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

[cn90]

My experience is...

In the early phase of failure, the temp gauge will sit at 3 o'clock position because most of the failure is in the "cold" setting.

For example, in the morning cold start, the ECT has a specific Ohm value for cold temp (the ECU does not "see" Ohm but it sees that in different voltage as the current passes through the ECT and affected by the resistance in Ohm). So during cold start, based on info fed from ECT, the ECU will enrich the fuel/air mixture for an easier start.

Now, if the ECT is messed up (usually shorting out at the sensor element), during cold start it sends the signal as if the engine is hot, so the fuel/air mixture is lean, this is why people with bad ECT have trouble only with cold start.

The only way to test the ECT properly is to remove the ECT from the car and heat it in a pot in the kitchen, then measure the Ohm values vs actual water temp. This is the correct technique but....

Knowing the 850, S/V70 issues with ECT, the "practical" people simply replace the ECT at the first sign of cold start problem and CEL code saying "bad ECT".

So, to answer your question, during the early phase of ECT failure, the normal operating temp is still at 3 o'clock position.

The petrol consumption question is difficult to answer b/c once the engine temp reaches 60C or so, fuel/air mixture enhancement is stopped.

[mikealder]

On a 99/00 the ECU monitors the rate of temperature change and if it the rate of change goes outside what it expects the car will throw the CEL on, run the radiator cooling fan for extended periods including when you stop the car and generate an error code of P0128.

Switching in a 1K resistor in series with the ECT will trigger the above condition, over cool the engine and waste fuel - Mike

[erikv11]

What about in parallel, that's what OP was proposing?

[mikealder]

In parallel set to 1K it will shift the ECT resistance presented to the ECU by 10 Ohms assuming the ECT is at 100 Ohms, anything less than 1K on the variable will cause a larger change in the resistance presented to the ECU - Mike

[badaitimur]

In parallel set to 1K it will shift the ECT resistance presented to the ECU by 10 Ohms assuming the ECT is at 100 Ohms, anything less than 1K on the variable will cause a larger change in the resistance presented to the ECU - Mike

i think do not connect with fix 1k resistor, because in 90deg, the ECT's resistance is about 150ohm. when its connected parallel with 1k resistace, the total resistance may reduce to under 100ohm.. so ECU detect its too hot, and 2nd speed cooling fan will on until ECT's resistance cold. But when they connected in parallel mode, the total resistance of resistor and ECT sensor may not increase significantly, maybe just 5ohm, so 2nd speed remain on..maybe your engine actually perform under the operating temperature..

better connect with 1k variable resistor..so we can vary the resistor until at certain ohm depend at cold or hot temperature..better connect with on/off switch to avoid cold start problem, so we can unconnected that the resistor anytime..

[badaitimur]

My coolant temperature meter is always at the three o'clock position.
If anything is wrong with the ECT will it still sit at the three o'clock position?
I presume if this item is faulty that it will also cause an engine to use more petrol.

better install external water temperature..more accurate..so we can see even the slightest temperature difference..

[deary5280]

I've got a 98 v70, starts fine cold or after sitting for at least an hour, but hot restarts require lots of engine cranking before it fires up. Any ideas, ECT?

[erikv11]

Those symptoms are very typical for a failing fuel pump relay. Try the jumper wire trick between the slots for pins 15 and 87, see if that fixes it.

[deary5280]

Will do. Thanks

[deary5280]

Tried it on the weekend, seems better. Thanks