highwayman

Hi everybody, I'm new here and I'm having a nasty problem with my modded LT1. I just heard about this site and hopefully somebody here will know how to cure my problem. Here's what's going on.

I've got a 1996 TA, LT1, that is giving me an odd problem. A TPS low voltage code, P0122 has thrown. The throttle position sensor and MAP sensor have stopped functioning. The problem does not lie with the sensors, they are mechanically and electrically fine. There is supposed to be a 5 volt reference voltage on both of these sensors and when the code is thrown, the reference voltage disappears, therefore the sensors stop working. What's going on with that? I cannot drive the car over 1900 RPM when the code is set. The car loses all power at that engine speed, regardless of gear. If I reset the computer by disconnecting the battery and pulling the PCM BATT and PCM IGN fuses the code usually clears. It will run flawlessly, the sensors work properly and everything is great, until the code comes back, then it goes to hell.
To provide a little more information on the problem(s), at idle, the ignition timing is approximately -30 degrees, and retards more when you rev it. Could whatever is causing that timing problem be causing the other problem? I know for a fact I installed the opti-spark unit properly. Also, when the voltage disappears, the MAP sensor reads 3 in/hg instead of the normal 29-32 in/hg. That value is fixed. Load value shoots up to 100% and that doesn't change, and throttle position reads 0% and that doesn't change even when you press the pedal.
I have checked for opens and shorts in every wire connected to those sensors and the others in the circuit and found nothing.
Here's my list of mods: The heads have been machined, 5-angle valve job, bowl blending, port-matching, new Erson valve springs, Comp Pro-Magnum pushrods, 1.6 stainless steel roller rockers, CC 305 cam, long-tube headers, off-road Y-pipe, etc. My build is not radical enough to require the use of an aftermarket MAF or throttle body. The PCM has been tuned to remove all emissions codes, remove rear O2 codes, etc. and change the air/fuel tables for maximum performance.

I have operational data charts I took with an AutoX-ray 5000 and can send them to you if you can interpret them. I can't, I don't have enough knowledge. I'll try anything at this point, I'm out of patience with this vehicle. Thanks in advance. MJD

eagle_eyes

iTrader: (1)

try replacing the sensors? i'm not an expert, but i think the PCM bypasses the sensors when it detects a problem, which is why the voltage disappears. maybe someone else will chime in.

highwayman

I tried replacing the TPS, but that had no effect. I believe the computer is getting junk readings from something and that's throwing off everything. Just a thought. MJD

highwayman

Problem still exists. I also have an update. I cannot get the code to clear no matter what I do. I could usually clear it by disconnecting the negative battery cable and pulling the PCM IGN and PCM BATT fuses. Now that doesn't work. It's almost like a permanent SES light has set. MJD

robsquikz28

iTrader: (246)

check ur grounds i knew someone with similar problems

sweetbmxrider

iTrader: (13)

could be a bad computer

kinglt-1

iTrader: (26)

^agreed x2.

RoAdRaGe912

I hate chasing electrical problems. If you had OBD-1 I could be more help.

In the OBD-1 system, the MAP and TPS use seperate 5v reference signals. Are you sure battery voltage is sufficient? You need to find out what all uses 5v references and if you have seperate 5v signals from the PCM. Check those other 5v signals to see if your problem is isolated to those two sensors.

I think it's going to be a continuity issue/grounding somewhere. Check the pins at the PCM. You may have to end up running new wires between the sensors and PCM, or jumper from another 5v signal if any are still good.

Armyman2445

iTrader: (17)

I had the same problem in my 96ss. I hunted and hunted and hunted for weeks and finally got tired of it and changed out the whole wiring harness. Problem went way and the car still runs stong (even though I sold it ).

lt1taracing

i had a similar problem years ago. after swapping sensors out and still having the problem. i noticed the wiring harness was frayed or exposed going into the t/p sensor. i had a spare harness and replaced the connector and the wires coming out of it. fixed my problem.

highwayman

There are multiple 5v signals leaving the PCM. The TPS, MAP, coolant temp sensor and AC refrigerant pressure sensor are all on the same circuit. There may be one or 2 more, but I'd have to check the shop manual. I have no refrigerant in the system right now, I had it evacuated when I did the mods and have not recharged it. Could that be messing with the pressure sensor? If it is, that might be the reason for the trouble. Some other sensors have a seperate 5v signal. No voltage is on those either. I have checked all the wires from the PCM connectors to the sensor connectors and found no opens or shorts. I can see the 5v signal coming off the PCM if the code is not present, but when it sets, the voltage does not leave the PCM.
When I sent the PCM for tuning, the tech told me he tested the computer and said it was still good. MJD

highwayman

SOLVED! I did some checking and found the root of the problem. Sure enough, I found a ground on both the black and gray wires leading to the MAP sensor. Whoever installed my passenger side valve cover pinched the wires under the cover and cut through the insulation. The wires were grounding out on the cylinder head. I couldn't find the problem before because there was a solid state device (A/C refrigerant temp sensor) in the circuit giving me false readings. Once I disconnected that, my readings were right and the trouble was obvious. All sensors are now functioning and the car is running like a raped ape. Thanks for the suggestions everybody. You all got me on the right track. Best regards. MJD

Formula96

good thread for later referance, i am soon to be doing a compleate rebuild and i might end up haveing this problem. Thanks!!!

sweetbmxrider

iTrader: (13)

glad you got it running

DriftZ28

I am having the exact same problem, but I can’t find any shorted or open wires in the circuit. I am not the kind of guy to just give up, but I may be replacing my wiring harness soon.

Please explain, “I couldn't find the problem before because there was a solid state device (A/C refrigerant temp sensor) in the circuit giving me false readings.”

I know this thread is old but I could use some help.

Thanks
Jonathan