TPS adjustment oddity

FYI: 1985 Caprice - 305 -- Q-jet E4ME (original) -- TPS sensor less than 1yr old and properly installed/seated

Hi all,

Yesterday I adjusted my TPS to the specified voltage at curb idle (.48V). It was off by about .30 volts and the factory manual says to correct it if it is off by +/- .05V... I did this and followed everything very carefully and used threadlocker to keep the TPS adjustment screw from moving per the instructions.

The issue is that the voltage reading will NOT stay the same. The throttle will stay at curb idle and the TPS voltage reading will go from .48 volts slowly down to .42 and eventually (several mins) .38 volts.

Even when I correct it back to .48 volts it does this again, and again...

I tried this with the engine off (IGN on) and also with the engine on-- same result either way. This removed the variable of vibration affecting the TPS reading while simultaneously adding the possibility of the battery slowly (slightly) draining and thereby affecting the 5V signal/source.

Based on the time it takes for it to go down to about .42 volts I do not think the battery slowly discharging could be the cause.

The only other variable I can fathom is heat but I don't see why this would change the voltage readout from the TPS...

Also, I'm positive that the adjustment screw is not loosening/tightening from vibration because it is difficult to move with the adjustment tool (whereas before it was very easy). The choke was fully open (full operating temp) and the idle speed solenoid disabled during all testing in order to be sure that the throttle position would not change at all.

Finally, I just thought it was worth mentioning that in another section of the FSM it says that the TPS voltage reading can vary by +/- .10 volts

Sorry for the long post, thanks for your time.

What is the voltage into the TPS doing during this time span?

Those tps sensors were netorious for never being good, along with other aspects of those carbs. Most people change out to an older carb without the computer control. The variance might be from rust inside the wire changing the resistance which changes the value on a meter.

What is the voltage into the TPS doing during this time span?

I will check this tonight or tomorrow and let you know.

quote snshddog: Those tps sensors were netorious for never being good, along with other aspects of those carbs. Most people change out to an older carb without the computer control. The variance might be from rust inside the wire changing the resistance which changes the value on a meter.

I would like to keep the current electric one so that I can connect my laptop to the computer for diagnostic purposes. I believe that rust in the wire or on the contacts/connectors could very well change the original resistance of the wire, but I don't think it would be variable nor would it cause the reading (voltage) to slowly decrease.

FYI: 1985 Caprice - 305 -- Q-jet E4ME (original) -- TPS sensor less than 1yr old and properly installed/seated

Hi all,

Yesterday I adjusted my TPS to the specified voltage at curb idle (.48V). It was off by about .30 volts and the factory manual says to correct it if it is off by +/- .05V... I did this and followed everything very carefully and used threadlocker to keep the TPS adjustment screw from moving per the instructions.

The issue is that the voltage reading will NOT stay the same. The throttle will stay at curb idle and the TPS voltage reading will go from .48 volts slowly down to .42 and eventually (several mins) .38 volts.

Even when I correct it back to .48 volts it does this again, and again...

I tried this with the engine off (IGN on) and also with the engine on-- same result either way. This removed the variable of vibration affecting the TPS reading while simultaneously adding the possibility of the battery slowly (slightly) draining and thereby affecting the 5V signal/source.

Based on the time it takes for it to go down to about .42 volts I do not think the battery slowly discharging could be the cause.

The only other variable I can fathom is heat but I don't see why this would change the voltage readout from the TPS...

Also, I'm positive that the adjustment screw is not loosening/tightening from vibration because it is difficult to move with the adjustment tool (whereas before it was very easy). The choke was fully open (full operating temp) and the idle speed solenoid disabled during all testing in order to be sure that the throttle position would not change at all.

Finally, I just thought it was worth mentioning that in another section of the FSM it says that the TPS voltage reading can vary by +/- .10 volts

Sorry for the long post, thanks for your time.

I would say this is because the TPS is defective. what can cause this is carb cleaner. this corrodes the electrical parts. as mentioned this component has had many failures. carb cleaner is usually the cause.

as the resistor [tps] gets hotter the resistance increases this means a lower voltage. most likely 5 volts is locked at 5volts.

with all power voltage /connector removed move the tps and measure the resistance . use a non digital meter is best see if the movement causes a loss of continuity.[open circuit]

5.oL engine it should be .5volts....

I would say this is because the TPS is defective. what can cause this is carb cleaner. this corrodes the electrical parts. as mentioned this component has had many failures. carb cleaner is usually the cause.

as the resistor [tps] gets hotter the resistance increases this means a lower voltage. most likely 5 volts is locked at 5volts.

with all power voltage /connector removed move the tps and measure the resistance . use a non digital meter is best see if the movement causes a loss of continuity.[open circuit]

5.oL engine it should be .5volts....

I measured the resistance w/ my analog meter and I didn't find any flat spots (i.e. infinite resistance or near zero resistance) but it was a bit "bouncy" for lack of a better word. When I carefully had the throttle nearer and nearer to WOT it would increase in resistance slowly but as I barely inched it further it would decrease and then increase and sometimes decrease quite a bit.

This also happened with my orig. TPS but I have never used carb cleaner since I installed the new one about a year ago or less... Did I just buy another defective TPS?

