Low 5 volt reference

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.

Plug them in one at a time monitoring 5V ref, whichever one pulls it down is the shorted sensor.

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?

With sensor unplugged i read 233 degrees f and 5.0 volts on meter.

i do believe the temp is a substituted value. Will check OBD direct.

OBD side has it as 78 degrees F.

Short it to ground (safe to do on thermistors) and watch for reaction on scan tool.

i connected the two pins together on the ECT harness and i got 0 voltage and 288 degrees F. on the scan tool.

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?

I did not expect that result haha, Can you try shorting it battery ground?

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.

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.

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.

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.

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

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.

Is this a Partial zero emission vehicle? just trying to pull up diagram

no its not.

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.

I thought the coolant temp sensor was just fueling adjustment and nothing to do with the function of the fans?