Coolant Gauge Stuck or Bad Sending Unit?

2006 4.7 4x4 SR5 DC

Owned for about 6 months and finally getting towards the smaller things on this truck that need fixing.

My coolant gauge works maybe 10% of the time. The other 90% of the time the gauge seems stuck, like it won’t get past a certain point. I confirmed the truck is running at the correct temperature from my scanner. I’ve taken it for 3 hour road trips without a stop in the summer and the temperature stayed perfectly in range. New radiator, pink coolant, thermostat, timing belt/water pump, fan clutch bracket, so all that stuff is good. Another thing to note is the 10% of the time it does work, the needle isn’t smack dab in the middle where it’s supposed to be, it reads slightly below half, like 2/5 if I had to put a fraction on it.

So my question is could a bad sending unit cause the gauge itself to sometimes work? Or couldn it be the actual little motor in the gauge itself is stuck and struggling to get past a certain point? Are the gauge clusters serviceable?

I attached a video. Btw the truck was at operating temperature when cycling the key. So the temp should have jumped to half, but you can see it gets stuck.

https://youtube.com/shorts/lFmnx6GMmfI?feature=shared

 

Good question. Do you have a reader that will show you the coolant temps, so you can compare if the ECU is seeing temps that match what the gauge is putting out?

I know the 2000-2004 trucks used two separate coolant temp (ECT) sensors and a different cluster, one sensor feeds the gauge, the other next to it sends data to the ECU.

We, with the 2005-2006 trucks, got an updated cluster and I would need to look... I know there's one ECT sender where the earlier years have two, but I don't know if the one that's to the passenger side of the throttle body feeds the gauge or the ECU, or both. If that sensor only feeds the gauge, it may be that's going bad. I'm not 100% sure though. I'd use my OBDII bluetooth dongle (OBDLink MX+) to monitor the temps the ECU is seeing, and if the ECU stays constant and the gauge doesn't, there's either another sender somewhere, or maybe the stepper motor in the cluster is tweaking.

 

Yes, but the gauge only works maybe 10% of the time. 90% of the time it seems to be stuck at the bottom. I’ll try to monitor it when it does work.

Maybe I’ll just go ahead and order the sensor and gasket and hope that’s fixes it. I believe these are the part numbers.

8942206010 sensor
9043012005 gasket

 

There's a 15% off sale right now thru https://autoparts.toyota.com at least here in the SE USA.

Meanwhile, I'll be interested to hear it, just know, that sensor can be tough to pull if it's never been removed. Expect it to cause some grief, use a good deep well, and hit it with penetrant like Kroil in advance. I've seen more sensors snap off where the plastic meets the base than I can count.

I'd need to run a VIN to confirm the part numbers. If you PM your VIN, I'm happy to look it up.

 

Yup I’ve seen that. Free shipping on 75 up and no sales tax from some dealers too. Lewis Toyota of Topeka has been my go to.

Yeah I’m worried about that…part of me just wants to leave it because it’s functioning in range and I can just read via my scanner. But it’s probably a good idea to get a working gauge.

I’ll go ahead and double check the part numbers on that site you recommend. I know how to use it now lol.

 

Funny enough went to test it and it was the rare time it worked. It seemed like the scanner reading and gauge was synced up. This is what I mean by the needle being a bit low, this is the highest it will go.

 

Possibly important to understand, senders for fuel and temp are often just reading resistance on the circuit. With fuel, floater in the tank lifts up and causes a contact on a circuit to raise/lower, which changes the resistance of the circuit, and thus as the resistance goes up/down, so does the needle. With some temp senders, heat at the sensor rises or lowers either due to the component inside being affected by heat, or even a coil winding/unwinding, and the heated thing increases/decreases resistance on the circuit, the needle moves in kind.

This isn't *every* sender, but many automotive senders work this way. If you have a DMM, you should be able to set it to measure resistance/ohms, and backprobe the connector to read how much resistance is showing across the terminals. Do this when the gauge is working, and either (A) compare it to what's in the FSM for accepted levels or (B) compare it to the reading you get when the gauge drops out. If you find there's no and/or low resistance at the ECT when the gauge is also flat, the ECT sender is *definitely* the issue. If it's not, then there's something else afoot, or another sensor somewhere you're not probing at.

 

If it snaps, worst case you have to buy new water crossover pipe.

 

Given there is only 1 coolant sensor on an 05/06....Engine Coolant Temperature(ECT) sensor....the ECU receives that ECT signal, uses it as a control signal and also supplies an output from the ECU of that ECT signal to the Combination Meter for the Coolant Gauge.

If the ECT thermistor is open for 0.5sec, it will cause the ECU to flag either a P0115 or P00118. If your gauge is stuck at the bottom 90% of the time, your ECT is not the likely culprit IMHO. You're Combination Meter Coolant Gauge is bad or you're losing the signal from ECU to Coolant Gauge.

 

No codes are being thrown.

This was my thought as well. Because it seems like the gauge tries to work, it just gets stuck. I'm still going to do what @shifty` recommends and test resistance to be sure.

How do I go about replacing this?

 

Doesn't seem like you can replace components of the gauges based on the parts diagram.

 

Look up taninauto, may be able to help.. I'm sure there is a way to take it apart...

 

No, but @bfunke will tell you, you can probably find someone that does cluster repair if you determine the cluster is at fault.

With @BubbaW's point, if the signal/circuit goes ECT -> ECU -> gauge cluster (something I wasn't sure of), if your scanner is showing correct temp data from the ECU while the cluster isn't showing correct temp, that would suggest the red circuit in that flow is OK, but you'd want to diagnose the blue leg. Which would mean tracking down the correct color for the ECT wire from the ECU, testing that pin, then finding the pin on the cluster side, and testing it there. Or performing a continuity test between the two wires.

Let me ask this: Have you popped out the glovebox to look behind for rodent activity, in case you had one of those fuckers in there chewing stuff up, or looked at the ECU wiring harnesses for evidence of water?