CD Smalley, the input source voltage stays at 5.02volts exactly no matter if the engine is on or off or if the throttle changes. When I disconnect the connector to the tps and then check the source voltage (from the connector itself of course) it reads 4.82 volts but does not change.

So you are not seeing a voltage drop to the TPS as the engine heats up then?

Hmm. Then I lean towards a defective part.

yes as you move the throttle the resistance should change smoothly and not jump.

when resistance wire gets heated it does increase in value. this should not make that much of a difference.

if the voltage where 12volts instead of 5volts on the input then this would get hotter and cause this effect. I have never seen that but anything is possible.

What kind of volt/ohm meter are you using?
The old resiter type or newer digital type good one?

The curb idle TPS voltage relies on the throttle shaft position being very repeatable. It is important to verify that the high idle cam on the right side of the carb is fully clearing the high idle adjustment screw when the choke is open. This will assure that the curb idle adjustment screw is controlling the throttle shaft position and NOT the high idle screw.

It is also critical that the throttle shaft is not wandering in the throttle body. I'd bet a nickel that the throttle shaft bushings are worn and the throttle position is not repeatable due to the throttle plates guiding the shaft to different positions every time the throttle closes.

You should also inspect the pivot arm, pivot pin, and linkage rod for the accelerator pump arm for wear and looseness.

Personally, I would agree that it is worthwhile to maintain the E4ME and not revert to the fully mechanical carb. Removing the E4ME and doing it properly would require a new distributor, a new scheme for converter lockup control, and new EGR control method.

The curb idle TPS voltage relies on the throttle shaft position being very repeatable. It is important to verify that the high idle cam on the right side of the carb is fully clearing the high idle adjustment screw when the choke is open. This will assure that the curb idle adjustment screw is controlling the throttle shaft position and NOT the high idle screw.

It is also critical that the throttle shaft is not wandering in the throttle body. I'd bet a nickel that the throttle shaft bushings are worn and the throttle position is not repeatable due to the throttle plates guiding the shaft to different positions every time the throttle closes.

You should also inspect the pivot arm, pivot pin, and linkage rod for the accelerator pump arm for wear and looseness.

Personally, I would agree that it is worthwhile to maintain the E4ME and not revert to the fully mechanical carb. Removing the E4ME and doing it properly would require a new distributor, a new scheme for converter lockup control, and new EGR control method.

your right on the throttle shaft bushing being worn would upset the adjustment as well as a constant correct idle RPM.

Wow thanks for all of your input. I used an analog meter for checking the resistance and a digital multimeter for checking the voltage. They also have new batteries and I'm sure are working fine. I actually have an issue with the bushing for the primary throttle plate (I think) and part of it is actually cracked... I will have to check the fast idle cam and order a new TPS again tomorrow.

FWIW, the resistance element in the TPS is not wirewound, but is a carbon film, so temperature is less of an issue. GM thought that one through about three decades ago.

When you are ready to fix the carb, check out THIS THREAD (http://www.automotiveforums.com/vbulletin/showthread.php?t=979533)

Wow thanks for all of your input. I used an analog meter for checking the resistance and a digital multimeter for checking the voltage. They also have new batteries and I'm sure are working fine. I actually have an issue with the bushing for the primary throttle plate (I think) and part of it is actually cracked... I will have to check the fast idle cam and order a new TPS again tomorrow.
You are welcome.
Sounds like you and posters to your reply have found you problem.
The analog volt/ohm meter can be used for a resistance check but only a digital volt meter should be used checking any PCM/ computer voltage or computer resistance testing.
The high resistance in the old analog meters can damage the computer circuits.
Not safe for computer voltage test.
But to get down to the nitty gritty on tps sensor tread out and testing only a good lab scope will show the TPS fine/true voltage readings and dropouts that a tps sensor may have.
The digital volt meter will not show dropouts in voltage very good.
Also disconnect the idle kicker solenoid and or fast idle stuff when checking TPS sensor voltage.
Good luck and let every know how it goes.

Actually, it a low resistance (ohms/volt) that can overload CMOS and older TTL circuits, but we knew what you meant.

Also, analog versus digital is no so much a question as as the resolution and calibration of a meter. I'll take a good, accurate analog VTVM over the newest, fastest, bestest EVAR Fluke digital out of the box. Digital meters read and resolve in steps (sample rate) whilst analog meters are, well, analog. Analog meters split hairs where digitals have to "step over" them.

In short, don't condemn all old meters just because they are not new. Remember, nothing with digital electronics has ever been to the moon and back.

Actually, it a low resistance (ohms/volt) that can overload CMOS and older TTL circuits, but we knew what you meant.

Also, analog versus digital is no so much a question as as the resolution and calibration of a meter. I'll take a good, accurate analog VTVM over the newest, fastest, bestest EVAR Fluke digital out of the box. Digital meters read and resolve in steps (sample rate) whilst analog meters are, well, analog. Analog meters split hairs where digitals have to "step over" them.

In short, don't condemn all old meters just because they are not new. Remember, nothing with digital electronics has ever been to the moon and back.

10-4 on that.
Just bought a Snap on Low amp probe EETA308D to use on the Snapon vantage 2500.
Dang thing needs driving me nuts trying to read out low mil amps.
Even read instruction manual 10 times but still a pain.
MT