Easy Coolant Temp Sensor Fix

[ls1nova71]

90's gauges use one wire, I believe he's talking about the two wires coming off the potentiometer, one to sender, and the other to the gauge. My concern though is that the gauge won't be linear in it's movement. I tried the same thing a long time ago by calculating the resistances and then put the correct resistor in line, and although I did get the gauge to read straight up at 195*, but when it got a lot hotter, the gauge didn't show it, if that makes sense. It moved a little more to the right, but not into the red part of the gauge, and clearly should have been.

[dayj1]

Originally Posted by ls1nova71

90's gauges use one wire, I believe he's talking about the two wires coming off the potentiometer, one to sender, and the other to the gauge. My concern though is that the gauge won't be linear in it's movement. I tried the same thing a long time ago by calculating the resistances and then put the correct resistor in line, and although I did get the gauge to read straight up at 195*, but when it got a lot hotter, the gauge didn't show it, if that makes sense. It moved a little more to the right, but not into the red part of the gauge, and clearly should have been.

The response curve of a temperature sender is not linear. When cold, the resistance is very high, often in the kilo-ohm or even mega-ohm range. As it heats up the resistance drops, but at an exponential rate. In other words, the added resistance from an inline resistor (or pot) is irrelevant at low temp, but skews the gauge more and more as temp goes up. The best you'll ever get with such an approach is a gauge that is accurate at a single temp.

Also, if I understand the OP correctly, the '91 gauge is spliced into the 2004 temp sender. If a separate '04 sender was used in the passenger head, then no harm, no foul (except a bad gauge reading). If the gauge was spliced into the driver's side sender along with the PCM, then all bets are off on what the PCM thinks the engine coolant temp is due to the load placed on the circuit. A bad temp reading could definitely effect how the PCM controls timing, etc.

[littlevictories]

Originally Posted by ls1nova71

90's gauges use one wire, I believe he's talking about the two wires coming off the potentiometer, one to sender, and the other to the gauge. My concern though is that the gauge won't be linear in it's movement. I tried the same thing a long time ago by calculating the resistances and then put the correct resistor in line, and although I did get the gauge to read straight up at 195*, but when it got a lot hotter, the gauge didn't show it, if that makes sense. It moved a little more to the right, but not into the red part of the gauge, and clearly should have been.

yeah thats how my fuel gauge works with the resistor fix works. reads nice at half tank, but above and below it's wildly inaccurate. more of a GO GET GAS DUMMY indicator than anything else

[scss02]

The response is not linear (and yes it a separate sensor in passenger side head). When I was testing, the temps above 195 were not dead on correct, but it did show enough of a response to show if I was starting to overheat. Example if my gauge goes above 210 and beyond I know I have a problem. At that point I hook up my lap top and double check what the computer is reading. My truck is not set up for racing or anything else. If that was the case I would be using Auto Meter Gauges. This is just an easy way to keep stock gauge (which are not that accurate to begin with).