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Part 2 - Its Importance


What It Measures

 Being just a pair of microswitches, the TPS does not provide constant information to the ECU, but it does inform it when the throttle is at two key positions: fully closed (engine at idle), and fully open (engine working as hard as it can). This is crucial for the following reasons:

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  • At idle, the ECU regulates air flow to the engine via the Idle Air Control (IAC) valve - this is necessary since the throttle plate is fully closed, thus cutting off the air supply to the engine, were it not for the IAC valve. Presumably, the ECU regulates the air flow to maintain a stoichometric (14.7:1) Air:Fuel Ratio (AFR), given that the LH-Jetronic 2.4 architecture employs an Oxygen Sensor (O2S), also known as a Lambda Sond/Sensor, mounted in the exhaust pipe.

  • At wide-open throttle, the ECU enrichens the fuel mixture so as to provide additional power, and prevent potential knocking/pinging/pinking/whatever you choose to call it.

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In either case, the ECU helps protect the engine from running under unpleasant conditions, and it could be thought of as an additional step of non-linearity in the air/fuel delivery. When all is said and done though, it stops the engine from cutting out - temporarily in the case of idling (by stalling) or rather more permanently when under full throttle (knocking causes all sorts of bad things like damaged pistons, cylinder blocks, valves, con-rods, etcetera).


When It Is Not Functional

When the TPS is out of action, the vehicle may still be driveable, in fact when the vehicle being studied here was first purchased, the idle portion of the switch was not functional since the throttle cable had been adjusted too tightly, preventing the throttle plate from closing fully and actuating the idle switch. This implies that the idle switch is not really necessary, and whilst this is true to an extent, it did have the effect of raising the idle RPM (burning more fuel, and placing more stress on the drivetrain), and giving a 'cruise control effect' at all times, making engine braking almost impossible, stopping harder, and wearing the brakes more quickly.

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Unfortunately, there is no substitute for the wide-open throttle switch, other than wiring in a kickswitch or similar underneath the throttle pedal, although at that point, one may as well just attempt to fix whatever the issue with the existing TPS is.

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