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beginner question

Discussion in 'General Electronics Discussion' started by GT66, Apr 15, 2010.

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  1. GT66


    Apr 15, 2010
    Hi All,

    Warning: I am an electronics novice. I can solder and identify various components but beyond that I am electronically illiterate. I am trying not to let that stop me in this case. :D

    I am trying to repair a digital gauge cluster on a 20 year old Dodge. The clusters use VFDs set up has horizontal bar graphs. Occasionally, one of these displays will "peg" meaning that all the bar elements will light up. Sometimes it will just be pegged all the time and sometimes it will display normally until it reaches a certain number of elements and will then peg. Tapping on the dash will occasionally cause the gauge to behave. Sensors and wiring has been checked out. It is definitely a problem with the cluster.

    So, not knowing anything about working on this gauge outside of the car, I thought that creating a bench setup would be best. That way, I can disassemble the boards and sit there with a multi-meter and try to figure out what's happening. To bench test, I need someway to simulate the voltage outputs of the sensors and vary that output in small increments. That has led me to believe I need some sort of variable DC power supply however, most of the ones I run across seem a bit heavy duty for my needs.

    I am thinking that using wall wart and adding in a circuit to vary the output voltage would do the trick. Most of the plans for variable PSUs seem to go from 1 volt up and don't really indicate the resolution of adjustment steps. This is where I'm at in the process.

    My questions: Would this even be the proper way to simulate those sensor voltages? I assume I could hook in the multimeter to be the display to see where my voltage is at. Could I take a starter kit design and change the voltage regulator to adjust from 0v to 3v in .1 steps instead of say 5v to 12v? I have no ability whatsoever to design my own variable interface but I think I could get a kit and make the modifications with the advice I might get here.

    So, what do you all think (other than this guy is in over his head)? Does it make any sense to build these low voltage PSUs from some starter kits and wall warts or is there a cheap device that can already do this sort of thing? Am I over complicating? Another car guy told me to use the dash light dimmer knob as the adjuster. Would that really work?

    I look forward to your responses.


    BTW - This is the sort of kit I am thinking I need as the starting point:
    Last edited: Apr 15, 2010
  2. Resqueline


    Jul 31, 2009
    Most sensors for gauges are just varying their resistance so you can probably get away with using a small fixed 12V power supply and a variable resistor (1k?).
    You'll need the wiring diagram for the cluster though.
  3. GT66


    Apr 15, 2010
    Resqueline: Thanks for the info. I have the shop manual to figure out the wiring. There are no schematics available as far as I know. This particular problem is common with these gauges. Previous and prior generations of the digital clusters for these cars don't seem to have this problem.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    One possibility is that over time vibration has causes a poorly soldered connection to fracture.

    Start by looking very carefully at the board, possibly under magnification. If there are any joints that look different, or broken, you may want to re-solder them (carefully).

    Once you have it set up, see if pressure or flexing of the board causes or corrects the problem. where you need to stress the board may help pinpoint poor connections.

    If it's a common problem, there may be a part of the board that is stressed and cracks (less likely if the board is fibreglass)
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