My coolant temp gauge on the dash is not working at all and I'm having trouble figuring out exactly why. Should be simple, but I think my brain is fried from everything else I've got going on.

Ok guys, let me bring you up to speed on where I am at diagnosing this thing.
I've checked the plug -- verified with the DVM that ground and power wires are correct. Put a DVM to both pins, it's showing 12 volts.
I ohmed the sensor -- it's got absolutely no resistance.
Got a brand new sensor -- put it in some boiling hot water, hooked up the plug, still no reading on the dash. Ohmed the new sensor and it's completely open too.
Took a 470 Ohm, 5W resistor and bridged the two pins on the plug -- dash gauge read near normal.

Now, after doing all that, I'm thinking the wiring is fine since the dash gauge moved with the resistor bridged across the pins. What I don't know is, what's the correct voltage to the sensor? I was thinking it should be 5V, not 12V.

Also, I'm wondering what the resistance range should be on the sensor? It seems to me that it shouldn't be completely open, but my old one, which was working fine in the 7.3, and the brand new one I got today both showed open. I'm guessing it's possible that the new sensor is bad out of the box.

Any thoughts here, or anyone know the correct voltage to the sensor and resistance through the sensor?

 

This might be helpful. I found it in the 2000 workshop manual. Its a test for the thermostat, but lists the approximate voltages for different temps.

Thermostat—Electrical Test

CAUTION: Always vent the exhaust to the outside when performing this test.

NOTE: The electrical thermostat test is most accurate if performed indoors at less than 37.8°C (100°F) ambient air. This test may be performed with or without the hood open and with the engine warm or cold.

Check the engine coolant level. Fill as needed.
With the ignition OFF, remove the engine coolant temperature (ECT) sensor harness connector and attach ECT Sensor "T" Cable as a jumper between the powertrain control module (PCM) (12A650) and the ECT Sensor. Attach the 73 Digital Multimeter to the ECT Sensor "T" Cable. Voltage values (0-5 V) may now be monitored while the sensor retains its connection to the wiring harness.
A scan tool or the Service Bay Diagnostic System (SBDS) may be used to monitor the ECT on vehicles equipped with data link connector (DLC). The SBDS sequence to use for the screen is: Toolbox-Electronic Engine Control and DCL-Item.
NOTE: Running this test with the vehicle in gear or with the A/C compressor clutch engaged (running) will cause incorrect diagnosis.

Place the transmission in PARK (P) or NEUTRAL (N).
Start the engine and allow the engine to idle throughout this test. Allow the engine to run for two minutes, then record the ECT voltage. Record the ECT voltage every 60 seconds. When the ECT voltage trend changes direction or only changes slightly (0.03 volt or less) from the previous reading, record this as the thermostat opening voltage. Use the voltage and corresponding coolant temperature chart listed below.

Coolant Temperature °C (°F) ECT (Volts)
22 (71) 3.00
43 (109) 2.01
71 (159) 1.01
82 (180) 0.75
91 (195) 0.059
97 (206) 0.050
105 (221) 0.040


If the thermostat opening voltage is greater than 0.75 volt and less than 88°C (190°F) on 5.4L engines or 82°C (180°F) on 6.8L engines, install a new water thermostat.
If the thermostat opening voltage is less than 0.75 volt and greater than 82°C (180°F), the water thermostat is good and should not be replaced. Refer to the Symptom Chart for further instructions.


linky to manual page

 

subscribing.... I am playing the same game with my EOT sensor, i should be able to T into the wire and get my (new)Water temp guage to read my oil temp(one a 2 pole toggle switch of course) but I am thinkng my guage does not like the reading I am getting from the EOT for some reason.

 

Jeremy I can grab a flashlight and my DMM and check my truck if you need.
Maybe the resistor is built into the guage circuit somehow

 

what happens if you heat it up? does resistance grow? Its possible they send 12V to it and the increasing resistance of the sensor bleeds off the signal to 5VDC or lower to make the gauge function and rise as it warms up....but this is a really wild guess.

 

Jim, that would definitely help. Probably no need to do it tonight, unless you're just bored. It's not critical to the truck running, just an irritation.

Two things that I'm needing to know from a truck that has a properly functioning ECT circuit are: 1)resistance value in the sensor, if any, and approximate temp. 2) Voltage across the female pins on the plug, with the key in the on position.

If the sensor is only supposed to receive 5 volts, and mine is getting 12, I've got something askew.

 

I'll check it now for you.

 

I checked mine and got 12 volts to ground on one of the pins. Did a resistance check on the sensor and got 542 ohms. The truck is still warm and the temp gauge is reading right below the normal zone.

 

I just checked it:
49* Outside
4.04 ohms on sensor
11.04 volts at connector (wire to wire)
12.18 volts on battery

Truck hasn't run in 6-8 hours (engine feels cool)

I hope that this helps

 

Trevor, Jim...thanks a bunch! You guys rock.

That makes me feel a lot better. 12V is normal across the pins. There should be resistance of some type across the sensor. That's what I needed to know.

Looks like both of my sensors are bad. I'll take the new one back tomorrow...and take the DVM with me to the parts store to be sure another new one isn't faulty. I think Kris is going to send me one though, so I may just wait for that one to get here.

 

I think that Leo may be right. When it heats up the resistance increases and the voltage drops causing the guage to move.

Glad that we could help Jeremy.

 

I agree Jim. Based on the info Trevor posted, that appears to be what happens. Resistance goes up as temp increases.

 

Need a little more thought on this one. It's may be tougher than I thought. What I'm finding is my gauge moves in just the opposite direction with resistance as what I think is supposed to happen. I tested it with a couple different resistors this time, still bridged across the female pins on the harness. First one, a 460 Ohm resistor and the needle moved up just to the low end of the normal range. Next one was a 10 Ohm resistor and the gauge shot up to read at it's max. Also, I bridged a wire between the two pins (zero resistance) and the gauge read max.

It's looking to me like my gauge is reading higher temps with less resistance and more voltage to the signal wire. I have no idea what would cause this, or where to even begin looking. Hopefully you guys have some ideas, because I'm completely lost again.

 

Looks OK to me what you've done logic is cirrect and results verify what I know

on my '00 the stupid thing has two positions: about 450 ohms and shorted at 250 ish degrees, could be more, but this is what I observe, and I can't verify the temps while running

I am looking at about 8/9 volts running to the gauge with heat coming out the passenger heater, coolant temp unknown. the only time the @#$^&* needle moves is when it is really hot outside and I'm running the Ac up the side of a mountain dragging the trailer. then it just kinda jumps to HOT which annoys the pcm enough to de-fuel somewhat.

I'm thinking you have a bad sensor even though it is brand new. you could always wire a variable resistor into the signal line and adjust it to reflect what the heater outlet see's

 

I'm going to give this some thought Jeremy.....