Rally dash gauge help

[chargerbob73]

Hi guy's new member I need some help please !!! I was adjusting my trans linkage on my 73 rally charger, when I was finished I started the car and my oil, temp, and gas gauges all pegged. They are rally gauges, I was told to test the limiter at the temp sending unit and I had voltage there so the repair manual is telling me the cluster mounting screws are loose but they seem tight. Any help would be greatly appreciated.

[Pete in NH]

Hi,

What voltage did you have at the temperature sender connection and was it unplugged when you measured it? If it was 12 volts the instrument voltage regulator is bad. This regulator in its original form is an electro- mechanical unit that outputs a kind of pulsing voltage that has an average value of 5 volts. If you put a voltmeter on the open temperature sensor wire you should see your voltmeter move around , if it is a steady 12 volts the regulator has welded up its contact points which is a common failure. Don't keep your car powered on, you will burn out the gauges if the regulator is bad.

And I almost forgot, welcome to the group!

[chargerbob73]

Thank you Pete. I checked the temp sensor still hooked up with the key on and the meter did fluctuate between 5 and 7 volts. I have owned this car for years and never had a problem with the gauges. I could see if I was working around wires and knocked one loose.

[XH29N0G]

I believe the issue is that the voltage regulator for the cluster should only be delivering a potential of 5V, which if there were no resistance in the circuit would give a full scale reading.  Otherwise the oil sender, temp sender, gas sender reduce the voltage by changing the resistance.  See how Pete responds. 

[Pete in NH]

The only thing I can think of that would peg all the gauges is the instrument regulator is not working or its output line some how got connected to 12 volts. Each gauge has its own sending unit and the chances of all three short circuiting at the same time is very, very remote.

The gauges are all thermal types. they have a small bi-metal strip with a heater around it. the more current that flows though the gauge the hotter the heater gets. This heat expands the bi-metal strip and moves the pointer.

Here is where things get a little tricky. The instrument voltage regulator pulses on and off. 5/14 th's of 14 volts has the same heating value as a steady 5 volts. So, what the system actually does is thermally average the pulsing output of the voltage regulator as if it was a steady 5 volts. It's actually a very clever system before the age of modern electronics. It was a way to do it electromechanically. The trouble is the contact points in the electromechanical regulator burn up and weld themselves together. When they do you get a full 14 volts on the gauges which burn them out quickly.

When you try to measure the regulator output with a meter you will see the pulsing voltage but you can't measure the actual voltage output. You have to use a very special meter that also works on heat.

[chargerbob73]

Thank you Pete I think it is the voltage limiter, I just ordered a solid state one. I'll let you know how it works out. Thanks again 

[chargerbob73]

And Thank you XH29NOG I appreciate the help. 

[chargerbob73]

Hey guys just wanted to let you know I replaced the volt limiter and every thing works fine Thanks again. 

[Nacho-RT74]

Nice you made it to fix way before gauges burnt out!!!!

need to note, if limiter was working yet, some could make it could be adjusted by a tiny screw on baquelite... need to be adjusted with an analog tester.

Or maybe just run out of ground on limiter?

some stuff to check to future references

[ACUDANUT]

Thank you Pete I think it is the voltage limiter, I just ordered a solid state one. I'll let you know how it works out. Thanks again 

Where again is this located. ?

[XH29N0G]

I think it is in the yellow box in this picture http://www.dodgecharger.com/forum/index.php/topic,1027.msg14711.html#msg14711 or here http://www.dodgecharger.com/forum/index.php/topic,22233.msg241016.html#msg241016  see what other say.