Hello. Looking for experience and wisdom. I have the famous reduced power mode on a 98.
Codes 1120 1220 1516 1518
Checked tps. Good
My main question is.... with the ignition on.... will the throttle blade move. This car does not. Car will start but will not Rev at all. I am leaning towards throttle actuator motor on driver side throttle body or the module under pass tire Does anyone know how to test the throttle motor voltage range? Is there a specific test to show if module is fried? Car is in a friends garage since that is where it quit...... I really wish it were home where I have a 2 post lift and my tools..... Working out of my comfort zone thanks in advance.

 

Might be the TAC (throttle actuator control). You're profile doesn't help out much. No idea of your model or year. But others have had this issue and it was caused by a variety of issues. One guy found corrosion in his connection the to TPS. Cleaned it and was good to go. Others were only perm fixed by replacing the TAC module. However, they did state they could clear the codes and still be able to drive for a while. YMMV....

 

I will update profile. It is a 98 6spd. D1sc. Procharger. Cam. FAST intake kooks headers. Approx 70k miles.

Tps connection is good. Voltage gets to module.

Is the throttle blade. Supposed to move when pedal pressed and car not running....just key in on position?

 

Also I am unable to clear codes to drive.

 

There were problems with the 97-98 TAC modules and GM did an upgrade for 99-04. If you find that is the issue you can use the later ones, no problem.


Mr. Sam

 

Also. Tps. Voltage is reading lower than service manual calls for. Seems like throttle body blade will not pull back initially. So kicks codes for voltage being out of range.
Soon as I press gas pedal I get the 1516 code because the computer says the gas pedal sensor doesn't match where the throttle plate is.

That is my theory. Now someone sell me a good working throttle body motor and we'll test theory

 

TAC modules are about $200 used to $300 for new on the auction site. Do you have a buddy with a C5 that you could borrow his TAC module to see if that fixes the issue? If you think its the TPS, check this out. https://www.corvetteforum.com/how-to...-sensor-368700 Fortunately, the TPS is less expensive, maybe $35-40. And it could still be in the wiring somewhere. The modules could be good. Just need to do some testing. If you do replace the TAC, use the later module from the 01-04. They're interchangeable.

 

In reduced power, the TB will not move. Clear the DTC and messages and see if they come back.

Check the actual voltage on the TAC Module FUSE test points and see if you have full battery voltage.

Any other DTCs of messages like NO COMMS????

BC

 

Bill Curlee, So glad you chimed in! Sounds like you are a living legend with C5 electronics.

Thank you for the info on the TB not moving. The codes immediately come back when cleared. TB sensor LOW 1 out of range. TP Sensor 2 out of range. P1120 and 1220.
As soon as I press pedal I get P1516 of course.

There is no other message such as NO COMM.

Can you tell me where the TAC module FUSE test Points are? Simple Fuse 17 in the panel??

Bill, I will be working on the car tomorrow night. (Monday) Is there any chance I could have you PM me a contact number if I get into a bind? Gladly feed your bank account for your time

 

Each FUSE has TWO small slots on top of the fuse. Those slots are TEST POINTS.. Test each one to ground. BOTH should have FULL BATTERY voltage if the fuse is GOOD.

97 & 98 C5s had documented WEAK/inferior TAC modules. There were TSBs that had the TECH replace the module if the DTCs that you have were displayed. Any 99-04 TAC module is a up-graded replacement for the early bad TAC modules. Before you replace the module, see if the module has a 97-97 module OR a 99+ replacement module What is the number on the module?

If you have good voltage at the TAC module fuse and read a good ground reading,,, the TAC module could be bad.. The TAC Module is mounted on the outboard side of the PCM in the passengers fender well.

OR

The Throttle Position (TP) sensor is a potentiometer. The TP sensor provides a voltage signal that changes relative to throttle blade angle. This signal voltage is one of the most important inputs used by the Powertrain Control Module (PCM). The TP sensor has a 5.0 volt reference, a ground and a signal circuit. It has TWO outputs in our Throttle By Wire system.

Do you have the proper 5 VDC supply voltage on the Throttle Position Sensor???

Check the Throttle Blade for free movement. If you have the ignition OFF and PUSH it open and let it go, it have a little spring resistance force and should FLOP back to a near closed position when you let it go. It will NOT flop completely closed.

If you turn ON the ignition, the TB should errect to the last known good idle position.

The TPS has TWO varaible resistors inside the sensor. There outputs are OPPOSITE in voltage reading outputs. The PCM expects to see the two opposite voltages as a CHECK SUM reading. If it doesnt,,,, it will put the PCM in Reduced Power so you dont get a stuck open throttle.


You MUST make sure that the TPS is working correctly.

