rdelboux

My c5 a4 1998 has been in the garage for the past four months waiting a solution from its dedicated owner. I live in Brazil, and there are no parts for corvettes available in the country. Sometimes I can find a similar part by size, but most of the time I have to wait for some months to get parts I order (for friends to go to USA and bring them or to have them posted here).
Codes are P1125 (APP System), P1275 (APP sensor 1 circuit), P1280 (APP sensor 2 circuit), P1285 (APP sensor 3 circuit), all suggesting my pedal sensor is bad.
Car starts but stays in idle with the message "reduced engine power" (and also traction control and ABS error, but this should be something else) and there is no response from accelerator.
To try to fix it I first opened the pedal sensor to check how it was made and if it was ok inside. Because i opened it, I broke some traces (they are made of ink instead of cooper and are very fragile) and they had to be repaired. I used silver conductive ink and after several attempts (traces are so thin!) I was able to fix it. The resistance from the three sensors seem to be working ok. Below are the values I read with the multimeter.
Because it did not work, I then bought another pedal sensor. No good also.
If one looks at the service manual they tell you that "when the TAC module detects a problem within the TAC System, more than one TAC System related DTC may set. This is due to the many redundant tests run continuously on this system. Locating and repairing one individual problem may correct more than one DTC".
I interpret this as a way of saying "the TAC module does not know what the problem is".
The service manual tells you to use the scan tool TechII in the TAC parameters menu and follow the disagree parameters, because this scan tool can see the voltages of the 3 individual sensors and the comparison between them (disagree parameters). There should not be a disagreement of more than 10.5% between sensor 1 and 2, and of 13% between 1 and 3.
I do not have the TechII, only the HP Tuners, but using the HP Tuners is not able to read such individual parameters. This is something folks in HP Tuners should still work on.
I then concluded that I had to buy a new TAC, because the decision tree in the service manual is to either change the APP sensor, the harness or the TAC module. I bought another used tac (newer) and problem continues.
I tested continuity for all wires from pedal to tac, and they seem ok. Tested continuity from the other 16 pins connector that gets to tac (cruise wires, throttle actuator motor, tps sensor, 12v, gnd and data link to ecu) and they are showing continuity to their destination.
I am now in the destructive and nervous fase and I cut out the connector of the pedal sensor and the connector of the tac, redid the connection bypassing car harness and problem remains. So I am 100% certain that the problem is not in the APP sensor and is not what TAC is indicating.
I then went into the second connector of the TAC and cut the 4 wires from cruise control that arrive at the tac to see if they are causing some badly diagnosticated problem within tac. Nothing got better (just got another code for the cruise).
I intent to fix the cuts with a soldering iron, insulation tape and maybe silicon glue and thermo shrink pipes to isolate the cut from any moisture and water.
Continuing my disgrace, I took out the throttle body and disassembled the tps sensor. I tested the 2 sensor terminals with resistance measurement in the multimeter and even though I found out that the center of the sensor is a little too loose, the 3 pins of each sensor have a varing reading according to position, approx 3.5k ohms to 0.6k ohms. I could not find any short or open circuit.
Then i disassembled the actuator motor. It is a simple constant current motor and there are three reduction gears to make the motor be capable of turning the butterfly. Out of the throttle body the motor is spining freely. The reduction gears are good, even though the spring that keeps the butterfly closed is very strong, which made me think that maybe the tac module would not be able to handle the current the motor will require. also, to keep the butterfly in a certain position, the motor needs to be energized and the demand of current needs to transfer power to the motor just enough to equalize the strong spring. The motor will spin continuously both sides if attached to power out of the body (I tested with 5v, but I believe the Johnson motor is rated 12v). It is strange that in the car the actuator motor is producing a fine continuous beep (or vibrating noise) for half a minute when key is turned to ON, I guess this is a result of the frequency of On and Off the Tac module may produce (instead of applying constant energy) to keep the motor in the same position. After a while the tac module takes out the power and butterfly closes because of the spring.
I was wondering if the problem could be the tac not talking to ecu, but it is, as the ecu would indicate lack of response if it wasn't and it receives the instruction to keep the "reduced engine power" and the APP sensors error codes were set and cruise codes set when I cut its wires.
I am out of ideas.
I am going through the route of bypassing all connector and harness for the TAC and maintaining the direct connection from pedal sensor to tac that I had made and create a new one from 2nd plug in the tac module to actuator motor, tps sensor, battery, gnd and ecu data.
I have found suitable replacement connectors (not perfect but makes the connection) for the actuator motor and tps sensor. I still need to find in Brazilian car a similar connector that will fit the 16 pins 2nd connector of the TAC (I found out the specs of such connector and it is a connector with 1mm pins, spaced 3mm from each other in the roll, 2nd roll 3.5mm from the other).
Also I am interested in intercepting the dedicated data communication between tac and ecu to try to see how it works, if it is vpw j1850 like the rest or something else...
I hope that I am not required to buy a new ecu.
An electrical mecanic told me that sometimes is some other sensor in the car that is attached to the ECU which also uses 5V that is messing with the 5V reference of the ECU. However, I believe that the communication to the ECU is not analog, it should be digital, so the problem should reside in the TAC.
Maybe the 5V reference for the TP sensor is causing some error. I will check if the replacement of the harness will be enough.
I guess I will have to find a trustable test to discart error in the TP sensor and actuator motor. Replacing the throttle body may be a solution.
I dont have codes for the throttle body (TP sensor and actuator) but I believe that if the motor is requiring too much current that the module cannot provide (a problem that is similar to the collumn lock error) maybe the TAC module could be going crazy....
If anyone has ideas I would love to hear. (please don't post the service manual instructions for the referred DTC codes as I have them).
If I wanted to go old school and get back the accelerator cable, which is possible in the LS1, instead of drive by wire, because the TP sensor is attached to the TAC rather than to the ECU, I assume that the TAC is also required, correct? but a different TAC type... Don't know if that would be a solution to my problems...

