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 Modding the Toyota temperature gauge (no zener dead-zone!!)

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Hiace Master
Hiace Master

Number of posts : 766
Home City : Cambridge, UK
Model and year : Model: KD-KZH100G-MRPGT
Year: 1996
Colour: 4K1
Trim: FN42

Registration date : 2016-07-16

PostSubject: Modding the Toyota temperature gauge (no zener dead-zone!!)   Thu May 31, 2018 9:43 am

The Toyota temperature gauge is designed to have a large dead zone in the middle, to find out how it does this and therefore how to 'fix' it, we need to understand how the gauge works:

There are 3 terminals on the back that screw into the dash, B+ (Ignition or Ign), Ground (Gnd) and T (from sensor). Only ground is connected to the gauge, B+ and T and just free terminals and if you disconnect the resistors and diode they connect to nothing.

There are ALSO 2 terminals on the needle side, these are the business terminals of 'meter power' and 'meter' position.

If you disconnect the diode the meter input now becomes disconnected, now if you then power up the meter the needle goes to the middle of the dial. This action is accomplished by two weak resistors inside the gauge that merely steer the needle to the middle in the absence of any input to the meter terminal. If you then connect (for instance) a 100 Ohm resistor to the meter input and with the other end touch ground the meter shoots up to the top needle stop, if you touch it to the meter power terminal it goes smoothly to the bottom or 'off'.

On the standard (original) gauge there is a 15 Ohm resistor from the B+ terminal to meter power, we can leave that there and ignore it. Then there is a 75 ohm resistor that goes from meter power back up to T, the dash connector where the sensor connects in on the head. This forms a voltage on the T pin that goes down with rising temperatures and up with lower temperatures.

Sensor values are:
1135 Ohm = cold
255 Ohm = just noticing on the gauge
100 Ohm = just right
19.5 Ohm = top of scale: very hot

Now the only thing on the standard gauge to connect the meter and swap if from its preferred centre position is that diode, forward it drops around 0.7 V, backward around 3 V (I think it's a 3V part, it's not that vital to know). This means that around the middle of the scale when the voltage on T rises to within 0.7 V of the middle the diode ceases to conduct, and when the voltage gets to +3V past the centre the diode again starts to conduct: this is what creates the large dead zone on the gauge, an unknown area of 3.7V. Two reversed parallel diodes would create a smaller dead zone, but I don't have time to analyse the zone position so I decided to eliminate the dead zone completely.

The key to my modification is that the 75 Ohm resistor and zener diode are entirely removed - snip them out carefully with small wire cutters, desoldering may damage the gauges.

In order to get the gauges to correspond to the same 'just noticing/just right/hot!!) positions I experimented and ended up with a slightly different topology. Connect a 220 Ohm resistor from meter power to meter. They are the two terminals on the needle side of the meter, and the 220 pulls the needle down to the bottom. Then where the diode was solder in put a 47 Ohm resistor. This is still between T and meter, and electrically adds a series constant 47 Ohm to the sensor resistance, the combined resistance then meets the 220 Ohm at the meter terminal and the resultant electrical tug-o-war will give a proper reading with no dead spot.

New resistors required
1) 220 Ohm  2W or higher rating, placed between the 2 lower face side terminals
2) 47 Ohm 2W or higher rating, placed where the old diode was (between T and meter).

My gauge is a 1996 Yazaki Meter one and the resistor values are based on characterisation of the old gauge, has not been tested on the van yet so there are no guarantees but when I fed in the same resistor values I'd seen on the original gauge it gave the correct indications on the new modified gauge so I'm confident it will be good, but I don't know quite where the needle will be pointing at 80C for instance, you'll have to get used to what is 'normal' for your own van.

You can see in the 2nd picture my fan of resistors I made with values of 255, 100, 47 and 19.5 ohms to simulate the sensor as I calibrated the gauge and selected the 220 and 47 Ohm resistors and the new topology (only one connection to the T terminal now).

I suppose at this point is is possible to add a warning lamp circuit but the gauge is quite clear to see and I didn't, but if anyone does please add to this thread!!
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Modding the Toyota temperature gauge (no zener dead-zone!!)
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