1984 Throttle Position Sensor (TPS)

[muskiemike]

Hello,

Have a 1984 Corvette, with 35K on it, have had the car for 29 years, always garaged and maintained.

About 200 miles ago, the car started to have a fast idle at a cold start up (1500 RPM's) - when the car was operating at normal temps (180* coolant). (900 RPM)

Then the idle would kinda come back to normal. (900 rpm)

Yesterday, I got an "Check Engine" light, so I pulled the ECM code, and it was 21, "Throttle Position Sensor" (TPS).

With the battery disconnected, I disconnected the 3 pin connector to the TBS- the (3) wires lead looked clean, as well as the 3 pin connector, I did clean these 3 pin connector - let it dry, then reconnected it back to the TPS, and still the Check Engine light went on, shortly after I started the car.

So I am wondering, what would happen if I disconnected the 3 pin connector that connects to the TPS? What impact would that have on running the car?

Also there's a metal pedal that is part of the TPS, with the car running, I pushed down on the metal pedal, and it did "not" change the idle speed. What the purpose of this TPS metal pedal? (see attachment - where the red arrow is at.)

In reading the FSM, it says that if the TPS is replaced that the TPS and both Intake Air Control's (IAC), have to be calibrated with a voltage meter- is this correct?

Again, the ECM error code is 21, (Throttle Position Sensor TPS) - could it be some other problem other than the TPS?

In advance, thanks and all comments are welcomed.

Mike

 

[djsroknrol]

The TPS talks to the ECM and controls the pulses to the injectors....you don't want to disconnect the TPS as you won't like the "baseline" the ECM sets for you

Sounds like you've got AIC valve problems as well...check them out after you put a new TPS in....if the Air Idle Control valve solinoid sticks in, they will raise your cold start idle

 

[Hib Halverson]

TPS stands for "throttle position sensor". It does not control the pulses to the injectors. The ECM does that. What a TPS does is tell the ECM how far the throttles are open, ie: it tells the computer how hard the driver is pushing on the gas. From TPS info along with the input from the MAP sensor, the O2 sensors and other sensors, the ECM "decides" how long to pulse the injectors .

If DTC 21 has set. Clear codes and road test. If it sets again, you need the 84 Factory Service Manual and the diagnostic instructions you will find there for that fault code.

Once you've solved DTC21 and made repairs, then move onto the IAC motors (stands for Idle Air Control). There are two of them, one in each throttle body. You need to remove them and make sure the pintles are clean. DO NOT immerse them in carb or brake cleaners. Spray it on the pintle then wipe off the crud with a rag or paper towel. If the pintles are really scuzzy, you can use a medium scotchbrite pad.

 

[muskiemike]

Folks,

Thanks for the replies . . .

As previously stated, I have an 1984, and the Check Engine, code = "21" - in the FSM, it specifically says check the Throttle Position Sensor (TPS).

There were no other codes.

And why do you suggest that it's not my TPS and is my Intact Air Control (IAC) Mods ?

Again, thanks . . .

 

[Hib Halverson]

Sorry, "muskiemike", a typo. I keyed-in 16 when I meant 21 which is: TPS Signal High. Start by solving that fault code per the instructions in the Service Manual.

As for the IACs, considering the age of your engine and that the IACs have a lot to do with idle speed and stability, it makes sense to, at least, pull them out and take a look.

One of the last service acts I did with the 82 that my Wife used to own, was replace both IACs on "GP". Just doing that noticeably improved idle stability. Engine controls diagnostics were "prehistoric" in the early 80s. In spite of no IAC codes stored, it turned out that one of the IACs was faulty and that was responsible for the crappy idle that car had for years before we sold it.

 

[muskiemike]

I have the FSM . . .

If I replace the TPS with a new one, and set the volts at .54 on idle, and 5.0 volts at WOT (per the FSM).

Do I also have to set / adjust the (2) Air Idle Controls (AIC)?

 

[Hib Halverson]

I have the FSM . . .(snip)

Then follow the instructions in the diagnostic info for code 21. Based on what you find in dong that, you will either:

1) replace the TPS
2) replace the ECM
3) repair opens or grounds in wiring and connections.

IACs are not adjustable. Simply install them per the Service Manual. I bought my replacement IACs from RockAuto. Good price. Quick shipping.

