A common call we receive concerns 4-cylinder Ford products such as the Tempo/Topaz and the Taurus/Sable body style. The model years in question range from 1985 to 1988 for the Tempo/Topaz and 1986 to ...
A common call we receive concerns 4-cylinder Ford products such as the Tempo/Topaz and the Taurus/Sable body style. The model years in question range from 1985 to 1988 for the Tempo/Topaz and 1986 to 1988 for the Taurus/Sable. The symptoms are usually an intermittent higher than normal idle, or a lack of power on acceleration.
Initially, the first check should be the Throttle Position sensor. A simple sweep test with your scope or multimeter will reveal if there are any opens or spikes in output. If the TPS checks out OK, the next item to check is the Idle Speed Control motor (refer to diagram #1).
This component combines a D.C. motor along with an Idle Tracking switch in order to control idle speed and monitor closed throttle position. There are four wires connected to this motor. Two wires are connected to the ECM through pins 21 and 41, which is a power and a ground. In order to change the direction of the plunger, the ECM reverses the polarity of these two wires.
The remaining two wires are used for the Idle Tracking switch. One of these wires is a ground connected to pin 46 of the ECM. The other wire is for the Idle Tracking switch, and is connected to pin 28 on the ECM in the Tempo/Topaz, while on the Taurus/Sable it is connected to pin 24.
The ECM supplies approximately a 10-volt signal on this wire. By monitoring the voltage on this wire, the ECM can determine if the throttle lever is in contact with the Idle Tracking switch. When the throttle is closed, the Idle Tracking switch is open. This means you should expect to see 10 volts on the wire going to pin 24 or 28, depending on the vehicle. As the throttle is opened, the Idle Tracking switch closes and is connected to ground through pin 46 of the ECM.
When properly grounded, this voltage should be very close to 0 volts. The ECM constantly monitors the voltage on this circuit. A high voltage reading on this circuit indicates that the throttle is closed. A low voltage reading indicates an open throttle.
Let’s see how a problem can arise. Suppose the voltage on this circuit remains at 10 volts at all times, even with the throttle open. At this point, the ECM will assume that the throttle is closed and the vehicle is decelerating. This will result in reduced fuel delivery. This will not have an impact on vehicle performance if the vehicle is decelerating. However, if the vehicle is actually accelerating, then you will definitely have a performance problem.
By monitoring the voltage on the Idle Tracking switch circuit, you can determine whether the ECM is seeing a closed throttle condition or not. You should measure 10 volts at closed throttle, and 0 volts when the throttle is opened.
On the other hand, if the Idle Tracking switch indicated an open throttle condition, even if it was not, then the ECM, seeing that that Throttle Position sensor voltage is low, will attempt to get a closed throttle reading by extending the Idle Speed Control motor. This would cause the engine to race at an idle that is substantially higher than normal.
By monitoring the voltage on the switch as before, we would see what voltage the ECM is seeing. If the voltage is always at 10 volts and the vehicle is suffering from a lack of power, try grounding the Idle Tracking switch circuit when the vehicle exhibits this condition. If the vehicle performance improves, the IdleTracking switch circuit should be checked.
Top Gun technician
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