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Temperature Gauge Investigation and Modification, Dave Mason, Aug 2002

Most owners are driving normally operating 'aces. We don't need to know the actual head temperature in degrees Celsius. What we do want to know is that we will get an indication if it deviates from "normal". Despite our careful maintenance the viscous fan coupling may fail or the thermostat may stick either open or closed. A water leak may develop. Or we want to know if "improvements" which we make have any effect.

Toyota's temperature gauge seems to be intended to fit the bill, but is generally agreed to be, uncharacteristically for Toyota, pretty useless.

Fitting an add-on calibrated gauge such as the Durite one makes a lot of sense. However experience has taught me to try to make minimal modifications to plant which has already been carefully designed. So I set out to try to understand what is wrong with the Toyota gauge and devise a simple, but useful, modification which owners might choose as an easier option.

Conclusions

When I started making measurements, the first thing that I realised is that the meter dial, with its 20 identical graduations, is very tricky to read. Unless the pointer is near the centre (or the red sector!) it's hard to say much more than something like, "just below half".

As many have already observed, its response has a strange flat spot, made worse by only having a hazy idea of where the pointer is anyway. Details below. You will hardly notice it move - it's always "just below half" - through the range 90C to 110C. Beyond 110C you'll notice it zoom away up into the red sector (at about 130C). Everything points to this being the way it was designed, but in "normal" operation you get the impression that the gauge isn't really working, and in a crisis the warning comes after a worryingly high temperature has been reached.

The best compromise for a modification is to use only the top 60% of the dial, where the gauge has a consistent sensitivity and the position of the pointer is easier to assess as it eclipses the picture of a thermometer. This can be achieved simply (this was written before the alarm modification was designed - ed.) by fitting one resistor under two screw terminals which are easily accessible behind the dashboard.
In normal operation you will then see the pointer move beyond "8" on the dial, beyond the flat spot, and indicate slight variations in the range 80C to 100C, for example the variation which cause the thermostat to open and close as the engine requires more or less coolant flow.
   

Meter connections

Removing the instrument panel is straightforward and described in the editorial in the Dashboard & Electrics folder of this Ace Answers website. Here is the arrangement of the three connections to the temperature indicator, as you see them at the back of the panel. The 1Watt 47ohm resistor is fitted as a shunt between the TU (temp. unit) sender terminal and the earth terminal. You can fit it in 20 minutes without even leaving the driving seat!

Standard Overall response

If your gauge usually sits about three graduations below the centre (at "7" on this scale although you'd be pretty observant to know whether it was 6, 7, or 8) then this explains why. The measurements and formulae on which it is based are described below.

Modification

Add a resistor as shown above. The immediate effect is that pointer will go to "7" as soon as you turn the key even when the engine is in fact cold. So the downside is that the lower third of the scale will no longer show you the engine warming up. The benefit is that choosing the value of the resistor now allows you to choose the temperature range over which you want the indicator to be sensitive. 47ohms is probably the best, starting at 80C. 39ohms (these are the standard resistor values) might be of interest - covering lower temperatures but the scale is more cramped. 56ohms and 33ohms are shown for illustration.The resistor must be rated for at least 1 Watt. Get a wirewound type with a temperature coefficient less than 300ppm/C if you can, it should cost less than a pound from any electronics shop. (Email me if you're stuck and I'll send you one).

Wiring detail

For reference, this schematic shows the arrangment of the sender and indicator. Pin numbers refer to the blue connector, see below.

Connector pins

There are 3 connectors to remove to get the instrument panel out. This one is the only blue one. I had to identify the temperature gauge wire, on pin 6, in order to break it.

Test arrangement

For the record, in case you want to see if my results are repeatable, this was my test setup using two digital multimeters. In all I brought 5 wires out, as shown, for test purposes. Below are the results which I obtained. I revved the engine for each reading, to bring the supply Volts up to normal

Indicator characteristic

Here is the most useful way to show the results of the test. The ohms load is calculated from the Volts and mA data above. Amazingly, it has two straight lines and so can easily be modelled by two formulae to derive the calibrations above.

Sender characteristic

I'm grateful to David Miller who supplied the measurements which he made, plus two (50C and 115C) from the Toyota test spec. These all fit very closely to a normal logarithmic thermistor characteristic, as shown by a trend line whose formula is given and used to derive the calibrations above.