Accelerator Pedal Position (APP) Sensor - Resistances

98 76 (looking at the APP plug)
12345

Sensor 1 - (678)
minimum position:
6-7 5.5k ohms
8-7 3.5k ohms (fixed)
6-8 1.9k ohms

maximum position:
6-7 1.9k ohms
8-7 3.5h ohms (fixed)
6-8 5.5k ohms

Sensor 2 - (234)
Minimum position:
3-4 0.64k ohms
2-3 4.2k ohms
2-4 3.6k ohms (fixed)

Maximum position:
3-4 4.2k ohms
2-3 0.67k ohms
2-4 3.6k ohms (fixed)

Sensor 3 - (159)
Minimum position:
1-9 7.3k ohms
5-9 2.5k ohms
1-5 5.7k ohms (fixed)

Maximum position:
1-9 2.5k ohms
5-9 7.4k ohms
1-5 5.7k ohms (fixed)


Throttle Position Sensor - Resistances - ABCDEF

Position 1

AB 3.02k ohms
AC 3.59k ohms
BC 0.66k ohms
--
DE 3.02k ohms
DF 0.41k ohms
EF 3.40k ohms

Position 2

AB 3.02k ohms
AC 0.53k ohms
BC 3.58k ohms
--
DE 3.02k ohms
DF 3.50k ohms
EF 0.48k ohms


-----------

Throttle body actuator motor - Resistance

5 ohms

-----------

Connectors part numbers from Accelerator pedal, TAC, Throttle body position sensor and actuator motor:
Supplier: Mouser (you can find in EFIConnection the repair harness (connector+wire) with bigger prices)

Accelerator pedal & TAC :
#: 12124264 Delphi - Automotive Connectors SECONDARY LOCK MEDIUM GRAY - $0.37
#: 12065425 Delphi - Automotive Connectors 10P FM BLK CON ASSY 150 SERIES 14 AMPS $2.67

TAC (2nd connector):
#: 12191067 Delphi - Automotive Connectors SECONDARY LOCK BLACK $0.38
#: 15460298 Delphi - Automotive Connectors STRAIN RELIEF LOCK GRAY $3.45
#: 12191065 Delphi - Automotive Connectors 16P FEMALE NATURAL 100W SERIES 7.5AMPS $1.50

Throttle actuator motor:
#: 12077900 Delphi - Automotive Connectors 2P FEM BLK CONN ASSY 280 SEREIS 30 AMPS $1.78

Throttle position sensor:
#: 12162261-B Delphi - Automotive Connectors CON MP 150 .2 6W FEM ASY $3.64

CactusCat

Glad you started your own thread. Let's get to work on your issue. I did some quick research and found this. I'd like for you to check yours out and see if this will resolve your problem.
https://www.corvetteforum.com/forums...280-p1281.html
With all the cutting you've done on your wiring, you might have this exact issue.

CactusCat

Also, while looking at the above, when you're checking the module connectors, verify that there is no corrosion inside them and that the male/female connectors are making good connection. GM was bad about using poor connectors. I would also assume you've checked the "normal" stuff like water damage, dampness, etc.

