Hi All!

I'm having some issues with the Temp and Gas Gauge on my 1968 Plymouth Valiant. Some time ago the Temp gauge quit, I didn't worry about it becauce I have a second Autometer gauge, and now the gas gauge has failed. Both gauges have failed to the low side.

I tested the wires to both sending units. With a test light and the key on the light blinks. I then grounded both wires and the gauges begin to climb. I changed the temp sending unit but nothing changed.

It appears the voltage at the sending unit is only about 2-3 volts. Is it possible for the IVR to fail this way or could I have a different issue?

Check the pass thru connector at the fire wall for corrosion and or poor connection.

It looks good to me. I pulled the middle plug off with the purple wire for the temp sending unit. It was clean and tight. The fuel wiring runs thru the kick panel and to the trunk.

make sure all the instrument cluster screws are tight. I don't know if it will make a difference but some dash issues are caused by a bad ground.

Those are probably the only two gauges on your dash circuit the limiter controls, and you are having trouble with both. I'm suspect of the limiter. However, when you grounded the wires, did the gauges peg to the end? If they did, the limiter is still good, and putting out five volts as it should. The sending units are actually variable grounds, eliminating them by grounding the wire lets five volts through the gauge, pushing each to max reading. If the limiter fails, gauges either won't read, or be on the low side.

Your on the correct track on suspecting the instrument voltage regulator as potentially the cause of the low readings
It is over 45 yrs old and who knows how many hours of use
I would buy a new one, NAPA and I think Rock Auto both offer them
I don't remember what the correct voltage is suppose to be going to those two instruments but I think it is higher, closer to 8 volts pulsing maybe
Hopefully someone else with a 1968 FSM will chime in and get you the straight scoop on this

I just double checked the temp gauge and it quickly climbs right to the end when the wire is grounded and the key is turned on.

Sorry to butt in but my van has this symptom.
What does this limiter look like and where might I find it?

type in mopar voltage limiter on Ebay. it plugs in on the back of the dash.

Thanks R.R.
"back o dash"?
I am assuming you mean the gauge panel?

If the gauges top out when you ground the sender wire the problem is the sender. The gauges won't move if it's the gauge or the VR and yes the VR plugs into the back of the gauge panel. Napa or Rock Auto.

On the gauge regulator, it can fail two ways. If it fails open, the gauges won't work, and if it fails closed, then the gauges will be burned up, as in dead, and ruined. I would not put another mechanical regulator back in the car either, especially because it probably would be made in china. There is a solid state regulator made by Real Time Engineering that if it falls it does it so the gauges won't get burned up.

OK so the gas gauge is at half mast when the tank is full.
That is the sender in the tank.
I'll try grounding the sender and see what it does.
At least I've identified the resister.
Thanks.

yes definitely get an electronic version & what Dave said, I was thinking the same thing that if you ground it & it pegs that it is the sender (or sender outer body not grounded) at least on the gas tank sending unit tho you would think the temp sender operates the same way but he did mention a very low voltage reading but maybe the pulsating method it gets 5 volts is misleading on some meters. jumpering the sender body (not the threaded stud) on it to ground would answer that (or ohming it) but he said he replaced the sender with no change. keep us posted.

Went out for a good rip tonight and got the car temp up to 200F according to the autometer gauge.The factory temp gauge moved from well below the "C" to the first mark. I'm kind of wondering if I got a temp sensor with the wrong ohm range or maybe a dud? Its aa BWD Automotive Part #WT127.

Now a days, you can't buy a sending unit that matches your gauge like back in the old days. We're stuck with what we get.

Fixed that for you

Yes, and they won't let me call this junk what it really is.