I have a 99 TJ 4.0. I am getting the code p0123 saying the voltage is to high for the TPS. I found a good video on how to check from jeepdsolid but my issue is not the same has his. I have had the ECM replaced for other issues and tested again to make sure that's not the issue. The jeep also has to be cranked 3 times to start every time. When testing I found that the TPS center wire is at 12v and does not change with throttle. In the comments of the video I found one person with that issue and he was told it could be the clock position sensor. Is this the next logical thing to check or is there something else I might be missing?
The supply voltage to the TPS should be 5v and is regulated by the PCM.
If you didn't source your PCM from @Wranglerfix you might want to send it in for testing.
Some folks have used a 12v to 5v drop-down converter (for USB) and intercepted the wiring at the PCM harness when the 5v supply fails.
If you're needing three cranks to start the Jeep now is a great time to check and monitor fuel pressure. You should have a Schrader valve on the rail. You can rent a gauge from most auto parts stores or buy a cheap one on Amazon. I'd tape it somewhere you can see from the driver's seat.
It's also entirely possible the ASD circuit inside the PCM is faulty given your other symptoms...but that really sounds like fuel pressure.
-Mac
If you didn't source your PCM from @Wranglerfix you might want to send it in for testing.
Some folks have used a 12v to 5v drop-down converter (for USB) and intercepted the wiring at the PCM harness when the 5v supply fails.
If you're needing three cranks to start the Jeep now is a great time to check and monitor fuel pressure. You should have a Schrader valve on the rail. You can rent a gauge from most auto parts stores or buy a cheap one on Amazon. I'd tape it somewhere you can see from the driver's seat.
It's also entirely possible the ASD circuit inside the PCM is faulty given your other symptoms...but that really sounds like fuel pressure.
-Mac
See if the 12 volts you’re seeing will actually light a test light.
Once a long time ago, I saw this very problem. Scan tool showed 5 volts on TPS signal, went to go verify it with a multimeter and the meter showed 12 volts. Turns out, the PID on the scan tool is only programmed to see the max voltage of 5 volts.
Anyways, it did light a test light which means it was shorted to battery voltage. After a few tests, it led me to unplugging the PCM connector which had a stupid tuner installed inline. When I unplugged everything, there was a bent male pin on the PCM which was shorting 12 volts to the tps signal wire.
Highly doubt it’s your issue, but, it is a step in the right direction.
Backprobe the TPS Signal wire, see if it lights a test light. If it does, keep it rigged up. Wiggle the harness all the way back to the PCM while watching the light. If at any point it goes out, you’ve found your short. If nothing comes of the wiggle test, unplug the PCM connector. If the light goes out, you’ve got something going on internal.
EDIT: an INCANDESCENT test light. Never LED.
Once a long time ago, I saw this very problem. Scan tool showed 5 volts on TPS signal, went to go verify it with a multimeter and the meter showed 12 volts. Turns out, the PID on the scan tool is only programmed to see the max voltage of 5 volts.
Anyways, it did light a test light which means it was shorted to battery voltage. After a few tests, it led me to unplugging the PCM connector which had a stupid tuner installed inline. When I unplugged everything, there was a bent male pin on the PCM which was shorting 12 volts to the tps signal wire.
Highly doubt it’s your issue, but, it is a step in the right direction.
Backprobe the TPS Signal wire, see if it lights a test light. If it does, keep it rigged up. Wiggle the harness all the way back to the PCM while watching the light. If at any point it goes out, you’ve found your short. If nothing comes of the wiggle test, unplug the PCM connector. If the light goes out, you’ve got something going on internal.
EDIT: an INCANDESCENT test light. Never LED.
Thank you for the response! I did get my PCM from Wranglerfix and got the upgraded version. They were nice enough to test it for me when I got the error. So I'm not worried that it could be the PCM. I will definitely check the fuel pressure.
First things first, test another circuit just to make sure your test light isn't faulty. I doubt that's the case, but Occam's Razor suggests you should always start with the simplest explanation.
That being said, if the test light stays on regardless of the PCM being plugged in or unplugged, it suggests that there is a short circuit to the power source somewhere in the wiring.
Here are some next steps you can take:
- Go through the wiring harness to the TPS and check for any signs of damage, fraying, or pinching that could be causing a short. Pay close attention to areas where the wires could be rubbing against sharp edges or where they may be exposed to moisture.
- It might also be worthwhile to check for any ground faults. If the TPS ground wire is shorted or incorrectly wired to 12v, it could lead to the symptoms you are seeing.
