Why *two* engine temperature sensors? - Page 1
Forum section: electronics / general

author: Circlotron
author_rank: Super Contributor
timestamp: September 12, 2024, 12:12:21 am
content: I drive one of the most exciting cars in the world, a white 1999 Toyota Camry. With the 5SFE 4 cyl donk no less. But the point is this - it has two coolant temperature sensors. One goes off to the ECU and tells it to do things like turn the radiator fans on and off, fiddle the spark table, go into limp mode if way over temp, and probably several other things. The other temperature sensor, as far as I can tell, only goes to the dashboard temperature gauge.The point is this - a while back the temperature sensor that feeds the ECU went intermittently bad. It would go open circuit and the ECU interpreted this as minus 40 deg C and promptly injected far too much fuel and the engine would stall and often refuse to start up again. The dashboard temperature gauge just showed things as normal. If they had both been fed from a single sensor then the gauge would have been going nuts and straight away given a clue as to what is happening.So... what would be the thinking behind having two separate sensors?

author: langwadt
author_rank: Super Contributor
timestamp: September 12, 2024, 12:36:20 am
content: Quote from: Circlotron on September 12, 2024, 12:12:21 ammost likely that the ECU and dash they pulled of the shelf from other cars they make/made each needed a sensor so instead of changing things and doing all the test and approval they just added another sensor.I've heard many stories of all kinds of nonsensical wring and connections made in cars just because it wasn't all build from scratch, then as time goes by and new versions of the car come out things slowly change

author: Andy Chee
author_rank: Super Contributor
timestamp: September 12, 2024, 01:55:30 am
content: The two temperature sensors is a consequence of having two coolant loops, one of which is bypassed when the mechanical thermostat goes into bypass mode.

author: Circlotron
author_rank: Super Contributor
timestamp: September 12, 2024, 02:35:33 am
content: Quote from: Andy Chee on September 12, 2024, 01:55:30 amWhich begs the question - why not have just a single sensor in the non-bypassed loop? What is the value of knowing the coolant temp in the bypassed loop if it is shut out of the game during the warmup cycle for example? Genuine question. Not picking fights.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 12, 2024, 02:53:50 am
content: Quote from: Circlotron on September 12, 2024, 02:35:33 amThe ECU requires engine block temperature for fuel mixture purposes.The radiator fan requires radiator outlet temperature (at the bottom of the radiator header tank) for turning the fan on and off.These two tasks require temperature to be sensed from different locations in the cooling system.

author: Circlotron
author_rank: Super Contributor
timestamp: September 12, 2024, 03:24:01 am
content: Well, seeing engine block temperature is the reason the radiator exists in the first place, why wouldn't it's temperature be valid for radiator fan control?

author: Andy Chee
author_rank: Super Contributor
timestamp: September 12, 2024, 03:39:40 am
content: Radiator fan on-off control requires a coolant temperature hysteresis swing.The engine block temperature is regulated and does not swing (certainly not enough for radiator fan control).Hence the temp sensor located at the radiator outlet at the bottom of the header tank.

author: magic
author_rank: Super Contributor
timestamp: September 12, 2024, 05:09:27 am
content: Quote from: Circlotron on September 12, 2024, 12:12:21 amSeen that too on a different car.ECU diagnostics should catch this condition. Or did in my case, at any rate.

author: Circlotron
author_rank: Super Contributor
timestamp: September 12, 2024, 05:30:49 am
content: Quote from: Andy Chee on September 12, 2024, 03:39:40 amOn my car the OBD2 data says the fans come on at 100 deg C and goes off at something less that 95 deg C. Can't remember off temp exactly. Just now I read that there is a fan switch at the bottom of the radiator, so that makes three. Here is a pic of the ECU and the gauge sensor side by side where the coolant exits the engine to the top radiator hose, so they both see the same temperature. ECU sensor is green 2-wire and gauge is white single-wire. Not my car but same setup .

author: Circlotron
author_rank: Super Contributor
timestamp: September 12, 2024, 05:34:17 am
content: Quote from: magic on September 12, 2024, 05:09:27 amYes, that's what I did eventually. Temperature graph would drop dramatically. Took a little while to realise why this would cause the engine to stall, but after the penny dropped it was clear.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 12, 2024, 05:43:21 am
content: Many cars also have a sensor for the coolant (water-) temperature and another one for the oil temperature.It´s possible that the dashboard display is for the oil temperature because it gives you the best indication when the engine is ready for hard use. The ECU probably needs the coolant temperature to control the radiator fan.

author: JPortici
author_rank: Super Contributor
timestamp: September 12, 2024, 05:52:45 am
content: Quote from: Circlotron on September 12, 2024, 12:12:21 amToyotas are weird. Some actions (speed, limiters, on/off) are still being done by wires between ECUs, if you know where to look at you can do all sort of stuff. And you know where to look at because they keep using the same pinouts/connectors from the 90s.Wouldn't surprise me you have two sensors not because of redundancy, but because at one time long ago a team did a car with no ECU but with a dashboard, then another team did a car with ECU but no electronic dashboard and the two were reused in a later model and the achitecture stood like this ever since.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 12, 2024, 06:05:19 am
content: Quote from: Circlotron on September 12, 2024, 05:30:49 amOkay that second engine block sensor I can't explain completely. From the beginning of the discussion, I thought you were just talking about two sensors; radiator & engine block.