By chance, if your "FSM" is an aftermarket book, such as Chilton or Haynes, use it to line your cat box then buy the factory 84 Corvette Service Manual published by GM. You can get them from CAC sponsor Zip Products.

Good luck.

 

[muskiemike]

Hib,

Thanks . . .

BTW, I do have the "Haynes" FSM . . . AND I also have a cat!

 

[muskiemike]

Hib,

Assuming I get a new TPS, how do I adjust / set the idle side at .54 volts and WOT at 5.0 volts on the TPS ?

 

[Hib Halverson]

Hib,

Thanks . . .

BTW, I do have the "Haynes" FSM . . . AND I also have a cat!

Haynes is not the publisher of factory service manuals. They publish "repair manuals" and, unfortunately, Haynes books typically are just awful as far as engine controls diagnostic info.

If you're going to DIY service on engine controls, get the factory books. CAC sponsor Zip Products sells them. That's where I get mine.

There is no WOT TPS adjustment. If the Haynes book says that, just set it next to your cat box for the next time you clean it out.

The spec. is closed throttle, 0.525v ±0.075.

Good luck!

 

[lt1george]

Hello,

Have a 1984 Corvette, with 35K on it, have had the car for 29 years, always garaged and maintained.

About 200 miles ago, the car started to have a fast idle at a cold start up (1500 RPM's) - when the car was operating at normal temps (180* coolant). (900 RPM)

Then the idle would kinda come back to normal. (900 rpm)

Yesterday, I got an "Check Engine" light, so I pulled the ECM code, and it was 21, "Throttle Position Sensor" (TPS).

With the battery disconnected, I disconnected the 3 pin connector to the TBS- the (3) wires lead looked clean, as well as the 3 pin connector, I did clean these 3 pin connector - let it dry, then reconnected it back to the TPS, and still the Check Engine light went on, shortly after I started the car.

So I am wondering, what would happen if I disconnected the 3 pin connector that connects to the TPS? What impact would that have on running the car?

Also there's a metal pedal that is part of the TPS, with the car running, I pushed down on the metal pedal, and it did "not" change the idle speed. What the purpose of this TPS metal pedal? (see attachment - where the red arrow is at.)

In reading the FSM, it says that if the TPS is replaced that the TPS and both Intake Air Control's (IAC), have to be calibrated with a voltage meter- is this correct?

Again, the ECM error code is 21, (Throttle Position Sensor TPS) - could it be some other problem other than the TPS?

In advance, thanks and all comments are welcomed.

Mike

Mike, you have to have the correct thermostat (195*) in for computer to operate in closed loop, otherwise you will have the problems you are having. Had the same problem on my 84 back in 84 when I thought I was doing the right thing by changing to a different themostat (180*) TPS sensor and IAC are are looking for correct 195 operating temp. Good Luck. Been there done that.

 

[Hib Halverson]

While it is true that some engine controls systems will not go into closed loop if engine coolant temperature (ECT) is below the OE thermostat opening temperature, that is not the problem in this case.

Neither the TPS nor IAC motors "look" for a specific coolant temperature. They can't do that because they are not devices which do any computing. They either (TPS) send data or (IAC) actuate some engine function, in this case bypass idle airflow around the throttle plates.

The problem is a code 21, which is a TPS fault with the signal high.

Depending on the what results the diagnostic procedure for code 21 reveals, the solution will be either replace the TPS, replace the ECM or fix wiring and connections.

With code 21 current, whether or not the coolant temp rises to 192-195 degrees is irrelevant because, with 21 set, the engine will likely not go closed loop, no matter what the ECT.

Solve code 21, first, then, if you determine the engine will not go closed loop, ECT is below the OE thermostat opening value and there is a lower-temperature thermostat in the engine; put the stock 'stat back in and test for closed loop.

 

[muskiemike]

Gents - all great posts . . .

Hib, Should I buy an ACDELCO TPS for 2-3X as much as a Standard Motor Products TPS ?

Does the TPS have an O-ring that connects to the throttle body?

In the pic I posted on this post, I highlighted (red) a "metal pedal" located on the TPS, - what's the purpose of this "metal pedal", does some throttle mechanism sit or connect to this metal pedal? What's the function of this metal pedal?

 

[Hib Halverson]

Gents - all great posts . . .

Hib, Should I buy an ACDELCO TPS for 2-3X as much as a Standard Motor Products TPS ?

Either the Standard or the Airtex are good choices. Get 'em at RockAuto.com

Does the TPS have an O-ring that connects to the throttle body?

