I've read all I can on the subject. My 260 gauge reads close to the red. I have checked temp with IR gun and it's about 195-200. I have tried to calibrate the gauge using resistors from 10-39 ohm with no change. I have run a dedicated, but temporary ground for the gauge, no change.

I'm wondering if the gauge can be defective inside and non-calibratable. Has anybody found this to be true by just changing the gauge and improving their readings? 

Asked another way, just because the gauge moves, is that proof it must be good all around?

 

Original Post

Here is a really great writeup for the gauge, according the data here somehow the gauge is bad.. http://www.panteraplace.com/page205.htm . But not too hard to do some measurements; 

   - disconnect the sensor and measure the voltage of the wire...this is the open "TERM" voltage which should be about 5volts to ground. 

 - attach that 39 ohm resistor between the wire and ground (not to the sensor), you should measure about 3v and the meter should be somewhere at the 'second mark' and 190'.  

 Bottom line, you should be able to change the resistance between the TERM wire and ground and make the meter read whatever you want, or the gauge is broke.....According to the PanteraPlace data, there is no way a healthy gauge with a 39ohm resistor in series should cause the gauge to peg out at the 260' (2:00 position), at most it should be around half way. 

  The gauges are so simple, I don't know how they would wear out to cause a swing to dead right...(Just thanks to the incredible writeup by Bill Taylor here! And good news, a replacement Temp gauge with any bezel but half-V black is easy to find...)--Lee

  

 

I purchased a bad gauge

I can't recall exactly how it responded, but found that by measuring the gauge internal resistances between the three terminals confirmed internal problem when compared to the other gauges

One of the "legs" was open.   Allowed it to "work" but was WAY off

You were right, it could be a ground to the gauge; But if the light works (the gauge backlighting), your ground is ok. Then, its possible that the ground connection between the case and the meter movement is open---if (at the first step, where you measure the voltage of the wire removed at the sensor) you see 12v, then the ground that connects the winding shown vertically on the right is open somehow in the gauge).  

  Another check, when the TERM wire is disconnected from the sensor instead of measuring the voltage from the wire, turn off the key and measure its resistance to ground, you should see about 180ohms (the resistance of that coil going to ground). Higher than this (say, over 300ohms) means the ground is lost at the movement. 

  Bill Taylor was pretty damn sharp to see the coils as they are, I've just now looked at 2 movements and still cannot see how the path to ground is made (well, except for the 2 tiny nuts on the back of the can, I cannot see how the coil touches the chassis). But measure it and its there...The use of 2 coils here probably explains why Veglia didn't need the voltage regulator as used on Smiths gauges (which is a longer story in the "mystery component" on the Mangusta schematic...the 2 coil design should be more insensitive to battery voltage changes...)

I had a similar issue, I painted the back plate on the rear oftheZF trans with black engine enamel when I detailed my trans during my recent motor rebuild. Steve Wilkinson said for me to clean the paint off where the ground strap connects the trans to the chassis and now my temp guage works correctly again. 

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