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Engine coolant temperature sensor (F)-40?

Tiny
CHASE1041
  • MEMBER
  • 1998 MAZDA B4000
  • 4.0L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 216,000 MILES
My coolant temperature is reading -40 degrees. I replaced the sensors and the PCM still no change. I have loss of power because of the low reading. Should I replace the wiring harness? Side question my A/C only comes on when I bypass my relay and it’s full of refrigerant!
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Wednesday, June 16th, 2021 AT 6:04 AM

8 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,179 POSTS
Hi,

Either the new sensor is bad or we have a wiring issue. The reason the vehicle is running poorly is that the computer is adding too much fuel so it will run at -40F and it isn't needed.

So, you could try another sensor, but if it is doing the same thing as it did with the old one, chances are that isn't the problem. So, I strongly recommend having the computer scanned for diagnostic trouble codes. Otherwise, we will be all over the place when testing the circuit.

Here is a quick video showing how it's done:

https://youtu.be/YV3TRZwer8k

If you don't have a scanner, most parts stores will do it free of charge.

If you look below, I attached the diagnostics for the coolant temp sensor. Take a look through them. The issue is, they are tied to codes which makes it a very extensive check without knowing the code.

Also, here is a link you may find helpful:

https://www.2carpros.com/articles/how-to-check-wiring

Let me know if this helps or if you have other questions.

Take care,

Joe

See pics below.

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Wednesday, June 16th, 2021 AT 9:38 PM
Tiny
CHASE1041
  • MEMBER
  • 35 POSTS
I have disconnected the sensor and tested the ohms when the sensor was cold it went up when the sensor was hot it went down. I think the wiring is bad, it looks like it’s been twisted so many times. I could replace the harness for now see if that works but I’m going to follow the wiring and see if it’s grounded anywhere. Because on the other hand the A/C only works when it is bypass it. But I’ll work on the sensor first cause that could affect A/C?
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Thursday, June 17th, 2021 AT 4:48 AM
Tiny
CHASE1041
  • MEMBER
  • 35 POSTS
I have two sensors one is flat tip the other cone shaped, which one controls the ECT?
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Thursday, June 17th, 2021 AT 6:23 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,179 POSTS
Hi,

It should be behind the thermostat housing. See pic 1 below.

As far as voltage, the sensor, based on temperature, produces resistance. Basically, the electrical resistance decreases when the temperature increases, and the opposite. So what I need to know is what the resistance in the sensor. Set your multimeter for ohms. Check the resistance when it's cold and then recheck it when the engine is at operating temperature. There should be a difference. If there isn't, the sensor is bad.

Let me know what you find or if you have other questions.

Take care,

Joe

See pic below.
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Thursday, June 17th, 2021 AT 8:53 PM
Tiny
CHASE1041
  • MEMBER
  • 35 POSTS
I fixed it by re-wiring the cable to the sensor.
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Tuesday, June 22nd, 2021 AT 4:31 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,179 POSTS
Hi,

I'm glad to hear you got it fixed. Please feel free to come back anytime in the future if you have questions.

Take care of yourself.

Joe
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Tuesday, June 22nd, 2021 AT 7:36 PM
Tiny
CHASE1041
  • MEMBER
  • 35 POSTS
Okay, I have a new problem. I switched my truck into 4x4 Low and when I turned it off now the O/D Light is flashing. It did this once and it finally jerked out and started to work. But it is not wanting to go past 3rd gear when that light is flashing:
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Saturday, July 31st, 2021 AT 10:53 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,179 POSTS
Hi,

I need to ask a favor. Could you copy and paste your new question to a new thread/post? We try to keep the threads specific to one topic so it's more helpful to others.

Here is the link to the new post:

https://www.2carpros.com/questions/new

I hope you understand. I will keep my eyes open for it.

Take care,

Joe
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Saturday, July 31st, 2021 AT 8:06 PM

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