit. The water temp is the easiest to try first.

4. Here's one that stumped me! If you had the clear lenses out, make sure they are installed properly. Mine were interfering with the sweep of the gauge needles and would not let the pointers advance to proper reading.

 

[RedFish]

The "daisy chain" for chassis ground path begins where screws mount boards to cast metal housing. Those ground points have more effect on the panel lights. Oil and temp gauges are not and should not have a ground path anywhere but though respective senders.
The fuel gauge is totally different. You already know it has 3 connections showing at the circuit board but you may not have noticed its 4th connection. On the backside of this gauge is a slither of metal (either brass or tin depending on year model). That is the chassis ground required by the internal limiter. So I read that you did something inside the fuel gauge. If you totally amputated that limiter, thats good. If you followed instrustions at RTE website, simply bent the contacts apart and it still has 12 volts going into it, it may still be a problem. If the winding on the limiters bimetal arm is shorted to the arm, and its chassis ground on the backside is still intact, it can function like a 4th and defective gauge wicking away some portion of the voltage supplied by any external regulated connected to the inst' circuit. It make be taking a lions share too. that would cause low gauge readings. If the fuel gauge is going to be like any common 2 post gauge it doesn't need 12 volts going to it. You can remove the blue with white tracer wire from the round harness connector and route it directly to whatever regulator you use and wherever you choose to place it. I can't hurt to lift the fuel gauge and cover that slither of ground metal with electric tape so it is mounted just like any 2 post gauge. No chassis ground path.
Nobody ever questions why the limiter was placed inside the fuel gauge. The standard panel has only 2 thermal gauges, fuel and temp'. Oil warning lamps operate on 12 volts. The same standard plug limiter off the shelf can operate those 2 just fine. Those panels with 3 thermal gauges (includes oil gauge) requires a limiter with a slightly different, increased load on its output side. The basic 5 volt solid state regular (was it 7405? 7805? I forget) isn't quite sufficient for this increased load. It will produce slower and slightly lower needle response. I used that regulator in 100s of solid state regulators but I calibrated every gauge to read right too.
And after all that typing... a proper ground at the regulator itself may be all you need.

 

[67Dart273]

And last do not just guess. TEST. Make CERTAIN you are actually getting proper full battery V to the cluster AND THAT IT's grounded, and THEN round up some resistors so you can check the gauge accuracy

 

[ESP47]

Yep I just wound up getting myself a 100 ohm potentiometer and adjusted it to those resistances for testing. My factory gauges weren't perfect but they were close enough for government work.