CactusCat

There was also GM bulletin on this issue for 97-98 Corvettes.
Document ID# 410906
Subject: REDUCED ENG. POW. MESSAGE ALONG W/ PARTIC. PWRTRAIN, DTCs P1516, P1518 and/or P1125 #87-65-16 - (03/24/1998)


SUBJECT: REDUCED ENGINE POWER MESSAGE ALONG WITH PARTICULAR POWERTRAIN TROUBLE CODES (INSTALL NEW TAC-MODULE IF STRATEGY IN SERVICE MANUAL FAILS)

MODELS: 1997-98 CHEVROLET CORVETTE WITH 5.7L V8 ENGINE (VIN G - RPO LS1)

CONDITION #1: INTERMITTENT REDUCED ENGINE POWER EVENT, ACCOMPANIED BY P-1516, OR P1518 TROUBLE CODES.

IF A SITUATION ARISES WHERE A P-1516 TROUBLE CODE IS ACCOMPANIED BY A

P-1120 AND/OR P-1220 TROUBLE CODE, THIS SERVICE BULLETIN DOES NOT ----------------------------------------------------------------- APPLY. IN THESE CASES THE TECHNICIAN MUST USE THE NORMAL STRATEGY ------------------------------------------------------------------ BASED DIAGNOSTIC. -----------------

CONDITION #2: INTERMITTENT REDUCED ENGINE POWER ACCOMPANIED BY TROUBLE CODE P-1125 AND ANY OF THE FOLLOWING TROUBLE CODES:

P-1276 P-1281 P-1286

CAUSE:

CONDITION MAY BE DUE TO DIAGNOSTIC SOFTWARE AND CALIBRATION IN THE TAC-MODULE WERE NOT OPTIMALLY UTILIZED.

CORRECTION:

IF THE ABOVE CONDITIONS CANNOT BE CORRECTED USING THE NORMAL STRATEGY BASED DIAGNOSTICS LISTED IN THE SERVICE MANUAL, INSTALL THE NEW TAC- MODULE LISTED BELOW.

PART INFORMATION:

NEW TAC-MODULE PART # 25319443

PARTS ARE EXPECTED TO BE AVAILABLE ON APRIL 6, 1998 FROM GMSPO.

WARRANTY INFORMATION:

FOR VEHICLES REPAIRED UNDER WARRANTY, USE:

LABOR OPERATION: J-5582 LABOR TIME: 0.5 HR

FIGURES: 00 ATTACHMENTS: 00

GENERAL MOTORS BULLETINS ARE INTENDED FOR USE BY PROFESSIONAL TECHNICIANS, NOT A "DO-IT-YOURSELFER". THEY ARE WRITTEN TO INFORM THOSE TECHNICIANS OF CONDITIONS THAT MAY OCCUR ON SOME VEHICLES, OR TO PROVIDE INFORMATION THAT COULD ASSIST IN THE PROPER SERVICE OF A VEHICLE. PROPERLY TRAINED TECHNICIANS HAVE THE EQUIPMENT, TOOLS, SAFETY INSTRUCTIONS AND KNOW-HOW TO DO A JOB PROPERLY AND SAFELY. IF A CONDITION IS DESCRIBED, DO NOT ASSUME THAT THE BULLETIN APPLIES TO YOUR VEHICLE, OR THAT YOUR VEHICLE WILL HAVE THAT CONDITION. SEE A GENERAL MOTORS DEALER SERVICING YOUR BRAND OF GENERAL MOTORS VEHICLE FOR INFORMATION ON WHETHER YOUR VEHICLE MAY BENEFIT FROM THE INFORMATION.

COPYRIGHT 1998. GENERAL MOTORS CORPORATION. ALL RIGHTS RESERVED.

CactusCat

I was re-reading your original post in the other thread. Both in that one and this one, you haven't told us how this started. It would really help to describe when and exactly what was going on when you discovered the problem. Did you get the TAC module or its connectors wet? Did it just appear out of the blue? Were you driving the car and it appeared? I want you to re-check your wiring. See Post #2 above and read that link. Your problem might have originally been a bad PPS but you took it apart, broke it, did some repairs, put it back together then (from your other message in the other thread) "I then cut out the connector of the pedal sensor and the connector of the tac, redid the connection bypassing car harness and problem remains. I then cut the wires from cruise that arrive at the tac to see if they are causing some badly diagnosticated problem within tac and nothing got better." You've replaced the PPS but you might have gotten the wires backwards when you did the cutting. Please verify and report back and let us know how this all started.

