Why are there two coolant temp sensors on many vehicles?

ford
f150

#1

Okay, as I’ve previously mentioned, my 1994 F150 had engine issues, and I’ve replaced the engine with a rebuilt unit. In the process of swapping over the sensors, I noticed that there are TWO coolant temp sensors: one by the thermostat, that sends a signal to the dash gauge, and one on the rear of the block, that sends data to the ECU.

This seems odd; that Ford would (seemingly) needlessly spend excess cash installing two sensors, when one would do, sending data to two places. Leading me to believe it isn’t really needless: there must be some advantage to such a setup, or they wouldn’t do it. Further augmenting this belief is the fact that another car I’ve owned (1998 Contour) had just such a setup.

So…why is it like this? What is the advantage?


#2

I’m no electrical engineer, but it seems to me that a malfunction in either the gauge itself, or the ECM itself, could feed back through the sensor and affect the operation of the other unit that’s connected to that sensor. That could compound the problem, leading to bad ECM function or an inaccurate temp gauge.


#3

Ford doesn’t do that anymore. Apparently in 1994 the dash gauges did not communicate with the ECU via CAN so they needed one for the gauges and one for the engine control.

An explanation of what CAN is;

Later Fords have “smart” dashboards with CAN so they can share sensors.


#4

That’s technology fer ya. In 1994 the computers weren’t “smart” enough. Nowadays you will NOT find two coolant sensors as the gauge is driven by the PCM, or maybe the BCM. That 23 year old computer is archaic compared to even ones from 10-15 years ago. It’s like the difference between the old Atari and Playstation4.


#5

Yeah GM did that too. You had to be very careful with the description to get the right one.


#6

Just looked up my '13 Mustang in the service manual. I have NO coolant temp sensor just cylinder head temp sensors. Not in the water jacket. My coolant temp is “calculated” from available cylinder head data and displayed. Huh!


#7

I understand that most gauges now just display a computer-generated output and are programmed to be steady unless way out of wack. Basically, a gauge version of the idiot light.


#8

Yes, Ford oil pressure “gauges” are a perfect example of what you just mentioned

As long as oil pressure is within normal parameters, the needle will ALWAYS be in the middle, to make the driver happy


#9

With all the useless features they are putting in cars now, like push button start, why are oil level gauges not standard. Can you think of how many engines could be saved? I also want a dipstick to confirm the gauge and bring back the transmission dipstick.


#10

I always had oil level sensors on my Rivieras in addition to an oil pressure switch that would shut the fuel pump off. Seems like a reasonable thing. On my G6, the schematic in the Chiltons actually shows an oil level sensor but I have found nothing in the manual or parts listings that talks about it. Sometime I’ll crawl under the thing this summer and take a look at the pan.


#11

My early 90’s Corolla, same thing. One sensor for the computer, one for the dash gauge. There’s merit to that configuration for the diy’er btw. If the dash gauge doesn’t work, it’s either the gauge or the sender. You don’t have to involve the computer and all its connectors into the diagnosis.

On my Corolla there’s even a third sensor that screws into the coolant jacket, used to control a separate cold start injector.


#12

George, you’re the only guy that consistently refers to his “early 90’s Corolla”

Why don’t you refer to your 1991 or 1992 Corolla, for example . . . ?

I could understand you referring to your early 90s Corolla, if it was a car you no longer owned, but you DO still own it, so I’m pretty sure you know what model year it is

I think you refer to it in that vague way, because you want to seem mysterious :space_invader:


#13

Many of those old systems had two “sensors”. One was a sender and that provided an analog output relative to temperature to drive a gauge that was completely independent of the engine controls computer. The other was a switch that provided a trip point into the ECM when the temperature exceeded a certain threshold and was used to indicate overheat condition. The ECM did not need to know exact temp, just that a threshold had been exceeded and this can be done with a simple logic input versus conditioning an analog input…

Now most are combined. The ECM takes in an analog voltage and drives the gauge as well as using it for fault logic…


#14

Now why would a company that is in the business of selling new cars possibly want to exclude a feature that would keep negligent owners from destroying their car and having to buy a new one? :wink:


#15

I was disappointed to find out in my 03 trailbazer with 180k, thinking man my oil pressure gauge is still like new to me numbers 90k ago, it is a calculated reading, how one calculates it I do not know, but there is an idiot light for actual low oil pressure.