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It’s fairly common knowledge that the OEM Toyota temperature gauge has a large “dead spot” in the centre of it’s range. This spot is deliberately engineered to reduce the apparent fluctuations and make the car appear to run at a constant temperature unless there is a significant problem. This works fine for most, but those of us that like to know what’s going on sooner rather than later demand a little more detail. Many people fit an aftermarket gauge somewhere in the car, I figure, if the factory gauge is already there and can be made better, do that.

One of the clever guys over at ih8mud figured out the circuit and how to modify it in his 80 series. He deserves full credit for the original article and all the work behind it. There is also a version for the Toyota Surf and Hilux.

The gauge circuit in my ‘98 HZJ105R was a little different to the 80 Series, so I had to re-do the calibration to suit. I had the opportunity to see the inside of and post ‘04 update dash, and unfortunately, it’s quite different. Someone will need to do their own testing and research on that one.

I bench tested various setups and found the following as a simple description.

Bench testing w/ Digital Thermometer

Heating the sender unit in vegetable oil

Mmm, wiring.

Resistors and Diode

We do not change the 15 Ohm resistor.

There are 2 components we replace, a resistor and a diode. The diode is what makes the gauge “non-linear”. Rather than explaining what they do in a wheatstone bridge, I’ll explain their effect on the gauge.

The gauge with no input actually sits in the middle of the scale. The 75 Ohm resistor we change “sets” what temperature the middle of the scale is to be. A lower value resistor sets it higher, a higher value resistor sets the scale lower. I found the following:

• 100 Ohms = 90c
• 120Ohms = 85c
• 87 Ohms = 95c

Gauge w/ no input – centres on scale.

The small glass diode gives a non-linear (dead spot) in the needle’s range. We replace this diode with a resistor to make the gauge react “normally”. The value of this resistor determines the “range” of the gauge. A lower value resistor gives large movement for small temp changes, a high value resistor gives less movement. Using no resistor with a 90C centre means the gauge hits the red at 94.5C – a little too low.

No Damping Resistor – large deflection

I found a value of 82 Ohms gives a good range with 115C touching the Red, 125C middle of the red and 65C touching the Cold. Properly mixed coolant boils at approximately 125C – 128C at 14PSI, and I’m not interested in below 65C, as the engine is not yet at operating temp.

This combination gives me the best combination of “”operating near the middle” and “enough movement to see what’s happening”. With the above detail you can adjust your own numbers if you wish.

The 100 Ohm resistor gets hotter as engine temps increase and will possibly exceed 1 watt. I recommend a 2 watt resistor. 5 Watt is very large and may not fit or be too heavy. The temp sender resistance decreases with heat, increasing current through the resistor.
The 82 Ohm resistor dissipates less than 1/4 watt, but I used a 1/2 watt to be safe.

	Original	Modified
130
120
110
100
90
80
70
60
50

You will need the following

• OEM ‘98-‘04 Landcruiser Dash
• 2 watt, 100 Ohm resistor
• 1/2 watt, 82 Ohm resistor.

Remove the gauge pod from car and disassemble. Be careful removing it from the car – there are 4 screws and 2 bolts. The bolts are captive and hold the plug connections, but need to be unwound a lot to release the plugs.
Remove the temperature / oil pressure gauge assembly.
Remove the 75 Ohm resistor and Diode.
Replace the 75 Ohm resistor with a 100Ohm and the Diode with a 82 Ohm.

Detailed pics below (5C steps)

	Original	Modified
130
125
120
115
110
105
100
95
90
85
80
75
70
65
60
55
50