SomeoneKnows
New Member
I’m trying to learn about electronics and have bought a number of books on the subject. I seem to learn better when I can apply things to real world applications. I wanted to start out with something seemingly simple. I want to tap into my Jeep's temperature sending unit where the dash gauge hooks into.
I know the temperature-sending unit is a variable resistor. I am trying to get some accurate measurements of the ohm values. I found that I get different readings from my ohmmeter if I use the grounding strap on the firewall vs. when I use the negative post on the battery. I understand now why this would happen because there is additional resistance caused by the longer path back to the battery through the vehicles body, grounding strap connections, and wires back to the battery.
I am also getting different readings from my ohmmeter when the engine is running versus off. With the temperature gauge in the dash showing somewhere around 215 degrees I am getting a reading of 65 ohms with the engine running and 158 with the engine off. The factory service manual says I should be reading 93.5 ohms at 220*F temperature. I don’t understand how to get an accurate reading regardless of whether the engine is running or not.
I would like to hook up both the ohmmeter and the factory gauge at the same time to analyze the data under normal operating conditions. I know the gauge works by comparing voltages between a constant source and the voltage through the sending unit. Maybe I need to rethink my approach. When I try getting a reading with my ohmmeter from the sending unit and hook up the dash gauge wire to the sending unit, the ohmmeter goes to an open reading. I assume this is happening because the circuit is being completed through the instrument cluster gauge. Is there a way to continue sampling data from the thermistor while the gauge is connected too? Is it possible to test the resistance within a circuit while the circuit is powered? Would putting diodes in the circuit let me take readings with both or is there a better way? I want to learn how to hook into the factory sensors without adding new sensors to do the same thing.
The books I have so far don’t seem to go into real world problems like this and it is hard to ask a book a question. I have the book “How to test almost everything electronic” but I didn’t find clues to these questions. That book seems to be geared more toward televisions than automotive applications. If anyone has a book suggestion I am interested in that too.
I know the temperature-sending unit is a variable resistor. I am trying to get some accurate measurements of the ohm values. I found that I get different readings from my ohmmeter if I use the grounding strap on the firewall vs. when I use the negative post on the battery. I understand now why this would happen because there is additional resistance caused by the longer path back to the battery through the vehicles body, grounding strap connections, and wires back to the battery.
I am also getting different readings from my ohmmeter when the engine is running versus off. With the temperature gauge in the dash showing somewhere around 215 degrees I am getting a reading of 65 ohms with the engine running and 158 with the engine off. The factory service manual says I should be reading 93.5 ohms at 220*F temperature. I don’t understand how to get an accurate reading regardless of whether the engine is running or not.
I would like to hook up both the ohmmeter and the factory gauge at the same time to analyze the data under normal operating conditions. I know the gauge works by comparing voltages between a constant source and the voltage through the sending unit. Maybe I need to rethink my approach. When I try getting a reading with my ohmmeter from the sending unit and hook up the dash gauge wire to the sending unit, the ohmmeter goes to an open reading. I assume this is happening because the circuit is being completed through the instrument cluster gauge. Is there a way to continue sampling data from the thermistor while the gauge is connected too? Is it possible to test the resistance within a circuit while the circuit is powered? Would putting diodes in the circuit let me take readings with both or is there a better way? I want to learn how to hook into the factory sensors without adding new sensors to do the same thing.
The books I have so far don’t seem to go into real world problems like this and it is hard to ask a book a question. I have the book “How to test almost everything electronic” but I didn’t find clues to these questions. That book seems to be geared more toward televisions than automotive applications. If anyone has a book suggestion I am interested in that too.