multimeter question

bill b va

Member
in trying to test a tempature sensor in a vehicle (thermosistor i guess) with my digital meter in the 200 range i get no reading . with my analog meter in RX 1 range i get 4 ohms ........... so i test both meters with a 1 ohm wire wound resistor and get reading in both . is there a reason the digital meter don't work on the sensor ? i had originally bought the digital meter to test motor winding and it works good for that....thanks bill
 
Your analog meter probably uses a higher voltage and current in the R X 1 scale than the digital meter. Your sensor could be a silicon junction type sensor that requires a higher voltage to create conduction. Bi-metallic thermocouples also generate a voltage that might counter your digital meter. It is common that digital meters do not work on temperature sensors. Still 4 ohms seems low unless it is a temperature sensitive diode. What is this sensor used for?
 
Sounds to me like you have some sort of semiconductor device rather than a thermistor. Try putting your DMM on the "diode check" scale (the setting with a diode symbol). Check for continuity in both directions; if the device is open in one direction and has continuity in the other, you have a semiconductor device.

Analog ohmmeters use relatively high voltage (typically a volt or more) to measure resistance, so they're able to forward-bias a diode. (It takes about 0.7 volts to forward-bias a semiconductor junction.) Digital multimeters operate at lower voltages on their ohms scales and can't bias the junction, so diodes look like open circuits. For that reason, most DMMs have a diode check scale that has enough voltage to forward-bias a semiconductor junction. Note that the reading on the diode check scale is in VOLTS, not ohms, and will typically be about 0.7 for a forward-biased diode.
 
it is the coolant tempature sensor in a jeep cherokee . the vehicle dies in idle when cold . one of sensors that could possibly cause the problem .
 
I did a google search for checking a Jeep Cherokee temp sensor and a digital meter should work. It shows a resistance of 11370-13610 ohms at 68 deg F. I would say your sensor is defective.
Click site below
Check Jeep Temp Sensor
 
My cheap, Free HF digital meter checks thermosistors just fine. The thermosister on my Kawasaki mule is in the 2.5k range at 68 degrees. More when the temp drops and less when its warmer. I had to trick my computer to make it idle right. I replaced the thermistor with a 1600 ohm fixed resister and it ran perfect. Then I made a thermistor to give me 1600 ohms at 68 degrees. So far so good.
 
Here is a thought.....
If the coolant temperature sensor has failed, it should set a trouble code. Most likely an "out of range" or "out of expected range" code. Normally, these sensors are backed up by secondary sensors. Your Jeep should also have an air temperature sensor. The CPU correlates the two sensors, and they should maintain some sort of relationship that is programmed into the computer. When a sensor gives a reading outside of expected values, a trouble code is set. Also, if a sensor is telling the computer that the engine has reached operating temperature, the computer may go into "closed loop" operation, which would be too lean for a cold engine.

Another thought is this - has the throttle body been cleaned? This could present a problem as you have described. Most modern throttle bodies are designed to never need cleaning. This is accomplished by sealing the outside diameter of the throttle plate(s) and using a calibrated orifice toward the center of the throttle plate. Cleaning one of the "do not clean" throttle bodies will damage it to the point that it will have to be replaced to get the engine to run or idle properly.

Yet another idea is if you have a mass air flow sensor. This is the MAIN mixture control feedback device. If it gets out of calibration for whatever reason, it will cause drivability concerns. This sensor can be cleaned.

Just a few thoughts that came to mind when I read through this thread.
 
(quoted from post at 06:11:34 11/06/15) My cheap, Free HF digital meter checks thermosistors just fine..

Buy quality and you wont have to ever buy another or question its readings.
I was at my Dad's working out a wiring problem and getting erratic results with his cheap HF test light, and discovered it only worked when you held the wire just right. He said, "yeah, thats how it works". You cant effectively work with crap like that, so I loudly announced "I'll fix that for you", and set it on the anvil and crushed it with a bay sledge. I thought he was going to have a heart attack, and his two buddies were rolling on the floor, making comments that sounded sorta like "that'll show you, ya cheap SoB.".
I bought him a good one at lunch time.
I bring my own meter, his 3 HF multi meters look like toys.
 

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