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 accurate, use it.
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 earlier 80 Series, so I had to re-do the calibration to suit.
I also had the opportunity to see the inside of a 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
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
- 82 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.
Detailed pics below (5C steps)
Original | Modified | |
130 | ||
125 | ||
120 | ||
115 | ||
110 | ||
105 | ||
100 | ||
95 | ||
90 | ||
85 | ||
80 | ||
75 | ||
70 | ||
65 | ||
60 | ||
55 | ||
50 |
Thanks for the guide. It is very well detailed.
I carried out this mod and my temp guage now works accurate instead of going straight to half and not moving.
Thanks again.
You are the best. Awesome mod. Thanks for sharing
great tech article can also be applied to supercustom vans, mine had a 130ohm in one model van and a 75 ohm in another so seems there are many variations