Author Topic: Testing coolant sensors  (Read 127 times)

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Offline jabalv

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Testing coolant sensors
« on: October 29, 2019, 08:16:12 pm »
Hi.
I'm looking at https://vdo-webshop.nl/en/temperature-sensors/457-vdo-coolant-temperature-sender-120c-m14-4103590689788.html
It says - Contact closes with rising temperature. What does it mean? How to test if it really works?

1) I tested gauge, by plainly attaching it to ground and +, sensor and it went close to end 110degree (120degree gauge). So technically it is faulty, cause it suppose to go till end?
2) I tested some resistance of sensor and it seems decreasing when temperature grows. So it kind of works, except, I'm not 100% sure of accuracy and either gauge or sensor might be faulty. Cause 110 degree after 5-10 minutes of idle engine work was too unreal. And technically this sensor should alert on 96 degree.
 

Offline StuartA

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Re: Testing coolant sensors
« Reply #1 on: October 30, 2019, 06:50:07 am »
You could get some fixed resistors, chosen by looking at the table of T and R, and use those in place of the sensor to test the gauge. For example, a resistor of 100R should have the gauge read somewhere between 65 and 70'C. Once you know for sure if the gauge is good, you can then assess the sensor.

It looks as if the sensor is grounded through its body and one terminal feeds the gauge and the other terminal a switch which operates at the warning temperature, possibly activating a fan.
 

Online Rerouter

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Re: Testing coolant sensors
« Reply #2 on: October 30, 2019, 07:26:33 am »
The link is a Sender+switch

The case is ground, 1 of the contacts is "W" which is a warning contact that closes to ground when the temperature exceeds 90C,

The other contact "G" is a resistive output, 282 ohms should read 40C, 22 Ohm should read 120C (For a temperature gauge)

on your gauge, shorting the sender wire to ground should make it swing to 120C, and open circuit should read 40C, There is more than 1 sender calibration, but this one should match most VDO gauges

Could you take a photo of the gauge / describe what the gauge is for, I repair these things, so may know whats going wrong,

E.g. for a boat, they sometimes have a seperate sender negative wire, and if left floating makes the gauge read wrong.
 


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