change in gas and temp gauges

[str12-340]

for years my temp gauge has worked well, verified with infrared sensor at the thermostat housing. At 180 to 190 degrees it sat 1/3 of the way into the bracketed part of the gauge. Once up to temp, if you turned the car off and came back 20 minutes later the gauge read 2/3 or a bit more and when you stared it soon was back down where it belonged.

My gas gauge has always been off showing 3/4 tank when full, which is just fine as it keeps me from tempting fate in terms of running out of gas.

Suddenly, when up to temp the gauge reads at the bottom bracketed line, but sensor tells me it is 180-190. When you turn the car off and let it get hot, it never moves off the lower mark.

At The Same Time, gas gauge reads 1/8 tank when full.

My tentative diagnosis is that the voltage limiter has failed / is failing...

Car has a four year old M&H dash harness and a 6 year old under hood harness. Original Gauges and a transistorized voltage limiter.

What say the electrical gurus???

 

[RustyRatRod]

Are we to assume you're talkin about the car in your avatar? It does sound like something is going on with the cluster voltage regulator.

 

[67Dart273]

You need to treat troubleshooting cluster/ gauges like an end to end system

That is: Proper 12.6--14.2 power to the cluster pin that supplies the gauge regulator/ IVR, and ground----and proper gauge readings with a given test resistor. THEN you can check the whole thing to the original senders.

Get some resistors so you can test the gauges. If you still have an OEM gauge voltage limiter, replace it with a solid state one like from RTE

Otherwise, generally problem areas in no particular order

1...the cluster connector pins are crimped and can corrode/ come loose. Clean them and solder to the board traces
2...Of course the gauges can become inaccurate
3...As mentioned, the IVR can become inaccurate
4...The "socket" where the IVR plugs in, those contacts can lose connection with the board traces. Clean and socler jumpers across from the contacts to the board traces
5....The studs on the gauges can lose contact with the board traces. Consider replacing the fake nuts with real ones. Loosen/ tighten the nuts several times to scrub the board copper clean
6....Cluster grounding is poor, and depended on mounting screws. Find a common ground point on the cluster, connect a grounding pigtail, and bolt that to the column support or the dash itself.

TEST the cluster/ gauges with test resistors. Here are the figures:

Once you KNOW the cluster works and have used resistors to verify that, THEN you can go after bad connections in the sender wires, or bad senders.

 

[67Dart273]

Also CHECK YOUR charged/ resting battery voltage. Do this after shutdown after a few hours, and check your battery running/ charging voltage. A healthy fully charged battery is about 12.6, and an operating system running should be about 13.8--14.2, and in no case below 13.5 or above 14.5

 

[str12-340]

Are we to assume you're talkin about the car in your avatar?

Yep, that's the car - 70 Dart standard cluster - not rallye dash

You need to treat troubleshooting cluster/ gauges like an end to end system

yeah, I hear you, but before I go through the special hell of pulling the cluster to do that, I'm trying to figure out if it's worth snaking my arm up there and replacing the voltage limiter. Since the only things acting up are the 2 gauges that are attached to it and they both went wonky at the same time...

 

[RedFish]

The wild needle swings are a symptom of failing mechanical limiter but I can't say I've seen similar from a solid-state regulator. The little 7405 chip for example Amazon product ASIN B08YNNBYB7 will output 5.2 to 5.4 volts with input anywhere between 9 and 35 volts. The output will climb as they overheat though. The heatsink and the transference paste between the chip and heatsink are players. The thing is, they don't get goofy and/or continue to function half assed. They just die.
I'll lay my coin on a failing temp gauge. Pull the connector off that sender and see does the fuel gauge go back to proper responses.

 

[67Dart273]

^^What Red is saying here, is that the IVR does not have enough power to compensate for one gauge which is (probably) drawing too much current, and the output to the opposite gauge sags, affecting it's reading as well.

 

[devildog]

I had a similar problem with a rally gauge cluster in my '67 Barracuda. Installed a new M&H dash harness and did the IVR thing and still had dead or incorrect gauges. Whoever owned this car before me butchered the wiring and troubleshooting is a bear. I finally decided to send the complete dash over to Redline Gauges in Santa Clarita, CA. They will clean, repair, restore everything including a rebuilt speedometer for a very reasonable price. If I still have a problem when I get it back it will have been function tested already so I can start trying to look at the rest of the wiring (again). I have a feeling there is something amiss at the firewall connection. When I got the car somebody had ran a 10 Ga. wire from the battery into the amp meter connection so anything is possible... frustrating. At least I will be able to eliminate the dash gauges, lighting, left turn signal indicator that connects to the 4 way flasher switch, etc. A real head scratcher for sure.