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98 Discovery Throttle Position Sensor Diagnosis

14K views 0 replies 1 participant last post by  nissandon83 
#1 ·
This is a guide to help diagnose a Throttle Position Sensor. NOTE: throwing a code for this part is rare and it can only correctly be diagnosed by either Test book or a Live data OBDII CAN code reader. Code readers can be purchased at Harbor freight or on ebay for under $80 now with live data. These work wonders. Just make sure you follow the connect and disconnect instructions so you don't fry your ECM or ECU.

These sensors typically go out at around 110k to 150k or more. Just don't touch it or wash your engine or get it in mud ( so let your wife drive it).

Symptoms: Hesitation an high idle are common. If you floor the thing it might die down or sputter. Also stalling may occur. Miss-fires at cruising speeds have also been seen.

Diagnosis:
Old school method: Check your TPS using a volt meter set on ohms. Many videos on youtube for this test.

OBD II Code reader:
Go to View Live data and view the RPM's and the TP% (Throttle position percentage) You will see the TP jump and then the RPM's.

The data may be 5-7 at idle but it should be CONSTANT unless you push on the throttle.

The graph view should not change unless you press the throttle or your throttle is stuck. The graph is the best way to test this because you get a real-time view.

With the engine off but in position 2 ( lights on, View live data and press the throttle to the floor. Your reading should vary by only .05%. If it goes from 7.0-5.0 you have a problem. The WOT (Wide Open Throttle) should read 7.0 or higher on the scanner and it may fluctuate with your foot.

Try moving the throttle slowly to full and see if the meter drops. Mine would go from .6 at idle to 1.0 then to 2.0 then drop to 1.0 and go back up again.

A new Throttle Position sensor with lifetime warranty was $86.19 with CA tax. Part number TPS4126.

HARNESS CHECK:

You should also check the voltage on your harness to the TPS with a volt meter. It should be a constant also. If it varies, you have a wiring issue and it is not your TPS. Check grounds first on battery and lambda sensors.

To do this, ground on the battery and test all the pins on the harness. One should read 5-12 volts and it should be CONSTANT. this voltage should not jump at all with a good meter set to 20 volts.

I plan on having a video and pictures posted as soon as next week (02-15-12) My next day off
 
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