Coolant temp gauge not working - SOLVED

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LS1FJ40

LS1FJ40

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Ok. So, my coolant temp gauge isn't working. Needle all the way to the bottom. I suspect the sending unit or wiring is bad. Leaning more towards the wiring because it quit working for a little bit then came back and has quit working again. Coolant level is good. I burped my system and all seems good.

However, another issue is that the fan comes on when I start my truck and stays on. So I'm thinking the thermostat is also an issue. I replaced the thermostat 5-6 years ago when I replaced the radiator. I ordered a new thermostat and gaskets.

It seems like I am burning "rich" and I get horrible gas mileage. I've heard this could be related the ECU and a reading from the temp sender. But is there a different temp sender for the gauge then there is for the ECU? Should I just forego the OEM temp sender to the gauge and get a digital inline temp sender (can't do ScanGauge because the 3FE is not OBDII).

Anyone have any suggestions? Thanks.

1992 FJ80 3FE 233,000+ miles.
 
Same sender. ECU thinks it's still cold and in open loop dumping the fuel in it. Replace it and check the wiring.
 
That's what I figured. Probably not the only cause of my poor gas mileage but definitely a contributor. Thanks.
 
The radiator fan.

Could it be that both senders went out at the same time? Maybe the one to the ECU went out a while ago and I just didn't notice?
 
The radiator fan should "roar" like a DC10 when you first start the truck cold. After about a minute it should quiet down, but it will ALWAYS spin and is strictly mechanical.
The water temp SENSOR is for the ECU; the water temp SENDER is for the dash gauge; water temp SWITCH controls FPR vacuum solenoid.

3FE StatHousing.webp
 
Ok. So, the SENDER (or wiring from sender to gauge) is bad. So, should I mess with replacing it or should I just go with an inline temp sensor instead?

Does the SWITCH control how much fuel is being dumped? Or is that the function of the SENSOR and ECU?
 
Thanks jonheld! I missed that it was a 92!

As usual, you are correct!

LS1FJ40: Believe everything jonheld has to say!
 
@BILT4ME I already do. :)
 
@jonheld Thanks for the detailed picture with part numbers on it. And for the clarification. :)
 
jonheld said:
The radiator fan should "roar" like a DC10 when you first start the truck cold. After about a minute it should quiet down, but it will ALWAYS spin and is strictly mechanical.
The water temp SENSOR is for the ECU; the water temp SENDER is for the dash gauge; water temp SWITCH controls FPR vacuum solenoid.

View attachment 1046995
Click to expand...

How many times have you posted this now? I think I have seen it enough that I'll never forget... as usual, thanks for sharing the knowledge and not getting upset that this comes up so much. The 3FE guys have to stick together.
 
LS1FJ40 said:
Ok. So, the SENDER (or wiring from sender to gauge) is bad. So, should I mess with replacing it or should I just go with an inline temp sensor instead?

Does the SWITCH control how much fuel is being dumped? Or is that the function of the SENSOR and ECU?
Click to expand...
I'm a big believer in finding the problem first and then making decisions based on that. Right now you're guessing. An inline temperature gauge will be more accurate, but I would still assess the issue and try to make it work as designed.

When you say "dump fuel" I assume you mean the injectors. That's all controlled by the ECU and the feedback it's getting from sensors in the system.
The O2 sensors are one of the primary feedback circuits to the ECU in regards to injector duty cycle, and can often cause poor fuel economy. They will not always throw a code even though they are well past their lifespan.
 
Golgo13 said:
How many times have you posted this now? I think I have seen it enough that I'll never forget... as usual, thanks for sharing the knowledge and not getting upset that this comes up so much. The 3FE guys have to stick together.
Click to expand...
Stuff like this doesn't bother me, and I only get upset when I'm hungry ;)
 
jonheld said:
I'm a big believer in finding the problem first and then making decisions based on that. Right now you're guessing. An inline temperature gauge will be more accurate, but I would still assess the issue and try to make it work as designed.

When you say "dump fuel" I assume you mean the injectors. That's all controlled by the ECU and the feedback it's getting from sensors in the system.
The O2 sensors are one of the primary feedback circuits to the ECU in regards to injector duty cycle, and can often cause poor fuel economy. They will not always throw a code even though they are well past their lifespan.
Click to expand...

Ok. So, in order to identify the problem I went ahead and ordered a SENDER so I can check the wiring (first) and then the sender (second). I've parked it until this weekend since I can't see my temp gauge. I've been thinking about an inline temp sensor for a while. They look pretty cool and seem really reasonable as well as giving me accurate temps (but will get put to the back burner after...)

I'm pretty confident my O2 sensors are crap and need replacing. I will do that next. Before I do an inline temp sensor since money is tight. :)

Thanks for your help @jonheld! I truly appreciate you always willingly sharing your wealth of knowledge. :)
 
Finally had time to dig in to this a little bit. Went to pull the connector off and the wire broke at the connector. Put a new connector on and still doesn't work. The gauge fuse had blown at some point so I was figuring this shorted it out. All other gauges are working fine. Just not the coolant temp. Ugh.
 
Ok. So I have power to the sender. (Checked via test light). And all other gauges are working. So it HAS to be the sender itself. Correct @jonheld?!
 
If the gauge fuse was blown, none of your gauges would work as that is supplying the reference voltage across all gauges.
The sender itself is a thermistor to ground. Put a meter on the terminal and measure resistance to ground as the motor heats up. If there is no change between cold and warm, then the sender is bad.
If you change the sender, have some coolant standing by.
 
Sounds good. Will check this morning. Thank you!
 
WOOHOO! It was the sender. It didn't have any resistance. I had ordered a new OEM one so I picked it up at Toyota this morning for $19. Swapped it out and crimped and soldered a new connector on and shrink tube. Lost about a cup of coolant (fast fingers). Cranked her up and waited a few minutes and she's working. Thanks @jonheld for your help and having patience with me!

ImageUploadedByIH8MUD Forum1426959794.008067.webp
 
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