The wire harness that runs down the drivers side of the engine on top of the manifold can get chaffed on the metal bracket at the back of the fuel rail and short out wires. The TPS wires are a PRIME suspect inside that harness!! Check those wires for damage at that bend point.


PM SENT

BC

 

Thanks in advance for any input. I appreciate it dearly.

okay, I borrowed an enclosed trailer low enough to get the car home.
printed out a 97-94 corvette electronic throttle control system diagram and got to work.

here are the voltages I got. I used a digital multimeter and grounded to the car battery with a gator clip.

1. At the throttle position Sensor harness (close to throttle body)
key on.
dk green 5 volt ref 5.01 volts
dk blue tp1 signal 0.65 volts
pink tp2 signal 3.9 volts
yel/blk 5volt ref 1.13 volts

2. key on, throttle blade opened nearly 100%
dk blue tp1 signal 4.10 volts
pink tp2 signal 0.94 volts


3. then I pulled passenger tire and splash panel to access Throttle Actuator Control Module wiring harness

yellow tac motor control 11.9 bat volt
brown tac motor control 11.9 bat volt
dk green 5 volt ref 4.01 volts ***
dk blue tp1 signal 1.06
pink tp2 signal 5.02

4. then checked continuity of white/black ground. it beeped.

strange readings..... how could dk green have 4 volts right out of module and 5 volts at tps sensor.
also the signal voltages changed from checking them at tps to checking them at the module.


5. I clear the codes and immediently get back
p1120 hc tp1 sensor out of range
p1220 hc tp2 sensor out of range.
p1221 hc
p1516 hc

doing this 3 or so times I also got a p1518 in there.

keep in mind a did swap out tps sensor, old one is back in because symptoms never changed.

opinions on this? What does the low Ref white and low ref purple wires do? what voltage should they check out?

 

Tell me all the DTC codes you would like the diagnostics on and will post them for you. They will have all the info for testing.

 

Thank you
P1120
P1220
P1516
P1518

 

The Throttle By Wire System uses descrete circuits. That means,,,, They DO NOT use chassis grounds as the return path to circuit ground in the calculation circuits.

You have a REFERENCE Voltage 5 VDC and a Low Ref Return to circuit card ground

On one of your readings you posted " yel/blk 5volt ref 1.13 volts"

If,,,,,,,,,,,,,,,,,,, its a 5 VDC REF voltage,,,, It should be 5VDC..


TSP=1 and TSP=2 are the variable resistor pick offs that should be equal and OPPOSITE voltages.

Depending on the state of battery voltage or voltage available at the TAC FUSE the reff voltage may be lower than exactly 5 VDC.

dk blue tp1 signal 0.65 volts
pink tp2 signal 3.9 volts

These two voltages are suspect!!!!!



Bill

 

Aw,, heck..... I thought I read it was 0.39 volts...... Never mind on that ststement..



If you have yel/blk 5volt ref 1.13 volts ,,,,,,,,,,,,,,,,,,,, the REFF Voltave is being drawn down because of a bad component in th circuit.

Bill

 

By bad component..... you mean TAC module?

 

Bill , I came across a write-up several days back that you had posted on how to Check for a bad TAC module.
I hand wrote the instructions and now can't locate the thread.
Does this sound right?

remove Fuse 17
Disconnect c2 connector on TAC
Measure pin 7 to ground should read infinity.

c2 Pin #7 to fuse holder - then I can't make out what I wrote. lol.

 

Should read ZERO OHMS... You are reading #7 back to the fuse holder to insure that you have a good power from the fuse to C2.

You can install the fuse and turn on the ignition and read battery voltage at #7 to chassis ground..

My thought is,, when you LOAD DOWN the yel/black reff voltage side, it dropping down to something lower than true reff voltage. REFF VOLTS should ALWAYS read 5 VDC!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Regardless of what you do to the throttle position. IMHO,, thats where you are triggering the REP..

BC

 

[QUOTE=Bill Curlee;1590703017]Should read ZERO OHMS... You are reading #7 back to the fuse holder to insure that you have a good power from the fuse to C2.

You can install the fuse and turn on the ignition and read battery voltage at #7 to chassis ground..

My thought is,, when you LOAD DOWN the yel/black reff voltage side, it dropping down to something lower than true reff voltage. REFF VOLTS should ALWAYS read 5 VDC!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Regardless of what you do to the throttle position. IMHO,, thats where you are triggering the



Bill. I screwed up. I was checking the wrong connector at the module. I'll regroup and post findings......

 

This link has the codes. If you need anything else such as schematics or connector views let me know.

https://www.dropbox.com/sh/u24x0ojdb...FcM4dqaIa?dl=0


Mr. Sam