Throttle Position Sensors

Problem

At 50-75k miles depending on driving style the potentiometer in the TPS may emit volatage spikes causing a Code 12 to be logged. Symptoms are a general lugging, miss-firing, poor performance and such. The condition of the ignition leads should be checked first, along with Code 12 verified.

Cause

Throttle movement wipes across a potentiometer track, wearing a groove according to your particular "driving pattern", eg, usually 2250, 2750 & 4250rpm. When driving behaviours moves outside this range there is a glitch, felt as an acceleration dip - yet pressing the pedal to the floor sharply a second time will achieve full acceleration.

In a code 12 condition computer goes into limp-mode & performance drops off with the engine losing it's rapid-response & smoothness. It can also buck under deceleration (the TPS is also part of the anti-buck programming).

Solution

A TSB exists regarding sensor adjustment, although it is possible for the TPS to be worn sufficiently to require a new one. The problem is that a Voltmeter may pass the sensor, but the sensor may still produce glitches only visible on an Digital Storage Oscilliscope rather than a smooth voltage transition as it wipes the carbon-track. Very few non-UK cars appear to be affected, and it is a relative failure in itself. The TPS should be adjusted if suspected but no Code 12 has been logged by the ECU.

Adjustment

The Throttle Position Sensor actually comprises two sensors within it, the Idle Switch and the Throttle Position Sensor itself. Both devices must be in specification for proper operation, if both can not be set in spec then the sensor itself is out of specification and must be replaced (circa £90/70$US).

Tools required:
Voltmeter (covering range 0-5V)& Continuity Meter are required, along with feeler guages of 0.15mm & 0.50mm. The procedure should only be done by people familiar with electronics.

Setting TPS portion of TPS:
1. Leave engine keyswitch in "ON" position to power the ECU, but engine "OFF".
2. Ensure handbrake is properly applied and no metal watches etc are worn.
3. Locate TPS under the bonnet/hood, there are two 10mm bolts with philips screwdriver centres, these will be loosened and the sensor rotated to effect adjustment.
4. I am only going to list adjusting the TPS via working at the ECU end, as it requires only one person. It is possible to adjust the TPS at the TPS end (harness must remain plugged into sensor) but inserting probes into its connector can cause damage or moisture entry points, as well as risking an accidental short and such like.
5. Locate the ECU: unscrew gearknob, unclip trim under handbrake by lifting it up at the rear, slide this plastic trim backwards with the cut-out U going around the handbrake lever. When this is moved backwards as far as possible, ease front part out and away from the dash trim and swing it to the left or right carefully (it will fit just) and there you will see the ECU with three yellow connectors in front of you.
6. Connect the voltmeter ground to a point on the chassis, the screws holding the ECU are grounded. The signal/+ve signal of the voltmeter (0-5V setting) is to be connected to ECU terminal 2F, the ALL-YELLOW wire on the bottom row near the right of the MIDDLE yellow connector to the end. Before touching this, ensure the voltmeter is set to VOLTS and not Amps.
7. With your foot off the accelerator, at Fully Closed Throttle the voltage should be 0.1-1.1V. With the accelerator floored, the voltage should be 3.1-4.4V. If it is out of this range, loosen the bolts on the TPS and rotate it slightly. Now we have to address the Idle Switch component which must simultaneously be in specification.

Setting Idle Switch portion of the TPS:
It is critical that the Idle Switch is in spec otherwise the car will act as though it has cruise-control which is only disabled via the brake or time.

1. Ensure ECU/ignition is OFF and the harness is unplugged from the TPS itself.
2. Remove the Airbox/VAF and black rubber tube before the Throttle-Body, take care not to damage the VAF.
3. Remove the harness from the TPS and locate the 4 pins inside it.
4. Set the Voltmeter to a Resistance or Contuinity Range. Connect the two probes across the two bottom terminals of the sensor - note there is no power to the sensor thus no damage can result (this is the benefit of adjusting the TPS component of the TPS-adjustment at the ECU end of the harness).
5. With a 0.15mm feeler gauge between the throttle plate stop and the throttle arm (end of throttle cable, back of Throttle Body), the guage should indicate continuity. With a 0.50mm feeler guage in place there should be no continuity. If this is not the case, rotate the TPS slightly to achieve this, then you must repeat the first procedure to verify the TPS section is still in adjustment.

It is critical that the Idle Switch is in specification. If you can't get both in specification (takes patience & time) then the TPS is faulty and a new sensor is required. The TPS is used to set ignition timing & advance in conjunction with the knock sensor, anti-bucking on deceleration, acceleration and in the application of the various VRIS settings which alter the torque profile of the car via it's variable rate tuned port intake runners. It is also used in achieving a 650rpm +/- 50rpm idle (Idle Switch), helping emissions & fuel economy. On ATX cars gearbox operation, if you do not adjust it correctly to within specification you will not achieve proper engine & or gearbox performance.

When finished reconnect everything, including VAF. If you suspect a TPS code has been set (Code 12) you should disconnect the -ve terminal from the battery for 2 minutes to clear the ECU of codes. The car will take 20mins to relearn its idle when reconnected, however normal driving over a few days is sufficient for this.

The caveat:
Ideally when testing the TPS part of the TPS sensor, instead of a voltmeter a Digital (Storage) Oscilliscope should be used set with a very slow timebase to show the waveform of the varying voltage as the accelerator sweeps from no-throttle to full-throttle. The reason is that TPS sensors carbon tracks can wear into a normal-driving groove and glitch, giving an eroneous voltage spike or drop-out which confuses the ECU - yet it might not set a code 12 initially.

A typical sign of a miss-adjusted Idle Switch is the car only slowly decelerating at Zero-Throttle from 2000rpm and below, and a wandering or high idle (as the car is getting no Continuity from the Idle Switch). A typical sign of miss-adjusted TPS component is the car will not accelerate fully at WOT, or will buck on throttle lift-off. A faulty TPS will give a code 12 or bouncing WOT a few times will give WOT.

Final tightening torque of TPS adjustment bolts is 1.6-2.3Nm which is very low - snug tight (aluminium is soft).

Cost:

TPS Dealer adjustment: £39/60$US, TPS replacement £94/145$US (or less from Roebuck Mazda, circa 70-80$US).

Last Upload: 31st January, 2002. V1.50a
URL: http://homepage.ntlworld.com/dorothy.bradbury/probemx/p_p9.htm