rallye gauge testing

[jdscofield]

Earlier today, I tested my non working gauges with my multi-meter and I wanted to make sure I did it correctly.  on the temp gauge and the oil pressure gauge (I think that was the other one), I disconnected the two leads to the two posts on each gauge and checked the ohms from one post to ground and then the other post to ground.  I had zero ohms (technically 1 ohm because my multi-meter starts at 1 ohm).  I then went from post to post and would read anywhere from 14.5 ohms to 20 ohms.  I'm assuming the read through means I have a short and the gauges are bad.  Did I test them correctly and is my conclusion correct.....bad gauges?

[XH29N0G]

I am not sure about this because I just worked on gauges from a 70 and I am not altogether certain they operate on the same principle.  

My understanding is that the gauges work on a circuit that has a constant voltage (5 V) and the sending unit generally has variable resistance.  This ends up sending a different current to the gauge, which heats up the inner workings of the gauge and the needle deflects depending on how hot it gets.  

To me, it makes sense that there should be some resistance to the gauge itself because that will generate heat if it is windings that do this.  So you may be fine with the resistance you measure.  

If you gauge works on the same principle, you may be able to use a 5V power supply (with appropriate resistors) to test the full reading on the gauge.  I assume if you use a different voltage, you will get a different deflection since Voltage=current x resistance.  

I used an old phone charger to improvise a tester, but some other people have some very nice power supplies.  These are described near the end of the present the cluster restoration thread.

http://www.dodgecharger.com/forum/index.php/topic,37999.0.html

[RAC95054]

XH29N0G is correct.  The best way to test the gauges is use a 5V supply.  If you don't have that, get 2 "D" cell batteries and put them in series (+ to -), then take a lead from the + end of the batteries and attach it to the left post of the gauge, then take a lead from the - end of the batteries to the right post of the gauge, and you should see the needle start moving towards the full/hot/high reading.  If it doesn't move at all, the guage is toast (or you have the leads backwards).  The resistance is in the sending units, so that is how the reading changes when the car is operating.

[Pete in NH]

Hi,

The reading you saw post to post looks correct. However, you should not see any reading from either post to ground with the gauges disconnected from the dash wiring. So, I'm a bit puzzled about the continuity from either post to ground on both gauges.

Br careful using those two D cells, just briefly touch the wire to the gauge post. If the gauge starts to move take the wire off, you don't want to leave it there and damage the gauge.

A 5volt DC source is the best way to test the gauge. Using a 10 ohm resistor  between the gauge and a 5 volt supply the gauge should read full scale. A 24 ohm resistor should be half scale and a 73 ohm resistance should read zero on the gauge. 

[RAC95054]

2 D batteries in series are just 4.4 volts, so there should be no way they would damage the gauge (plus the amperage is also very low). And you want to see the needle sweep, so you know it can.

[XH29N0G]

I think the key point (and I missed it in my post too - but fixed it now) is that one needs to put a certain resistance in the 5 V circuit as well for a given current and gauge reading.  It may be that at 4.4 v, the resistance on the gauge alone is enough to make it possible without a resistor.      

[Pete in NH]

Two D cells in series would actually be 3.1 volts if they are new cells. IF the gauge internal resistance is actually 14 ohms the current using the D cells would be around .21 amperes through the gauge which is likely around its operating range. So, yes the gauge resistance will limit the current using only 3 volts. But, that will depend on if the gauge internal resistance is really around 14 ohms.

[Dans 68]

Here is a sketch showing the set-up. Change the resistances to get the different set-points for the gauge.

Dan

[gibber]

I have a small side business repairing these Mopar gauges. If you get approx 20 ohms across the two posts on either a fuel, temp. or oil press gauge, it only means the insulated nichrome wire wrapped around a bimetal strip inside the gauge  is not broken. It does not mean the gauge will function correctly as the wire could have been subject to more than the 5VDC that the voltage limiter is supposed to provide them. The only way to know for sure is to open up the gauge and check the condition of the insulation on that wire. If it is blackened at all it throws off the calibration and if it is burned off enough, the gauge will drive to full range (Hot, High, or Full) as appropriate once power is applied to it. Of course if the ohm reading is near zero it indicates the wire has already burned in half.....

Thanks,

Mark
www.thegaugedoc.com

[RAC95054]

Two D cells in series would actually be 3.1 volts if they are new cells. IF the gauge internal resistance is actually 14 ohms the current using the D cells would be around .21 amperes through the gauge which is likely around its operating range. So, yes the gauge resistance will limit the current using only 3 volts. But, that will depend on if the gauge internal resistance is really around 14 ohms.

I stand corrected.  Dry cell batteries are usually about 1.5V.  I'm not sure where I got the 2.2V number from, but could have swore that was the max of normal dry cell batteries.  Anyhow... thanks for setting me straight.     BTW, it's the sending unit that is providing the variable resistance of the circuit.  If it fails as an open, then your gauge reads nothing. If it fails as a short, it will peg the gauge.  The voltage limit comes from the regulator (the little can device) plugged into the gauge circuit board.  If that goes bad, then 12V goes to the gauges and fries them, which is why I converted the old-style points-type regulator to a solid-state model to avoid that.