- Follow the wiring from the TPS back to the PCM and check each connection point. You might consider unplugging other sensors and devices on the same circuit to see if the voltage changes.
- If you have any aftermarket accessories or modifications, make sure those aren’t interfering. This includes tuning devices, modified wiring, or anything else that might affect electrical loads and voltage.
Very strange. Just so we’re all on the same page, you say the “test light doesn’t go off”.
So, the test light does illuminate, but you can’t get the light to ever stop illuminating, even with the key off.
Am I reading this right?
Also, did you verify that 5 volts is present on one of the other two wires with the key on?
You are correct them light stays on with or without the key. And with the pcm disconnected. And yes I have 5v on the correct side. I know sounds really weird!
Hmm. Well, the test light is still going to be your friend. It’ll give you a visual for when you eventually find it. This may require you to rip the harness apart if wiggle tests don’t work.
I’m trying to think where constant, key off 12 volts even lives… the starter, alternator cable, and power to PDC is all I can come up with. I’m sure there’s more but I haven’t had my coffee yet.
Being that the light never goes out, figuring out where constant, key off 12 volts are on this vehicle will narrow in your search.
Also, cutting the wire at the PCM and TPS connector and splicing and overlaying a new wire is always an option if you can’t find it and need to get going.
I’m trying to think where constant, key off 12 volts even lives… the starter, alternator cable, and power to PDC is all I can come up with. I’m sure there’s more but I haven’t had my coffee yet.
Being that the light never goes out, figuring out where constant, key off 12 volts are on this vehicle will narrow in your search.
Also, cutting the wire at the PCM and TPS connector and splicing and overlaying a new wire is always an option if you can’t find it and need to get going.
Also, being that you’ve verified the light stays on with the PCM unplugged, I would highly suggest finishing your troubleshooting with the PCM unplugged. I’m not sure how good it is or how long the PCM can handle battery voltage on a 5 volt signal return circuit.
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Posted this in the wrong thread somehow 
But, got some coffee in me.
With the test light installed and the key in the run position with the PCM connectors connected, start pulling fuses one by one. Install the fuse if the light stays on. If the light goes out, you’ve isolated your circuit.
Skip the PCM fuses. You’ve already eliminated it by unplugging it, and it’ll kill power to other circuits that you might need.
But, got some coffee in me.
With the test light installed and the key in the run position with the PCM connectors connected, start pulling fuses one by one. Install the fuse if the light stays on. If the light goes out, you’ve isolated your circuit.
Skip the PCM fuses. You’ve already eliminated it by unplugging it, and it’ll kill power to other circuits that you might need.
Posted this in the wrong thread somehow
But, got some coffee in me.
With the test light installed and the key in the run position with the PCM connectors connected, start pulling fuses one by one. Install the fuse if the light stays on. If the light goes out, you’ve isolated your circuit.
Skip the PCM fuses. You’ve already eliminated it by unplugging it, and it’ll kill power to other circuits that you might need.
Ahh great idea! That will narrow it down fast
If I’m not mistaken, the TPS wiring runs with the O2 heater wires. Can’t remember, but I thought the heaters were a 12 volt duty cycle controlled by the PCM when the O2 activity was low and it was in open loop. Either way, it’s a quick check to unplug the O2 sensors one by one and see if the test light goes out.
Not yet, through out my back somehow but going to get to it today.
So finally got out there. The rain has been all day but I was able to test as you had suggested and found that the in F6 under the hood is the one. A large 30A fuse. Trying to figure out what it goes too.
Looks like thats the Automatic Shut Down Relay, Powertrain Control Module which makes sense to me if thats the normal fuse for the tps among other things. But not sure how its giving 12v all the time.
So just a recap of where I'm at now. I am getting the error P0123 TPS high voltage. The middle wire of the TPS is reading 12v with the key off/on and disconnected/connected to the TPS. (the other wires are normal) Pulling the PCM does not change this. Pulling the 30amp F6 fuse under the hood removed all power to the center wire. According to the documentation it is for Automatic Shut Down Relay, Powertrain Control Module it also read 12v at the fuse with or without the PCM connected. The PCM is from wranglerfix and they tested it for me and it works without problems. I'm not sure where to go from here but I grabbed the wiring diagrams from the service manual. Looking at them I am not real good at reading these but I cant tell were 12 volts would come from or where the fuse is. Looking for ideas on how to narrow this down. Thank you all for your help so far!
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