New parts arrived - a couple of thermostats at original 92ºC and 88ºC and ECU coolant temp sensor.
Standard 54mm/2" thermostat, drilled with a 4mm bypass hole to allow circulation before it opens - I thought I'd replace this anyway while the housing was open.
Confirming the failure point
Next up comparing the temp sensors, the new one is from Emerald however took a few minutes to double check before replacing. The part number is E450 on both, I think the 4011 and 0418 on the reverse must be manufacture dates - week number and year.
The old sensor read approx 9k ohms and the new one - in a similar room temp environment only 3k ohms. They drop resistance as temp rises so that would be consistent with the old one thinking the engine was colder than reality and therefore not turning on the radiator fan.
Double checked in the ECU software - the old sensor (left) thinks the ambient temp in the garage is 4ºC below and goes up to around 70ºC max. The new showing a much more sensible number - QED - sensor failure but within range so the ECU just wasn't turning on the radiator fan.
While I was here I checked no sensor connected - at which point the ECU reports 80ºC but also turns on the radiator fan.
Reassembly
A little fiddly to get the ECU sensor in under the dash one - requires removing both of them - but relatively straightforward.
Thermostat
I'm going to try a slightly lower thermostat value to let coolant into the radiator a little earlier at 88ºC which should let the radiator smooth the top of the warmup curve before the fan has to kick in 10ºC later. Should help things when the car is stationary and hopefully doesn't affect things significantly when moving airflow is present.
(Horizontal scale here doesn't mean anything - its just representative, Yellow where coolant is admitted to the radiator, and green the fan kicking in.)
(Horizontal scale here doesn't mean anything - its just representative, Yellow where coolant is admitted to the radiator, and green the fan kicking in.)
Thermostat back in its home.
That was good - nice clear failure component identified, hopefully the new one lasts another 7 years.
Option/thought - there is a thermostat mounting point in my radiator - would just need a radiator mounted switch which grounds the lead on ECU Pin6 (or the other end of it on the fan relay).