Testing Coolant (IAT/ETC?) sensor

Post by **scot850** » 22 Aug 2016, 15:14

I have separated the coolant sensor connector by the back of the power steering pump)

: Coolant temperature sensor connector

I have read the thread below stating the different resistances at different temperatures. I am not confident in electrical testing, so how do you test the resistance of the sensor? Do you just probe across the 2 pins with a meter set to resistance? Does the connector have to be connected together or the engine running to do this test?

Appreciate this is a basic question but like I said I am a mecheng not an electeng!

Thanks,

Neil.

Post by **tryingbe** » 22 Aug 2016, 15:42

V70R?

You be able to get the coolant temperature by using a OBDII scanner.

Post by **scot850** » 22 Aug 2016, 16:12

Sorry should have been more specific. This is on the 97 850 NA, but technically the same question would still be appropriate if you don't have a scanner for any P80 I guess.

I re-checked with my electrical tester and realised I had not got it set high enough for the resistance levels I needed to be checking for. I tried it on 2 sensors, the one in the car in the coolant, and a spare one I removed from the V70R when I first bought it as I thought it was faulty as the needle was not registering at 3 o'clock. Brand newVolvo IAT/ETC (?) sensor made no difference, so I am assuming the old sensor should be a reasonable cross reference. The only concern isthat I believe there is a different sensor used up to 1998 and 99-00 used a different part number for some reason. Strange thing I my Vida copy shows the same part number for all, but when you chec the actual vehicles and years it lists different part #'s.

Anyway, I have just placed a thermometer in the garage to check the temp out there, but both were tested out there. I have also just checked with my hand held digital thermometer. It read at 65F at the thermostat housing and at 67.5 at the tip ofthe non-fitted sensor

1) old sensor from 2000 V 70R resistance: 2,400 Ohms (65F = 18C approx.)

2) Sensor in 850 in coolant resistance : 3,000 Ohms (67.5F = 20C approx.)

Not sure if this test is relevant or not.

Either way, does this sensor control the cooling fan operation through the ECU? The sensor in the 850 reads at 3 o'clock when the car is warm, so is this a reasonable assumption it is reading correctly?

Thanks,

Neil.

beigg · Post by **beigg** » 22 Aug 2016, 16:14

Reading should be done with meter set to ohms (resistance) and display a "+" positive readout number and if it has the dash pre-number, reverse connections and be sure to wire appropriately to the pigtail and not mix or cross the negative/minus/"-". Car will run like poop if it'll start at all before fouling the plugs should the wires be crossed incorrectly.

Post by **scot850** » 22 Aug 2016, 16:32

Went back and re-checked as I wasn't sure I got the readings the right way around.

The ambient temp in the engine area was registering at 18.4C (65.1F) on a digital sensor placed there.

Using the hand held digital laser sensor, I measured the temp of the thermostat housing in the 850 engine bay in the garage and it was 65F so the same. Re-checked the resistance and it measured at 3000 Ohms this time. Based on a chart I found it states it should read 2,400 Ohms at 20C (68F) and at 0C (32F) it should be at 7200. So not knowing if the resistance is linear, I would think this would look about right. Is this a fair assumption? If so it would suggest the sensor is ok?

Neil.

Post by **abscate** » 22 Aug 2016, 19:15

Those readings look ok. They are documented here in the P80 forum. I think typical failures are the curve goes non linear which is much harder to diagnose. The ECT and resistance changes in the 1999 model, of course

Post by **erikv11** » 22 Aug 2016, 20:48

Everything you mention points to the sensor being fine but if you want to test it you need to pull the sensor and make up a little curve with the resistance at different temps.

Post by **scot850** » 22 Aug 2016, 22:59

I'm fairly confident it is correct. As there are only 2 wires going to the ECT all they can be measuring surely is a resistance change as temperature changes so all it is, is a thermocouple. This sends a signal to the engine ECU to determine fuel settings and that in turn must send a signal to the fan relay to determine if the fan needs to run at what speed. Have to believe if the temperature on the gauge is where we expect it to be, then the same signal goes to the ECU. So I am going with it is ok for now. Will now have to check the relay operation and then if that is ok then I'm assuming the electrical side of things is ok. The onto looking for leaks, or hope it is just another burp!

Thanks again,

Neil.

Post by **MrAl** » 23 Aug 2016, 05:04

Hi,

Is that a resistance type sensor, and it is the same on the 98 v70 ?

Cees Klumper · Post by **Cees Klumper** » 23 Aug 2016, 06:05

If based on the above we assume that the sensor is ok, then the needle not sitting at 3 o'clock could mean the thermostat is faulty? At least that's what happened in my S80. Engine would not reach proper operating temp. Replaced themostat (ahich BTW came with its housing and a new sender) and all was well again.