Temp sensor resistance decrease? - Page 1
Forum section: electronics / beginners

author: syphear
author_rank: Newbie
timestamp: June 19, 2013, 03:54:18 pm
content: Hi all,My motorcycle came from the factory with a inactive temperature gauge, I would like it to work.It is digital and located on the bikes main LCD.It cycles (flashes on) when you turn on the ignition then disappears.This is normal for my bike, they all do it. The gauge is for the water temp on a 650cc bike that shares the same LCD display. My bike has a 250cc oil cooled engine.My bike has a temp sensor that goes straight to the ECU and not the temp gauge. its output is 28K Ohm at 0 degC the water cooledbike is 5.7KOhm.The gauge will be expecting the resistance range for the 650cc water cooled engine but my range is higher resistance.I can't change the temp sensor as my ECU is using the resistance range beginning at 28KIs there a way of shifting a sensors output decreasing the resistance?rough table: temp I have I want0 28.7K 5.7K20 12.2K 2.4K40 5.7K 1.15K60 2.8K 0.58KThe two outputs decrease at roughly the same percentage each step so this should be possible I hope.Any help would be great, thanks!

author: Fraser
author_rank: Super Contributor
timestamp: June 19, 2013, 04:02:41 pm
content: You need to be careful here. Is the LCD controller supposed to be placed in parallel with the ECU input from the NTC thermistor ? If not you could upset the ECU. Resistance is measured by applying a voltage across it and/or a current through it. I would normally expect to see separate oil and water temperature sensors on a modern 4 stroke water cooled engine. I am not aware of a method of changing the temperature / resistance curve that would not also effect the ECU input. A better solution would be to fit an additional NTC thermistor of the correct specification that serves only the LCD display. That is, of course, providing that the LCD controller does not need to be configured to show temperature via a change to firmware settings. In a car I would modify the sump oil drain plug to provide an easy installation for the thermistor. Not the most accurate location for oil temperature but acceptable.

author: syphear
author_rank: Newbie
timestamp: June 19, 2013, 04:12:02 pm
content: Looking at the wiring diagram for the 650cc bike the temp gauge and ECU wiring are in parallel. The 650 has a different ECU to mine and obviously is designed to work with the different sensor output.I did plan on using a correct spec thermistor but I wanted to know if there was a way to morph the output of the one that's already mounted in the perfect spot in the engine first. I planned on leaving the wire from the sensor > ECU intact and taking another from thesensor manipulating it and then feeding it to the temp gauge.

author: SeanB
author_rank: Super Contributor
timestamp: June 19, 2013, 06:49:51 pm
content: As you have an ECU it most likely is reading the temperature from the ECU and displaying it on the display, thus you need to go into the ECU and tell it ( a setting bit in the ECU or the display) to show the temperature on the display. The sensors and wiring are already there, just it is a disable feature.

author: syphear
author_rank: Newbie
timestamp: June 20, 2013, 03:18:38 am
content: The temp gauge is completely stand alone and separate from the ECU. It just has a single wire input.The single wire in question is not connected to anything. All I want to know is how to take the output resistance from the temp sensor I have, make some kind of amp circuit, and feed the correct range to the gauge input.

author: SeanB
author_rank: Super Contributor
timestamp: June 20, 2013, 04:16:09 pm
content: In that case you just have to buy the other sensor and place it on the engine block, and connect it to there. It is hard to have both done by the sensor when both are driving them in different methods.