Coolant temperature sensor, it have a purple top with 2 prong, [car wont start]

Tiny
ATKINS
  • MEMBER
  • 1986 BMW 528E
  • 2.7L
  • 6 CYL
  • 2WD
  • MANUAL
1986 bmw 528e 2.7, this is the sensor that run the ecu. Iam getting no ohms reading when I check the sensor, should it show some type of resistance, so the ecu can read
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Friday, February 6th, 2015 AT 2:40 PM

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Tiny
CARADIODOC
  • EXPERT
Are you measuring at the sensor or from the computer's connector? The typical coolant temperature sensor, as I recall, will measure somewhere between a few thousand ohms to perhaps as much as 100,000 ohms.
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Friday, February 6th, 2015 AT 2:57 PM
Tiny
WRENCHTECH
  • EXPERT
This may help you.

These are a couple of charts that very slightly but will get you close.
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Friday, February 6th, 2015 AT 3:20 PM
Tiny
WRENCHTECH
  • EXPERT
Redo
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Friday, February 6th, 2015 AT 3:23 PM
Tiny
ATKINS
  • MEMBER
Measuring the sensor it self, unplug
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Friday, February 6th, 2015 AT 4:28 PM
Tiny
CARADIODOC
  • EXPERT
Thanks Wrenchtech.

The thing to understand is it is extremely uncommon for a two-wire temperature sensor to read a wrong value because there's just one component in it. When they do read too high, it's more likely to be due to severe corrosion on the terminals.

An instructor I used to work with forced his kids to memorize these resistance values, but that is a big waste of time. Not every sensor will measure exactly the same resistance at a given temperature. Their individual characteristics is part of what Engine Computers learn by comparing various sensor readings and operating conditions. What you need to look for is if you have something or an open circuit. Even an open circuit isn't common. If you DO get a resistance reading, you can compare it to the charts, but don't get excited if it isn't exactly what is listed.

A better test is to plug the sensor in, then back-probe through the rubber seal next to the signal wire, and measure the voltage. On all the cars I'm familiar with, the voltage will start near 4.2 to 4.5 volts, and go down as the engine warms up. It may get to around 2.0 to 2.5 volts well before full operating temperature is reached, then it may pop back up to, as I recall, around 3.5 volts, then go slowly down again.

That transition back to a higher voltage is due to the computer kicking in a different internal resistor to cause the sensor's reading to have more accuracy. As you can see, I know the theory better than the actual operation because we only test these circuits after the computer has already detected a problem, set a fault code, and we're just verifying it or looking for the cause.
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Friday, February 6th, 2015 AT 8:18 PM
Tiny
ATKINS
  • MEMBER
Coolant temperature sensor only, by itself
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Saturday, February 7th, 2015 AT 12:37 PM
Tiny
CARADIODOC
  • EXPERT
And by "no ohms reading", you mean you're getting an infinite reading on the meter's highest scale? You don't mean a reading of 0 ohms, do you? If you're reading infinite right on the sensor's terminals, I would have to agree that it's open and needs to be replaced. The proof would be by back-probing the wires when they're plugged in, you'll find approximately 0.2 volts on the ground wire and 5.0 volts on the signal wire instead of something between 0.5 and 4.5volts.
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Sunday, February 8th, 2015 AT 3:37 PM
Tiny
ATKINS
  • MEMBER
Thank you, that what I wanted to know, iam getting a infinity reading on my ohms meter at the sensor, when I unplug, to test it, instead of a resisitance reading
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Monday, February 9th, 2015 AT 3:46 AM
Tiny
JOHNNY G.JR
  • MEMBER
Bad sensor
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Sunday, January 1st, 2017 AT 6:20 AM
Tiny
WRENCHTECH
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The previous responder is not a registered expert at this site.

Since this question hasn't seen activity in 2 years, the problem is likely resolved already and shouldn't have been answered anyway but before condemning the sensor, lets make sure you actually have either a self ranging meter or have it set to the correct range for the reading you are expecting.
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Sunday, January 1st, 2017 AT 6:52 AM

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