TPS response...

[Schrade]

Originally Posted by Franke

Vettetech1, throttle position sensors are famous for off idle sag. They can be tested with a ohm meter to see if there is a dip in the resistance just as the sensor starts to move off idle. Its a potentiometer and varies the voltage to the ecm.

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edit:
There are NO problems with the engine, no codes, nothing.
There is only the 'off idle sag' / slow response in rpm's.
5V Voltage FROM THE TPS, back to the ECM, is LINEAR; half throttle = approximately 2.3V.
TPS resistance, in mine, is exponential / logarithmic, as snappics show here.
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What's the solution to this? Is it common also in '90?

And do you mean a SPIKE in resistance; not dip??? edit: ain't no SPIKE in mine; it's just VERY LITTLE CHANGE, as the arm swings to midpoint!!! Then it GOES. See snappics below

I give mine SLOW pedal, and rpm's still respond slowly, until it GOES...

Exactly which is the lead to test / procedure?

Alright - I pulled the TPS, and put a multimeter to the signal feed, and signal return pins.

Here are results:

BASELINE



RANGE OF MOTION PEGGED



AT LEAST HALF ROM



This gives at least 1 NEGATIONS...

1) Impedance of MY TPS is not linear (is it broken, and IS supposed to be linear??? )
this could account for the delay in rpm increase with a SLOW pedal

If my TPS IS working, then it is safe to assume that the 5V REFERENCE signal INPUT, FROM the ECM, is inversely exponential, and is calibrated for the non-linear resistance of the internal potentiometer. OR, it is calibrated AFTER the 5V REFERENCE signal RETURNS TO the ECM.

[Franke]

Schrade, since the TPS is a potentiometer it has 5 volts and ground from the ecm on the outside leads with the center lead as the signal wire. TPS signal voltage varies from about .54 volt at idle to approx 5 Volts at WOT.
According to the FSM the Blk wire is sensor ground from the ECM, the Dk Blu is the TPS signal, and the GRY wire is the 5 volt reference from the ECM. As you know the resitance should be linear so any spike/dip in Ohms is a problem as the voltage back to the ECM will fluctuate with the resistance. The solution is to replace the TPS as you probably cannot replace the guts of the TPS potentiometer. This is all from my 1990 FSM. Wire colors may be different on later years. Hope this helps.

[Schrade]

Thanks...

Is there a TSB for '90 TPS?

Do I need an osscilloscope to test it? Or just a multimeter?

[Schrade]

OK - if the TPS potentiometer response is LINEAR, then there should be a way to rig an arm of a sort, get a range of motion for the arm, then to confirm impedance at 1/4 range, half range, etc.

That is, as long as it is LINEAR...

[Franke]

Its a sealed unit from what I can see so as to protect it from the enviroment. A voltmeter or Ohm meter can be used to check it. Since it is attached to the throttle shaft then it could be verified off the car with an "arm" to watch the resistance change. Most of the defective ones i've seen are bad just off idle position. The FSM doesn't state that it is linear, that is my assumption. If it was logarithmic then the throttle response would be somewhat erratic I think.

[Schrade]

deleted to post #1...

[We Gone]

I may be wrong here but I have always tested them on the car with a jumper. they sell a test jumper to hookup so you can watch the voltage as you cycle the throttle.

http://www.ecklerscorvette.com/asset...ette/A8906.pdf

http://www.mamotorworks.com/corvette...0test%20jumper

You can also watch it on a scan tool if you have one.

[Schrade]

Went to Advance to test a new TPS; they didn't have one.

Tested a '90 'K' platform [V8] TPS (new, off the shelf), and got almost identical results.

THis calls for some modification to the throttle body / linkage cam pulley...
Which is of course, the answer to the :

Vettetech1, throttle position sensors are famous for off idle sag.

Ideas???

[Franke]

It appears not to be linear based on your arm positions. Please remember I assumed it may be linear but the FSM doesn't state one way or the other. It just states the voltage will vary with throttle position from .54V to approx 5.0 V WOT. Also, I always use a analog ohm meter to check the ohms from the potentiometer wiper so that I can actually see if there is a dip/spike in the resistance. The problem with digital meters testing pots is that they don't respond fast enough to a very small ohms change and is very dependent on how fast or slow you move the wiper arm. Usually the problem with a defective TPS is a bad spot on the pot just after you go off idle.
I know your TPS is not shorted or open as you would get a code 21 or 22 set.

[Franke]

Schrade, Something else I just read in the driveability and emissions section states that..." voltage should increase at a STEADY rate as throttle is moved toward WOT about 4.6 volts. The ECM has the ability to auto zero the TPS voltage if it between .46 volt and .62 volt. This means that any voltage between .46 and .62 will be determined by the ECM to be 0% throttle". From that statement I gather that the TPS should be linear. Correct?