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Throttle Position Sensor: Friend or Foe?

By Mike Fleming

The only user-adjustable input to the electronic engine management computer  (EEC-IV) is the Throttle Position Sensor (TPS).  It also happens to be the most critical adjustment for proper drivability.  A properly adjusted and functioning TPS can make a car a pleasure to drive - far better than any carburetor-equipped vehicle (and I've worked on them all including the three-, four- and  six-Weber Ferrari's!).  This sensor signals the EEC module of the three throttle modes: closed throttle (CT), part-throttle (PT),  and wide open throttle (WOT).  If  the TPS gives an incorrect value  to the EEC module, several drivability symptoms may arise: loss of power, surging at steady throttle, poor idle, continuous high idle, pinging, high fuel consumption, etc.

The TPS is connected though three wires: VREF (5 VDC reference from  the EEC module), SIG RETurn (the ground reference to/from the  EEC module and all powered sensors) and the TPS signal (varies between the two depending  on  the actual throttle position, hence the name Throttle Position Sensor). The sensor normally has an output value ranging from slightly below 0.90 to above 4.50 volts with no high or low jumps in between.  A good sensor will show a smooth increase and decrease in voltage as the throttle is opened and closed - whether hot or cold.

Typically, a defective TPS will develop an open spot  that will produce a range of TPS movement with either no voltage output or no change in output.  This can make the EEC module do strange things,  as it immediately (well, within about  10 milliseconds) recognizes that the signal is not valid and therefore will not use any more signals from that sensor during the existing key-on cycle for most strategies.  The result is that there is no indication of CT  or WOT anymore [to the EEC] and drivability problems occur.  If the ignition is switched off then on again, the EEC module will once again see a functioning sensor and operate normally until it once again detects a bad signal and the failure mode is initiated again.  An EEC self-test will usually produce a Code 23 (TPS Out of Range) for this condition.

A very interesting function of the TPS is to inform the EEC module that the  throttle has just opened.  As soon as the throttle is sensed to open, some ignition advance is added and EGR flow is initialized.  The exact values depend on several factors, including calibration, but for example let's look at the 1985.5/86 SVO calibration (aka PE) in the "Premium Fuel" mode, which adds 10 degrees (crankshaft) timing advance to the base timing (which  is 28-30 degrees BTDC).  This helps the initial response of the engine at small throttle openings.  When  CT (Closed Throttle) is sensed at any engine speed, the ignition timing is immediately taken back to the base timing (28-30 degrees BTDC) to provide a stable idle, lower HC (hydrocarbon) emissions and maximum engine braking.

To replace the TPS: remove the throttle body, then remove the ISC/TAB (Idle Speed Control/Throttle Air Bypass) solenoid from the throttle body assembly.  Using a #1 Phillips screwdriver, remove the two screws that mount the TPS.  Place the new TPS on the throttle body using new screws if they came in the kit.  Do not tighten the screws fully, leave loose enough for a slight adjustment. 1984-1985 use  E5FZ-9B989C (comes with the attaching hardware) and 1985.5-86 SVO's use E6ZZ-9B989-A (may or may not come with the hardware). The sensor is identical, only the wiring harness connector is different.  After reassembly, use a high-impedance digital voltmeter and set the CT adjustment to less than 0.90 volts with WOT going greater than 4.50 volts.

Ideally, adjustment of the TPS should allow the tuner (or advanced tinkerer) to  set the TPS so the first 1/2 degree of throttle movement  from CT would add the 10 degrees of advance.  In reality the mechanical linkage and cable drag are not quite precise enough to allow that kind of accuracy and 3-4 degrees is more likely to cause fewer problems.  If the line is drawn too  thin, then  the difference  between CT and PT may not be well defined, with high idle, poor running and higher fuel consumption being the result.

I like to set the TPS's for about 3 degrees of movement before the timing  advances - check it with a timing light to see what I mean.  Timing should be 28-30 degrees at idle with the throttle closed and will jump 10 degrees higher when just barely moving the throttle without any gain in engine RPM.  After getting this setting proper, verify the actual values with a digital voltmeter to make sure that CT is less than 0.90 VDC and that WOT is greater than 4.50  VDC.  Record the CT setting for future reference.  If  the  EEC module senses PT instead of CT, the idle speed will appear to stick around 2000 RPM.

In this order: clean the throttle body around the  throttle plate, clean the PCV system, adjust the base idle then adjust the TPS.   If the closed-throttle angle is changed for any reason, the TPS must be re-adjusted.   If you recorded the voltage as was suggested in the last  paragraph, the adjustment will be quite quick and can be done on a cold engine. As the cars age and the throttle shafts and housing bores wear, it’s always a good idea to have engine vacuum applied when making the final TPS setting.

BASE  IDLE SETTING:
1) Get the engine stinking hot and set the base ignition  timing to 10 degrees with the SPOUT wire disconnected.  “Stinking Hot” is defined as after a ten+ mile drive.

2) Disconnect the ISC/TAB (Idle Speed Control/Throttle Air Bypass) solenoid and adjust the throttle plate stop screw to an engine speed of  750  ± 25 RPM with all accessories OFF.  If  the cooling fan comes on, wait until it stops.

3) Re-connect the ISC/TAB and wait for the engine speed to drop to normal idle. This may take a couple of minutes as it gradually brings the speed down. Let it find it's own idle at it's own pace.  Most calibrations will make a final idle speed between 900 and 975 RPM.

4) Finally, adjust the TPS as outlined above.

Happy SVOing!

 
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