Temperature Gauge Mod
Article: Rob Hall Photos: Rob Hall

Modifying the stock Surf temperature gauge to read accurately


 

Why is the modification needed?

There is a deliberate "dead zone" on the stock temperature gauge (see picture).

On the stock gauge, all engine temperatures between about 70 C and 100C give materially the same position on the gauge.

At over 100C the stock gauge moves very quickly to the red zone. (It has been "held back" and is now "let go"). This means the driver gets no or very little notice that trouble is developing.

The dead zone is deliberate, so that Toyota is not bothered by service calls by owners complaining that their temperature gauge is moving around.

For 4 wheel driving and towing and in the case of a failing radiator or fan clutch, this is really dangerous. An experienced Surfer knows the implications of temperature moving between 70 and 100+ and it is misleading for the gauge to display all of these temperatures in the middle of the dial as "normal".

After the modifications the driver gets better information. The needle comes on to the dial at about 70 degrees, about when the thermo opens.
At 90 degrees it is at dead centre and at 100 degrees it is right next to the red zone. 110 is in the red zone and 120 is off the dial.

The critical window from 90 (normal running) to 100 (getting a tad hot) is now advertised to the driver. The needle moves responsively as the temp changes, giving the driver a much better picture of what is happening.

 

Parts and Tools

Small phillips screwdriver.

Needle nose pliers.


Soldering iron with a sharp point, silver solder and flux.

Two resistors, available from Jaycar, at about 30 cents each (see picture)


56 Ohm 5 watt resistor
120 Ohm 5 Watt resistor

The resistors you need are clearly marked:

5W 56 ohm

5W 120 ohm

The resistors are about 3cm long.


  Step 1: Remove the instrument cluster from the truck and take to a work area.

 

This step is not shown in this article. It is covered elsewhere on the forum.

You need to take the cluster to a table where there is good light, a clean environment, and facilities for some fine soldering work.


In other words, the dining room table with a bright table lamp and a dinner plate as the soldering station.

Step 2: Open up the cluster.

 

The front "glass" of the cluster comes off reasonably easily. Looking at it from the back, there are a series of push through clips around the circumference. You just need to push these down to release them. A potential problem is that as you release one clip, the previously released one clicks back in. You can avoid this by inserting the back of a dinner knife between the "glass" and the cluster to keep the previous clip open. Once a couple of clips are free, the rest are easy. Just be patient and don’t force anything.

The picture shows the top clips highlighted. There are also clips on the bottom.









Step 3: Take out the temperature gauge


There are three screws on the back of the cluster that hold the temperature gauge to the circuit board. The are marked + , - and T (battery positive, battery negative and Temp Sender).

It pretty obvious which 3 screws we need - the temperature gauge is directly on the other side of the circuit board.

Remove these 3 screws and the temp gauge can be lifted out. In fact be very careful that the temp gauge does not fall out when you remove the final screw. It is delicate and would be damaged if dropped, even a short distance.






Step 4.1: Guided Tour #1 - Existing Resistor

This is the left hand side of the gauge looking from the front.

The component circled is the existing resistor which we are going to replace with the new 120 Ohm 5 Watt resistor.





Step 4.2: Guided Tour #2 - Existing Zener Diode

 

This is the right hand side of the gauge looking from the front.

The component arrowed is the Zener Diode which we are going to replace with the new 56 Ohm 5 watt resistor.

The diode is tiny - you can compare it to the fingernails.


Step 4.3: Guided Tour #3 - Circuit board

 

It is crucial that the new resistors go in the right places after we pull the old components out:


The 56 Ohm 5 watt resistor replaces the zener diode

The 120 Ohm 5 Watt resistor replaces the existing resistor.

Fortunately, there is a printed symbol on the other side of the circuit board (underside of the temp gauge) showing what goes where on the other side of the board.

.

Step 5: Unsolder old Components

 

You now need to unsolder and remove the old components.

The picture shows the solder points which are holding the components which are on the other side of the board. (See Steps 4.1 and 4.2)

Three hands are needed to do this job:

Hand 1 gently uses a pair of tweezers or needle nose pliers to hold the component on the other side (top) of the board.

Hand 2 holds the gauge with the underside uppermost as shown (so the solder does not run everywhere).

Hand 3 applies the soldering iron to one of the soldering points arrowed until the solder melts.

At the same time Hand 1 gently tugs the component until that connection breaks free. (Two connections for each component as shown)

 

Step 6: Expose the holes in the PCB/ remove old solder

When the old components are pulled out they should leave a tiny hole through the board at each position where we unsoldered. These are where the legs of the new resistors go when they are inserted from the other side of the board.

Invariably though, the hot solder from removing the old components runs into the holes and blocks them up making it impossible to insert the legs of the new resistors through the board.

So the solder has to be removed so that the legs of the new resistors can be inserted.

The right way of doing this is to use special solder removal tape where you use the soldering iron and the tape in conjunction to soak up the old solder. Another way may be to use a tiny (repeat tiny) drill and drill the hole through. The size of each hole needs only to be big enough to accept the wires (legs) on the new resistors.

The method we used was to dab some flux on each solder point, hold the gauge the right way up and from underneath melt the solder at each of the 4 points. Gravity then caused the solder to run off the board and onto the soldering iron, exposing the hole. The soldering iron barely touches the solder point - you use heat and not pressure.

Don’t push or you might melt a large hole in the board.

 

Step 7: Shape the resistors so they fit in the PCB

 

The new resistors are much larger than the components removed and their legs have to be bent so they will fit onto the board.

The picture shows the new 120 Ohm resistor which will replace the old resistor. You bend the legs to fit the relevant pair of holes in the circuit board. (Step 6)

The legs of the 50 Ohm resistor, which replaces the Zener diode need to be much closer together as the Zener diode is tiny.

Bend the wire around a small round screwdriver or pencil so that there are curves rather than sharp bends.

The wire is very soft and bends easily so you can have several goes at getting right, each time checking that it fits snugly - see Step 8.

 

Step 8: Final Check, resolder and reassembly.

 

It is very important to check all clearances before soldering the new resistors in. This view is looking from the top of the gauge.

The resistors have to be fitted snugly as shown in the picture Before soldering adjust them so that:

a) each resistor (the white bit) does not touch anything. They get warm - especially the 120 Ohm one.

b) the gauge can fit back into the cluster. There is enough clearance if the resistors are positioned as per the picture.

The wire legs are soft and its easy to adjust them until the fit is perfect.

All you have to do now is hold the resistor in place, turn the gauge upside and apply flux and solder to where each leg pokes through the circuit board.

Then snip off the surplus bits of the legs which are standing proud of the new solder- an ordinary pair of scissors does this.

Then, reassemble everything and you have an accurate temperature gauge.

 

Disclaimers and Acknowledgement All disclaimers apply - this mod was bench tested (both Gauge and Sender) and tested in a 2.4 LTD Surf - but no responsibility is accepted.

The authors acknowledge that a similar mod for the Landcruiser gauge has been previously published at http://forum.ih8mud.com/showthread.php?t=64252