The 3 circuit board connection are indeed as above.

I just noticed that the brown wire is also part of the 2 wire temperature switch which is next to gauge sender on the engine block. Not sure if this brown wire goes there.

There is some very strange thing going on. Today I hooked precision potentiometer (500 ohms as opposed to 10K). The gauge cold to hot range is 300 ohms to 200 ohms respectively. This is not right. When engine warms up, the sender resistance is around 100 ohms. 250 ohms is where the gauage reads in the middle. I then checked the resistance of the sendor wire with ground with sender disconnected, it reads 60 ohms. I thought this should be OPEN.

I wonder this 60 ohms is acting like a parallel resistance. 60 ohms in parallel with 500 ohms makes the equivalent resistance to below 50 ohms (that is why it reads HOT all the time)

If the ground is missing, wouldn't it be like OPEN/high resistance between the ground and sender? if that's case, it should read COLD all the time.
The problem I'm having is exactly opposite, it reads HOT all the time

Is it possible to post the circuit diagram?

 

The brown wire of the 2-wire temp sensor eventually routes to the ECU and is then internally grounded within ECU. Thus the brown wire eventually goes to ground, but not the same grounding point as the temp gauge.

The temp gauge most likely works on a magnetic principal. Think of the gauge as a tiny electric motor, but the motor shaft (armature) only rotates the length of the H/C range of gauge (maybe 120 degrees).

The motor has two field coils (call them A and B) both powered by the battery voltage. One coil (A) is grounded (the brown wire). The other coil (B) routes to the sender unit then to ground via the sender case.

The armature is like a permanent magnet needle mounted on a pivot point and responds (moves and rotates on the pivot) to the magnetic fields generated by the coils.

If coil B is not grounded or the sender has a very high or low resistance, then coil A magnetic force will dominate and move the needle to one extreme. If coil A is not grounded, then coil B magnetic force will dominate and do the same.

As such a bad gauge ground will cause the gauge needle to peg out.

Sometimes if the gauge has electrical components such as resistors these can fail, burn out, bad solder joint, etc. But two gauge units having the same issue is strange.

The link below has lots of photos, drawing, etc of electric temp gauges.

https://www.google.com/search?q=car+...sp%3B600%3B600

 

Well guess what when I jumper brown wire to ground reference Gauge works like a champ !!! So we know the ground is missing. What is puzzling is the same ground reference is used for fuel gauge but that seems to be working fine.

Instead of finding where this brown wire is disconnected(which could take hours !!) is it possible to jumper the PIN 2 and PIN 8 on blue connector which will be easy to do. I checked, PIN 8 is ground just not sure without schematic what's its purpose and whether it is going to mess other stuff. Can you check what PIN 8 is?(red/black)

Thanks again for your help.

 

Pin 8 on same plug is for Reverse light. Why it indicates ground is unknown. Suggest not using this wire for ground.

Pin 5 (brown) on another connector goes to ground but at a different location.

Pins 9 and 16 (white/black) ditto.

Don't know the effect of grounding pin 2 together with above pins. Suggest not using pin 8 red/black.

Suggest looking at the ground point for pin 2 and determine if one wire has a bad connection to ground or connection to the terminal end.

Options:

Try the other pins that go to ground.

Run a wire from pin 2 to chassis ground or back to area close to sender.

 

Hi Toyo,

My gauge is working with ground point right behind the cluster. It now reads slightly above mid mark as shown in 1st picture (I thought the normal is a tad below mid mark)

When the temperature is as shown in 2nd picture, the radiator fans kicks in. I checked the coolant level is good, I burped the system. The Thermostat seems to be working as I can feel the bottom hose getting hot.

I don't know if the gauge is off slightly or fans kicking in late or thermostat is not working accurately.

 

Typically the gauge needle stays in the same spot even if the fans are running.

You might place a thermometer in the radiator fill point and let engine heat up. Fans should start up at approx 199F at coolant switch on lower tank of radiator.

 

Meaning take the cap off and idle the engine? Isn't coolant going to flow out if you let it reach hi temp?

Also I checked the radiator top and bottom by touching it, there is considerable difference in top and bottom and this may be normal as that is the job of radiator to cool the coolant. If I checked the coolant temp at the top like you suggest and bottom is cold ( I don't know the usual delta) then it may not work properly.

Even if when I touch both coolant hoses, they are hot, does that rule out thermostat issue? If the thermostat would remain open all the time, I would have had different problem (excessive cooling), if it stayed shut, then I won't be able to feel the heat on bottom hose.

Is it possible that thermostat is malfunctioning, despite it is not completely open/closed.

 

Did the gauge and engine cooling system ever work correctly. As in the gauge needle reach operating temp indication and stay there regardless of engine operation?

Take cap off when engine cold and let engine heat up. A properly working cooling system will not boil over at engine idle. The fans will keep the coolant temp low enough.

 

When it was working, it would read at half way mark.

I checked the temperature you described with infrared thermometer. Here are the readings.

With engine idling for 15 mins.

170 F coolant temperature measured at the fill cap pocket.
177 F where the sender is mounted
Fan comes on around 188-190 F coolant temperature
183 F upper hose
178 F Bottom hose

Bottom hose started getting hot, even before CT hit 180F, it seems the thermostat is opening up little sooner.
Instrument cluster was reading at 6 on 0-10 scale.

I again simulated resistance with potentiometer

at 24 ohms the cluster gauge reads between (6-7)...what I consider as running hot

at 29 ohms the cluster gauge reads between (4-5)..what I would consider as normal reading.

It seems to me may be the sender gauge I replaced is off and not working per OEM. What do you think?

BWD/Intermotor Engine Coolant Temperature Sender WT654: Search no more for the best Temperature Switch/Sender at Advance Auto Parts

Any recommendation on OEM part?

 

Would be unusual for an aftermarket sender to cause issues, but there was a comment about the sensor in the link not working out of the box.

If original problem was a bad ground wire, then old sender may be OK. Compare new and old sender ohm readings at same temp.

Or may still be a grounding issue. Toyota did ground the gauge at the engine and there must have been a reason for this.