Fuel Gauge issues

[73nme]

First time posting so just wanna say I find all these threads and posts interesting and insightful. I am not mechanically inclined first off. My very first car was a 1973 Charger base model, 318 engine. Had body work as well as mechanical work done back in 1997. Upon moving in 1999 had car garaged until I along with my son who I gave it to got it running during 2020. Since then we’ve been fixing minor issues along the way. My question is I brought it to a restoration shop(the owner is 80 and loves to work on just older model cars). My gas gauge never worked for me when I first bought it many years ago. They said wiring checked out ok but the gauge was the issue. They replaced it, from where it was purchased I do not know. Upon picking it up and starting  the gauge went to 1/4 of a tank. I thought it odd as when I brought it to the shop I filled up b4 driving the 28 miles to the shop from my house so I was already thinking there should definitely be more fuel in tank. So upon leaving after picking it up, first stop gas station, however after taking 5 gallons the pump stopped and now gauge read 1/2 full. Went back to shop and left it. They informed me a few days later it was the sending unit, which was replaced along with fuel tank in 2020. So I waited until part was in and they called me.  Now I’m told it’s the circuit board as gauge was working but then went to “E” on test drive, and it was found that a wire was run from alarm system(my son had it put in) to board and I guess fried it and the new gauge.So now circuit board and fuel gauge have to be ordered and reinstalled. I do have doubts and just wonder if anyone has ever run across issues similar. I know it’s 50 years old and 1 issue can lead to others but I’m concerned because of my lack of automotive knowledge whether or not I’m being suckered…thanks for reading the long post and appreciate any feedback. MOPARS RULE!

[DynoDave]

Welcome aboard 73nme.

If I recall correctly, my '72 still had hard wired gauges....no printed circuit board. That may have changed in '73, so I'll let other more knowledgeable of that model year chime in.

[Nacho-RT74]

A general view on the diff setups and how they work:

All 71/74 standard cluster are with PCB

and all 71/74 Rallye cluster are wired straight to the gauge.

On standard gauge setup the cluster/voltage limiter is energized from RUN circuit

On Rallye cluster setup, the cluster/voltage limiter is energized from ACC circuit.

There are two diff Gas gauges on rallye clusters, both in combo with oil function (one assembly for both) the basic Rallye clusters with this gauge at a side of the speedo (big opening), and the cluster with tach or clock with this gauge bein small for the smaller openings.

All gauges works on the same way thought, with same voltage rates and senders resistance, but no one interchangeable.

All gauges works the same (temp and oil for rallye included ) getting a “constant” (actually pulsating) 5 volts positive signal coming from the voltage limiter attached to the cluster housing and what makes the gauge to measure is the ground variation signal from sender.

All clusters get the same color code for every gauge signal… blue from gas sender, gray from oil sender and violet from temp sender.

In ALL cases the voltage limiter MUST BE grounded to work correctly without burn the gauges. If voltage limiter misses the ground, the full 12 volts voltage will run straight to the gauges without being regulated/limited, burning the gauges.


Fortunatelly, the clusters are grounded with a wire, no need for a chassis ground althought better. On PCB the ground is included on the multipin conector and the harness ground is attached to the dash frame with a J nut close to the driver side bottom end. On Rallye cluster the Voltage limiter gets a straight wire and plug, wider than the rest to a prong the limiter includes. On both cases we must be sure the harness ground must be attached, not loosen or broken.

If just one gauge works, but some other not, it means the voltage limiter is good, but there is something wrong with sender, sender wire, terminals or gauge itself.

On Standard cluster IS NOT UNCOMMON to get some pin slightly loosen from the PCB which could make the signal coming and going (BTDT)… that’s something to check and usually easy to fix soldering with lead the pin to the PCB track.

You can test for working conditions safely with any DC power source below 5 volts… even with an AA 1.5 volts battery just for checking needle movement, althought will moves barelly to around 1/4 of the reading scale. If moves, is still in working order.