Help needed...What should I do next?

[mikeheff]

Thanks in advance to any suggestions/help anyone can give me. I have a 1996 Tahoe that keeps getting code p0121 (throttle position sensor circuit A low output) I have replaced the tps with genuine ac delco part, checked voltage , grounds. I installed a new connector on tps, rewired it straight to pcm in correct terminals, soldered wires. I did this in case there were a wire rubbing in harness that I couldn't find. Still get code ! I got a used pcm with same serial number as original. I plugged it in and runs rough, I assume it will need reprogramming. Does anyone agree replacing pcm is the only thing left? nothing else left in that circuit that I know of. This is getting frustrating to say the least, so if anyone has experienced this please give me some good advise.

 

[SunlitComet]

if nothing is wrong with the tb or cables themselves then replace the pcm and reprogram for proper drivetrain setup.

 

[mikeheff]

if nothing is wrong with the tb or cables themselves then replace the pcm and reprogram for proper drivetrain setup.

Thanks, I am taking in this Thursday for programming. Is it normal for the donor pcm to run very rough until programmed. I got one that had same serial # as mine, but off different vehicle they told me.

 

[SunlitComet]

if same drivetrain then it should not be an issue. but perhaps yours came off an engine with a different engine size. and it is part number not serial number.

 

[mikeheff]

You would be correct, I called the used yard I got it from and they said was off blazer with 4.3 L.

 

[SunlitComet]

so yeah it isn't exactly firing on all 8.

 

[mikeheff]

Thanks again for your help, I will post how its doing after the programming later this week

 

[mikeheff]

Ok guys, I finally got the ecm programmed at GM dealership. The same problem is there, sets tps code low voltage. I'm lost! new genuine tps, rewired straight to ecm, replaced ecm and programmed. any ideas? even GM had no positive input.

 

[mikeheff]

today I put on new coolant temp sensor, just cause t shares ground with tps. I also installed new ac delco spark plugs, just because, I knew this had nothing to do with tps problem, but why not, right? I double checked all pins and still, The Damned TPS code still comes back, runs like dog pooping razor blades when you accelerate and shifts hard 1-2. Any ideas folks...I'm lost once again

 

[SunlitComet]

how does the pcm data output on temperature reading compare to the dash? are the grounds on the thermostat housing in good shape as well as engine to frame to battery?

 

[mikeheff]

temp on tech II reads 188 degrees, dash reads right at 200 ish.. grounds from motor to frame appear good, I need to check the one on tstat, didn't know one was there, thanks..can spider injectors , intake or throttle body cause this?

 

[mikeheff]

I will check it out , thanks for helping !!

 

[SunlitComet]

the tech 2 is showing you the one on the thermo housing. that feeds the pcm. so that one is at 188. the dash reads off the left cylinder head so it is not to far off then i would suspect something. do you have a wiring diagram to all the sensors to ensure you are isolated from any faults.

 

[mikeheff]

I have them, but understanding them is my problem. I have had the gm guys at my work look at it several times, they are stumped as well. Their guess is another sensor is causing the issue, something that does not require a 5 volt reference

 

[mikeheff]

the wires at the tstat housing are secure, a little green in color, but not corroded like you would see on some battery terminals..no fuzzies or anyting

 

[mikeheff]

today I spent another few hours just piddling on this thing, I cleared codes, figured I would start unplugging sensors to see what made a difference. After unplugging mass air flow, it runs 90% better. No Tps codes set for the first time, trans shifts normal and actually accels great. Thought I actually fixed it when I replaced the mass air flow sensor....NOT...tps code P0122 now, was po121..same symptoms as before. I unplug mass air and runs ok....any ideas. I really like this Tahoe, BUT, I'm at my wits end

 

[mikeheff]

today I spent another few hours just piddling on this thing, I cleared codes, figured I would start unplugging sensors to see what made a difference. After unplugging mass air flow, it runs 90% better. No Tps codes set for the first time, trans shifts normal and actually accels great. Thought I actually fixed it when I replaced the mass air flow sensor....NOT...tps code P0122 now, was po121..same symptoms as before. I unplug mass air and runs ok....any ideas. I really like this Tahoe, BUT, I'm at my wits end

 

[mikeheff]

today I spent another few hours just piddling on this thing, I cleared codes, figured I would start unplugging sensors to see what made a difference. After unplugging mass air flow, it runs 90% better. No Tps codes set for the first time, trans shifts normal and actually accels great. Thought I actually fixed it when I replaced the mass air flow sensor....NOT...tps code P0122 now, was po121..same symptoms as before. I unplug mass air and runs ok....any ideas. I really like this Tahoe, BUT, I'm at my wits end

 

[mikeheff]

has anyone ever experienced this before?

 

[mikeheff]

I have unplugged every sensor I can think of, maf, iac ,iat, egr, its runs better with any of these unplugged and the tps code does not come back unless I plug the one I had unplugged back in. Any ideas?