My electric temp guage reeds about 25* high, is there any way to adjust this?? I am running 2 guages now, & I want to ditch the aftermarket mechanical guage.
Thanks for the help.

 

Other than replacing the sender with one that has more accurate resistance, no. The resistance from min to max on the sender is not linear. If you add or remove resistance to get the gauge to register correctly at one temperature point, it will be off everywhere else.

What you can do is to measure the resistance of the sender at a sweep of temperature incriments. Then get a variable resistor and connect it to the sender wire and measure the various points of resistance to the gauge reading and compare the two (i.e.; the sender has a resistance of 300ohms at 180 degrees while the gauge registers 280ohms for the same temperature reading). In a perfect world you could match a sender to the gauge's readings but the most you can hope for is that you come close.

 

Willcox has been working on this. As gerry72 said you can't change it on the sender wire but I think if you change the resistor on the back of thegauge you can calibrate it.

In your case because the gauge is reading HIGH you can add a resistor in the sender wire and lower the needle reading , but it will only be accurate for that temp.You might want to check the sender output ohms to see if maybe its outputting the wrong ohms for the temp.

 

I think adding the resistor in the sender line is a good idea. The only temperature you really care about is at "normal" operating temp. If it is reading very low or very high, you still know there is a problem. If those extreme points are 20 degrees off from actual temp, who cares? Go to Radio Shack and buy a resistor near 10 ohms and another near 20 ohms. While you still have the other gauge installed (to cross-check), stick the 10 ohm resistor in series with the signal wire [sender terminal to resistor to sender wire]. When engine stabilizes at normal level, compare the gauge readings. If that doesn't lower the reading enough, replace it with the 20 ohm resistor and check again. You can put both resistors end-to-end in the line (30 ohms) if it is still not enough. You get the idea. When you find what you need, just use some shrink tubing to interconnect the resistors in the wiring. I would suggest that you insert the resistor(s) away from the engine as much as possible (ie, just before the dark green wire gets imbedded into the alternator harness).

 

Making an adjustment in the ground resistance on the back of the gauge can be done to dial in the gauge. Doing it this way will create a non linear adjustment that is correct over the entire gauge vs. the fixed adjustment method associated with resistance induced in the sending unit wire.

Making the correct change in resistance is time consuming and you must know the output resistance on the sending unit at fixed temperatures. Once you know the output you make an adjustment in the resistance to allow those output readings to work the dash gauge.

Doable, yes! Worth the time at home, I’m not sure. The easy way is as state above, insert resistance and get one fixed accurate point. If interested in having your gauge calibrated, contact sales and they can give you a cost estimate.

Willcox

 

Thanks guys, my problem is probably the sender unit I purchased from NAPA, it is probably generic. If I order the correct one from a corvette vender for a 75 do you think it will make a differance?

 

I'd like to add for those reading this. You can ONLY make an adjustment putting a resistor in the sending wire if the gauge reads HIGH. If your gauge reads LOW you cant adjust the gauge this way.
Adding a resistor can only make the gauge read LOWER.

 

"Wells" seems the way to go. Asked the same question in May.

http://forums.corvetteforum.com/c3-t...ding-unit.html

Also, take a read of the link I posted. At the end they talk about tape. Someone actually tested it.