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Low 5 volt reference

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4 years 9 months ago #30964 by trsvan
Low 5 volt reference was created by trsvan
Hello Paul, I have a 2004 toyota camry with a 2.4ltr engine. Symptom is coolant temp reads over 150 degrees f when vehicle is powered on and engine cold. Cooling fans come on instantly and Car also is hard to start. After several attempts i unplug the coolant temp sensor and the car starts most of the time.
I replaced the temp sensor (no help). I have been reading you book, particularly section 6 regarding 5 volt reference signal. I followed your testing prucedure and have been unable to find a fix.
On my last attempt i found that every 5 volt reference signal from the computer reads low, less than 2 volts with the sensors plugged in. When i unplug the sensors ie. MAF, ECT, TPS they all read 5.04 volts.
Not sure where to go next besides trying another computer. Any help you can offer would be greatly appreciated.

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4 years 9 months ago #30965 by jreardon
Replied by jreardon on topic Low 5 volt reference
Plug them in one at a time monitoring 5V ref, whichever one pulls it down is the shorted sensor.
The following user(s) said Thank You: John Curtis

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4 years 9 months ago #30967 by trsvan
Replied by trsvan on topic Low 5 volt reference
From what i see in the schematics the sensors do not share a 5 volt reference. I back probed the ECT harness (unplugged ) and plugged in the others one at a time as you suggested. Nothing changed, the voltage only dropped when i plugged in the ECT. I then unplugged them again and back probbed the MAF harness (unplugged) while monitoring the ref signal and plugged in the TPS first no change. then i plugged in the ECT and the voltage did not change. It only changed when i plugged in the MAF.
Should the computer drop the voltage when the system is powered on?

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4 years 9 months ago #30970 by trsvan
Replied by trsvan on topic Low 5 volt reference
With sensor unplugged i read 233 degrees f and 5.0 volts on meter.

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4 years 9 months ago #30971 by trsvan
Replied by trsvan on topic Low 5 volt reference
i do believe the temp is a substituted value. Will check OBD direct.

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4 years 9 months ago #30972 by trsvan
Replied by trsvan on topic Low 5 volt reference
OBD side has it as 78 degrees F.

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4 years 9 months ago #30973 by jreardon
Replied by jreardon on topic Low 5 volt reference
Short it to ground (safe to do on thermistors) and watch for reaction on scan tool.

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4 years 9 months ago #30976 by trsvan
Replied by trsvan on topic Low 5 volt reference
i connected the two pins together on the ECT harness and i got 0 voltage and 288 degrees F. on the scan tool.

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4 years 9 months ago #30977 by jreardon
Replied by jreardon on topic Low 5 volt reference
So let's see 288-233 = 55. You're saying you can only swing the temp sensor by 55 degrees by shorting it and opening the wire?? You've confirmed you have a good ground? Touching it on and off ground real fast and that's the only swing you're making?

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4 years 9 months ago #30978 by jreardon
Replied by jreardon on topic Low 5 volt reference
I did not expect that result haha, Can you try shorting it battery ground?

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4 years 9 months ago #30980 by jreardon
Replied by jreardon on topic Low 5 volt reference
What scan tool are you using? I can't believe it would show faulty numbers in obd2 global mode, it's an ECT we're talking about lol.

Start over and monitor the 5v ref at the TPS and only the TPS, that voltage should not change whether TPS is unplugged or plugged in (so long as key is on).

Use that 5v ref as YOUR reference point and find out which sensor is pulling down your computer's 5v ref.

I back probed the ECT harness (unplugged ) and plugged in the others one at a time as you suggested. Nothing changed, the voltage only dropped when i plugged in the ECT.

Seems like you were monitoring the 5v ref at the ECT when you plugged it in? That's normal for it to drop. What would be NOT normal would be if the TPS's 5v dropped.

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4 years 9 months ago #30982 by trsvan
Replied by trsvan on topic Low 5 volt reference
Thank you very much for your input. I am using a Snapon Modis scanner with the built in 2 channel scope. I feel its very reliable. I also checked it with two other scan tools and got the same results. I was leaning toward getting another computer, just wanted to throw the problem out there to see if anyone had any other ideas and to make sure i wasn't missing something.

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4 years 9 months ago #30983 by trsvan
Replied by trsvan on topic Low 5 volt reference
I have tried two different sensors. One was a duralast and the other was a intermotor sold by Advance auto parts stores. Both sensors were no help.

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4 years 9 months ago - 4 years 9 months ago #30984 by jreardon
Replied by jreardon on topic Low 5 volt reference
Just thinking out loud: It can't be the sensor, as you replaced it already and you tested the wires without the sensor. It can't be high resistance on the signal wire, as your scanner would drop the voltage. It can't be open, duh. What about the ground wire, you got the same reading using the jumper wire and externally grounding it straight to battery, right?

Time to check computer power and grounds. Something's messed up alright.

edit: get to computer harness and jump the ect signal wire to ground, then check power and ground while you're there.
Last edit: 4 years 9 months ago by jreardon.

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4 years 9 months ago #30985 by trsvan
Replied by trsvan on topic Low 5 volt reference
from what i see on the schematics the ECT does not share a ref signal.

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4 years 9 months ago #30986 by jreardon
Replied by jreardon on topic Low 5 volt reference

trsvan wrote: from what i see on the schematics the ECT does not share a ref signal.

Correct, the ref signal (ect signal wire) cannot be shared with other sensors. Whether or not there's more than one 5v regulator inside the computer I don't know.

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4 years 9 months ago - 4 years 9 months ago #30987 by jreardon
Replied by jreardon on topic Low 5 volt reference
Is this a Partial zero emission vehicle? just trying to pull up diagram
Last edit: 4 years 9 months ago by jreardon.

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4 years 9 months ago #31005 by trsvan
Replied by trsvan on topic Low 5 volt reference
no its not.

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4 years 9 months ago #31015 by trsvan
Replied by trsvan on topic Low 5 volt reference
Thanks for all the input guys. i went and changed the computer and all is well. When i checked the ECT on the OBD side this time it went to -40 and the cooling fans are now cycling normally. So something was definitely wrong inside the computer.
The following user(s) said Thank You: Monde

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4 years 9 months ago #31017 by Crombie
Replied by Crombie on topic Low 5 volt reference
I thought the coolant temp sensor was just fueling adjustment and nothing to do with the function of the fans?

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