Drivers Auto

02/01/2022

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Repairing drivers side AUTO window switch.

As tolerances of components drift out over time, the auto switch on the drivers side window can stop working correctly and “stick” on. This can be bad news for the motor as it overheats and the safety trip cuts in.

The circuit works using a comparator. This looks at the voltage drop across a shunt, when the window reaches the end of its travel it draws more current, the IC in the switch looks at the voltage drop, compares it to a reference and if within limits shuts off the latch on the switch.

Because tolerances change over time, the trip reference no longer matches that of the motor so, does not shut off the latch system.

To solve the problem we need to re-calibrate the circuit to shut the system off once the window reaches full travel.

This mod is based on a 1991 turbo model but should apply to all. First thing you need to do is gain access to the PCB on the back of the switch. To do this remove the drivers door handle.

Once you have access to the back of the switch remove the plastic cover over the PCB.

All you need to do now is solder a 75 Ohm resistor across the pads as illustrated in the photo here.

There is plenty of clearance between the PCB and plastic cover for the resistor to fit on the back of the PCB without fouling.




		Click for larger Image

Once you have done this, pop the thing back in the car and check it works correctly. If it does then screw it all back together and enjoy a fully working auto drivers window!

As all circuits were set up the same in the first place from Toyota, by theory all component’s should age similarly so the value of 75 Ohm should solve most people problems. If you find this is not the case you could solder a 50K variable resistor across the pins and adjust it gently till you find the circuit functions correctly.

Once this has been achieved remove the variable resistor, measure what value it is set to and find a suitable resistor to put in its place .

Hint:

While you have the switch out, if you have not already, you can always convert the switch to give an auto up function too.

If you look on the back of the top part of the auto switch, you will notice a small tab that prevents you from pressing the switch down. If you cut this tab off you’ll find you can now push the top part of the auto switch down. This allows the window to raise automatically as well as lower.


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