It is possible the TPS has seals or o-rings to keep dirt out of it, but I"m not sure. The TPS' mechanical connection to the throttle shaft is via the throttle activator lever on the TB. That lever may not be visible with the TPS installed and that lever IS NOT the part shown in the image you uploaded with the OP. I do not know the function of that part in the picture. It is possible it may have no function for the Corvette application of those throttle bodies.

 

[muskiemike]

Hib,

Again, "many" thanks . . .

Do I have to be concerned if I order the Standard Motor Products TBS - as I may have heard bad things about this company?

Also take a look at the 2nd pic I have attached (Throttle Position Sensor-2), as I have highlighted a red arrow to this Metal Step, is this what you refer:


"The TPS' mechanical connection to the throttle shaft is via the throttle activator lever on the TB".

 

[lt1george]

While it is true that some engine controls systems will not go into closed loop if engine coolant temperature (ECT) is below the OE thermostat opening temperature, that is not the problem in this case.

Neither the TPS nor IAC motors "look" for a specific coolant temperature. They can't do that because they are not devices which do any computing. They either (TPS) send data or (IAC) actuate some engine function, in this case bypass idle airflow around the throttle plates.

The problem is a code 21, which is a TPS fault with the signal high.

Depending on the what results the diagnostic procedure for code 21 reveals, the solution will be either replace the TPS, replace the ECM or fix wiring and connections.

With code 21 current, whether or not the coolant temp rises to 192-195 degrees is irrelevant because, with 21 set, the engine will likely not go closed loop, no matter what the ECT.

Solve code 21, first, then, if you determine the engine will not go closed loop, ECT is below the OE thermostat opening value and there is a lower-temperature thermostat in the engine; put the stock 'stat back in and test for closed loop.

I found out the hard way back in 1984. The Corvette Needs the Correct 195 themostat, otherwise the check engine light will come on and the engine(computer) will "hunt" for correct idle. I had the techs pulling their hair out back in 84. Put the correct 195 themostat in and .......no more problem. Now maybe you have do a bad TPS or bad IAC valve. But you also have to have the correct 195 themostat for engine to run right. Just trying to give you alittle knowledge on problem. Nobody back in the day could figure why my idle was so "messed up". Good Luck.

 

[muskiemike]

It1-george

Thanks for the "heads-up" on the 195* thermostat . . .

The car has 35K miles on it, I bought in 1985, so it's running the original 195* thermostat.

I am planning to replace the thermostat and hoses shortly, so your comments are appreciated.

I too had been thinking about going with a 180*, since I live in Dallas, so now it's good to know, that I should stick with the 195*.

 

[Hib Halverson]

If you don't like the Standard Motor TPS, buy the Airtex or the ACD.

Much better picture this time. The one you attached to the OP was so bad it was hard to understand what you were asking about.

The TPS pick-up lever engages the throttle activation lever on the TB. That's how the mechanical link between TPS and TB shaft is accomplished.

If you're going to try a TPS replacement, first, you unscrew the old one, replace it with the new one, then adjust it per the FSM.

Once you've got the new TPS in and adjusted, clear codes and road test.

 

[Schrade]

The problem is a code 21, which is a TPS fault with the signal high.

It's a TPS CIRCUIT fault.

Most shadetree hacks don't think about the difference, and end up wasting money on Plug N PRay replace-a-part fully automatic assault wallet repairs.

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The TPS talks to the ECM and controls the pulses to the injectors....

And most shadetree hacks know EXACTLY what djsroknrol meant by the TPS [role in] controlling injector pulsewidth.

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I love teaching the 'teachers'. ESPECIALLY 'teachers' who focus on others' mistakes, instead of the issue at hand. I know of another group guilty of such behavior - politicians. THey don't have many solutions, but they can sure point out the other guys' error... :eyerole

 

[muskiemike]

Folks,

As noted in my OP, all comments are welcomed - as I may not have the technical skills to fix cars, I am eager to learn, and listen . . .

And I make mistakes along the way.

I greatly appreciate all of the comments here, as I attempt to isolate and fix car problems as they surface.

But I am not interested in any "fights", so if you want to help me fix my car problems - great, I am all ears, if you wish to pick a fight, take it to another forum.

I ordered the Standard Motor Products TPS - should have in 4-5 days, will report back. In the meantime, look forward to your comments