rdelboux

Thank you for the reply. Here is the history of my "project car". It is a "salvage" and it has been rebuilt twice in the US. From what I can tell it suffered a side collision that affected the driver door, but it must have had other things. I was not able to obtain the police report in the US for such accidents so far.
In Brazil it is not possible to import used cars. However this was an auction purchase. As it came running to a northern state of Brazil through the Venezuelan border with a US license plate (Massachusetts). After overstaying its temporary permit, it was apprehended by the Brazilian IRS. Stayed 4 years in the parking lot of the IRS (under rain and 100oF of temperature) and was finally sold to me in April 2014. It has 30.000 miles and it was in need of deep cleaning, a complete repaint, new seat covers but mechanically it was good, only the gas pump was stuck. I changed the battery, spark plugs, motor oil, air filter, wipers, brake pads, serpentine and A/C belt, pulleys (that were squeaking), drained the gasoline, fixed the gas indicator (need to change it to a new one, as it is not very reliable), drained the coolant, took out and had fixed the a/c compressor after taking out the water pump. The front window was broken and the driver outside mirror was missing. I had the collumn lock problem, the rocking seat, but those were simple fixes that teached me to disassemble basic things, and I am a very pacient guy.
It is being an interesting experience, as I like but don't have mechanical experience and the corvette is full of DYI explanation on the web. I like electronics, so I am enjoying learning how the EFI works and how to do the tuning.
The paint shop kept the car for 6 months to do the complete repaint. There was a crack in middle part of the firewall which caused leaks to the interior of the car (probably caused by a crash and the repair in the US did not replace the firewall). I had a person to disassemble the interior (including HVAC box) to fix such crack in the firewall. The paintshop left the car in a very dusty area without the hood and the dust got into all parts of the underhood. Then, in order to deliver the car back to me, they sprayed the motor with a hot fine mist and cleaning product (without water, which they said is indicated for motor cleaning), which I think may have caused the problem. Initially this problem appeared in the paintshop. I had the car towed to a mechanic shop that was not able to fix it, and when I was present in the mechanic shop the accelerator problem disappeared. After the car came back normally to me, the problem came back and never went away.
That is basically the history. I still did not have the opportunity to take the car to the road. I never spent a single tank running yet.
I suspect the motor is running too rich, fuel consumption may be too high, and I purchased the HP tuners because I want to adjust the air/fuel mapping and spark advance to make it more adequate to the mixture of gasoline and ethanol at 27% that we use here in Brazil. The octane is higher, but the ammount of required fuel is a little bit bigger than pure gasoline, and when car is in open loop (without the feedback from O2 sensors) I believe it is not adequate. Maybe in the future I will put a turbo in it...

rdelboux

That is not the case (wrong mapping of wires with same color). The problem existed before the cutting, and I only did the cutting to try to isolate the problem (I suspect of harness problem).

rdelboux

No, this is also not the case. Connectors seem ok visually and by testing continuity with multimeter. I will try to bypass the original harness anyway.

In relation to the bulletin of GM, the advice is to change the TAC and I already did that also...

rdelboux

Updating information.

I found the plugs I needed and improvised a new harness to the throttle body (AP sensor, actuator motor, battery, gnd, cruise control). This was for testing the TAC without the original harness to eliminate the possible harness problem. The 2 data lines from TAC to ECU went into the original harness, as I was not able to intercept this cables in the ECU. Result was the same. Still getting errors for accelerator sensor.

Then I talked to a Chevrolet authorized dealer in my town and they accepted to allow me use the TECH2 scanner to check tac parameters. I went there with the car (at 5 miles an hour and pushing the car to go up in inclined roads - embarrasing).

I used a MDI (model EL-47955-2) with a Tech2Win software. Went into the acquisition of data, TAC data, and saw that the disagree paramaters are ok (so the 3 sensor of the APP are not too different from each other), they are moving from 0% to 100% uniformly, but they all are being indicated as "APP sensor out of range - YES". Pictures are attached for illustration (sorry they are in Portuguese).

Voltages at 0% are 0.00V for APP Sensor 1, 5.00V for APP Sensor 2, 4.59V for APP Sensor 3.
Voltages at 100% are 4.96V for APP Sensor 1, 0.02V for APP Sensor 2, 0.43V for APP Sensor 3.

I believe this voltage is actually being converted/formed inside the ECU, because if you measure the voltages in the 3 signal pins of the TAC they are different from what is shown in tech2.

In any case, I found a reference on the internet that indicates that such voltages are outside the minimum/maximum, and will flag a DTC. The attachment should have the information (minimum for sensor 1 is 0.25V and mine is 0.00v).

