Veglia water temp gauge - Aftermarket sender unit

Discussion in '206/246' started by TonyL, Sep 1, 2018.

  1. TonyL

    TonyL Formula 3

    Sep 27, 2007
    Full Name:
    Following replacement of the temperature sender unit I decided I wanted to check the calibration of the two items, the old Veglia gauge and a new aftermarket sender unit. The Veglia water temperature gauge once you look at it closely is a very clever simple design. Basically you have three connections (apart from the bulb) numbered 12V, GN, BN.

    Now I thought 12V would be the 12v input, it isn’t, it’s the feed to the sender unit, (still cannot figure that one out) BN is the supply (12 volts) and GN is ground.

    Across each connection are two coils of different resistances. One is 344 ohms (GN to BN) the other is 122 ohms (GN to 12V) These two coils are positioned 90 degrees to each other and the current passing thru the coils creates a magnetic field

    The resistance of the temperature sender unit decreases on temperature rise so this resistance decrease causes more current to flow through the first coil and less current to flow to the second coil. This change in coil current causes a magnetic field across the two coils to move the needle.

    The new sender unit was placed on a test rig at 90C (195F) (optimum engine temp) the gauge read about 30% low which surprised me, I was expecting it to read higher, so then connected a small potentiometer across 12V(sender wire) to Ground. This effectively worked in conjunction with the sender unit which allowed the gauge to be adjusted to match the actual temp. Incidentally we also varied the voltage input from 8 volts to 14.2 volts under the test. Any variance in voltage is almost all cancelled out due to the 2 coil design. At 8v there was possibly a 10% error.

    To make a correction to the gauge the potentiometer measured 550 ohms and resistor was placed on the gauge. The test re-run and at 90 C (195F) the needle was in the exact position on the gauge. The temp was raised and lowered as required and it worked perfectly although it was difficult to assess as the gauge scale is rather vague at best.

    So the aftermarket sender units are close to being accurate but do need adjustment to give a true reading.

    Now this is the bit I don’t get, putting it all back in the car, running the engine to 90C (thermocouple inserted down the main flow pipe from the cylinder heads), the gauge now read higher. Perhaps there are some local interferences once refitted back into the instrument binnacle, to move the gauge back down to match what was going on with the actual water temp, a resistor was placed across GN to ground instead of 12V to grd. This corrected the gauge once again and am now confident that at when the engine as a 90C my gauge is telling me exactly that.

    The resistance values are not linear so a 90C it is 100% accurate, above & below that I would say a small error but insignificant. So long as my gauge is accurate at operating temperature a slight error either side of that is immaterial.
    synchro, DinoLasse and pshoejberg like this.
  2. DinoLasse

    DinoLasse Formula Junior
    Silver Subscribed

    May 26, 2009
    Full Name:
    Interesting post, Tony.
    I did a similar preliminary calibration on my oil pressure gauge, and I plan to document it here, too.

    As you say, these instruments are simple but cleverly designed, and it is certainly possible to get them to read accurately by calibrating the resistance as you describe.

    The seeming mislabeling of the connections to the gauge is probably not unique to your car. I have seen similar mislabeling on one of the instruments in the Duetto. I thought it was a one-off manufacturing mistake, but learned on the AlfaBB that a whole batch of instruments are labeled that way. I guess we will never know why.

    Strange that your gauge would read differently in the car, as compared to on the bench. Hard to imagine what it could be. But the important thing is how it reads installed in the car, and that is where you have it calibrated now.

    The temperature gauge is something we absolutely have to rely on. On the AlfaBB, I read about a 2600 owner who lamented the expensive damage he caused to his engine because he did not realize that it was overheating. His temperature gauge was reading 20 degrees too low, he later found out. So, good advice to get the temp gauge accurately calibrated.
  3. TonyL

    TonyL Formula 3

    Sep 27, 2007
    Full Name:
    Thanks Lars, it was a lot of work to set up a test rig but the curiosity was killing me!

    Some of the aftermarket sender units where basically crap, some pretended they were Veglia compatible and were not. The interesting thing was that in the range of prices wanted - cheapest was £13 and the dearest was £98 (from a Ferrari specialist)

    the cheapest was the best and most accurate.!!!!

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