DIY: 1998 V70 ECT (Engine Coolant Temp) Sensor Replacement

[cn90]

DIY: 1998 V70 ECT (Engine Coolant Temp) Sensor Replacement

Bad ECT (Engine Coolant Temp) Sensor Symptoms:

https://www.matthewsvolvosite.com/forums ... hp?t=11429

https://www.matthewsvolvosite.com/forums ... =1&t=30880

Most Engine Coolant Sensor works by NTC principle: as temp increases, the resistance (measure in Ohm) decreased:
For example, I measured the brand-new Volvo ECT and got these values
* -20C: 16,000 Ohms (16 kOhms)
* 0C: 7,300 Ohms
* 20C: 2,600 Ohms
* 80C: 300 Ohms
* 90C (Operating Temp): 200 Ohms
* 100C: 150 Ohms

So, when it is cold, the ECU injects extra fuel to start the engine. The problem is, when the ECT fails, it usually fails in “HOT” position (short-circuit inside the sensor), I have these 2 symptoms:
* Aux fan runs on its own for about 6 minutes or so.
* Hard start when COLD (ECU thinks it is hot and no need to inject extra fuel when it is COLD).

So, if you got stuck in the middle of nowhere with this problem and need to go home, you can use a resistor that simulates COLD engine (such as 20C with 2,600 Ohm or about 2 KOhms), to start the engine. Once warmed up, you can re-connect to the defective sensor to drive home.

VolvoECT01.JPG (120.08 KiB) Viewed 21041 times

Parts:
* Volvo PN 9186486, I got it for $40 + shipping from Waltrip Volvo (Franklin TN). Made by SWF (Germany).
* Ebay has same SWF for $39.

Tools:
* Coolant
* T40 Key and 19-mm Wrench
* Some random resistors in case you want to play with the ECU.

VolvoECT03.JPG (108.4 KiB) Viewed 21041 times

Procedure:

* If you have not replaced the tstat, then consider replacing it. I replace my tstat every 70K.
https://www.matthewsvolvosite.com/forums ... =1&t=50745

* With long T-40 Torx key, remove the bolts and push the tstat housing aside to expose the ECT.

* Use a screwdriver to push the connector outward. The connector is very difficult to pry apart. I broke the plastic tab, so I used Zip Tie, no big deal.

* I use a pair of cutting pliers to cut the old ECT plastic sheathing to remove it (it was bonded to the harness very tight!). Look at the new ECT so you know where to cut on the old ECT.

* Note that I already disconnected the Aux Fan (because it was running!).

VolvoECT04.JPG (118.46 KiB) Viewed 21041 times

VolvoECT02.JPG (120.59 KiB) Viewed 21041 times

* Use 19-mm wrench to remove the old ECT sensor. When installing new ECT sensor, don’t forget to remove the old Aluminum washer! Also don’t forget to use the new Aluminum washer.

* Due to tight space, turn the 19-mm wrench sideway to tighten the ECT. Just snug it then tighten it. Don’t over-torque it.

VolvoECT05.JPG (138.28 KiB) Viewed 21041 times

* Now re-install the tstat housing.

* Refill reservoir with coolant to Max, start engine, drive around blocks and check coolant level later. You will need to add a bit more coolant later.

* That's all folks!

[matthew1]

CN, super tutorial. I haven't had to do this on my 850, but when I do, I'm coming to this topic! Going in the Repair Database.

[erikv11]

Great tutorial!

That Prestone is not HOAT, I wouldn't use it in my Volvo.

[jreed]

Thanks for this informative write-up! I like it!!
I particularly admire the sideways use of the open end of the 19mm wrench, with presumably a screwdriver inserted through the closed end to give you leverage for tightening up the ECT.

[db130]

The procedure for 99-00 Cars is similar but the part numbers are different:

9125463 for the Coolant temp sensor MSRP $59.98 - and it does NOT come with the crush washer
11994 crush washer MSRP $1.27

[matthew1]

Thanks DB!

[JeffersV70]

Excellent write up, many thanks. Been having hard start problems, and after checking my starter, battery, alternator and grounds it turns out that all it was was this little sensor. Being close to the holidays I didn't put down the extra money for the thermostat and gasket, but wish I had. Not removing the thermostat housing makes changing this sensor 10x more aggravating than it needed to be.

[siboyslim]

Would Just like to say a big 'Thankyou' for this guide, I have just successfully diagnosed a coolant sensor fault, it was reading 53.5 with the meter set at 200k, with I take it means 53,500 Ohms? This was while the engine was warm. Have just ordered a new sensor from my local parts supplier. Thanks!



Simon (UK)

[badaitimur]

Helo, i'm Badai from Malaysia. Currently using Volvo S70 2.0T year 2000. This is rare model in US or EURO because i think this model for Italy's market, due to high price for road tax policy, its has same issue with Malaysia.

So i want to ask about the engine coolant temperature (ECT) sensor. according from this forum state the ECT will trigger cooling fan on/off.

I have OBD2 tools (android application), i have monitored and its seems the fan just on when temperature reach at 105C and off at 95C. I dont know its normal or not. But Malaysia's weather is too hot to accept that cooling fan trigger on at 105C. Its boiling level..!

so, may i know if i can change the ECT sensor that will trigger ECU to on cooling fan about 85C-90C? i don't interest to play with the ECU by change the ECT's resistance.

or, if we have another solution to makes ECU trigger the cooling fan more early., about 90C.
tqvm.

[erikv11]

I don;t know of a way to do what you ask about.

However, consider that for a 50-50 mix of coolant and water, the boiling point is indeed about 105 C. But the cooling system is under pressure, and that additional 10-15 psi will raise the boiling point another 20 degrees or more. So the coolant never gets even close to boiling over as long as the system is kept under pressure, as it is designed to do.

To make the car run a little cooler, you could use a thermostat rated to open at a lower temperature, that would help.