I did a test by connecting a 5k potentiometer to 5V, grn and the middle pin to the signal port of the APP sensor in the TAC, and I was able to see the varing position (% and Volts) of the individual sensor. However, the information "APP sensor 1 out of range - YES" was always there, regardless of the voltage (even above 0.25V). It could be due to the fact that this information is not updated continuously like the others, or it could be that the software running inside the ECU is somehow corrupted.

So my suspect now is the ECU.

I took out the ECU from the car and took it to a person that fixes ECU. They are not used to receiving Corvette/LS1 ECUs because this type of motor is not common here, and I am sure he will strugle with the VATS as immobilizer (I should have deleted it before sending).

They have a simulator, where inputs for several sensors are provided to the ECU in the bench. They have to customize a harness (160 pins take time to config!) based on the pinout I provided from service manual. They can test the ignition and injection pins, as well as other functions and repair the circuit board by replacing components, if they discover the problem (I am sure they would not have the components here in Brazil for this ECU). They will not have all required pins (transmission specific pins etc) and will get lots of errors, and I hope they do not burn my chear ECU.

What I want from this test is to see if the error for the APP sensor will appear in the bench, because if it does not appear, then this would mean something else is causing the problem (like earth to a sensor or a short for example). I need to isolate the ECU as the problem in order to buy a new one. If the APP sensor error is still there, then I guess the next step would be to get a new ECU.

I will continue to report in the future what is happening.

rdelboux

Just a report of what I had been doing so anyone with similar problems may avoid the hassle and go directly to what would be the solution.

I took the ECU out of the car and sent it to a person that tests/fixes ECU. Here in Brazil there are shops that have simulators (interesting machines that mimic all sensors the ECU needs to operate and test the injectors timing and all). The person said that he could put my vette ECU in the simulator and test it. He finally said there was nothing wrong with it (which I did not believe). I noticed he opened the ECU but I am pretty sure he was not able to bypass the VATs password via VPW and he probably did not wire the ECU to a custom setup as he said he would to test it. The Corvette ECU has 160 pins and it is quite complex, it is connected to transmission, A/C, cruise control, abs, etc. But then out of a sudden he said he had tested it and there was nothing wrong. He could not talk about what he has done, which for me was suspicious.

I have now disabled VATs in the HP Tuners, to would allow the ECU to work without a BCM, which I think would facilitate testing outside the car also.

I revised the ground points in the motor and in the chassis and unfortunatelly this did not solve my problem.

I was considering as a last resort to change the ECU to a programmable ECU (like Haltech, and others). This requires a huge work and you end up loosing functions, and sometimes you have to have both ECUs in the car.

I was thinking that some sort of faulty sensor was causing the ECU to read the APP sensor wrongly, and it was out of range because of a fault in other sensor (it does not make much sense because the communication between TAC and ECU is digital). I started to look into the wiring diagrams of the ECU in the service manual and noticed that just alongside the battery there are some connectors that I could disconnect to isolate the ECU from some of the sensors. I tested them out by disconnecting them each by each and writing down the DTC codes and checking if the APP sensor error would disappear. No luck.

I then thought about doing a test with my first APP sensor, the one that I had opened. I was testing it out of the pedal itself, for easier handling (this proved to be one of the cuprits). I removed the spring that keeps the sensor in the minimum position and left it around 10% pressed. Turned on the battery and key, and for my surprise, NO ERROR!!!

It makes sense, as the "out of range" message was because I was getting 0V out of sensor 1, and other voltages from the sensor 2 and 3. Then I moved the sensor until the end (maximum) and I got "reduced engine power" again. I consider that the project of the sensor is wrong as the limits in the sensor should never allow it get out of range. So my solution was mechanical. I screwed a small screw to the sensor to avoid it to get to minimum and that was it. When I bolted the sensor back to the pedal I realized the pedal does not allow the sensor to go all the way in and out. So the restriction in the pedal range avoids the sensor to get out of range, which in itself it may get. I have no idea why this started, or if I did something else that fixed the problem, but the solution is simple: you don't even need to open the sensor. Just add some sort of metal or rubber on top and bottom of the spot where the pedal defines its range, in order to reduce it. It will stay within what is expected by the TAC.

Both my TACs are working, as well as the APP sensor I got. Now it is time to put the new ones up for sale...





Screw used as internal spacer in APP sensor





Minimum limit





Maximum limit

jdoyle

what did you end putting behind the gas pedal to prevent the code?

did putting something behind the bottom of the gas pedal cause a high idle situation? thank you

Dads2kconvertible

jdoyle, the OP hasn't been active on the forum in over two years.

jdoyle

thanks