P0121, P0123, weird voltage values, possible faulty PCM?

Hey guys, trying to run down a couple of TPS code and other weird values with the Heep. Here's what I've gotten so far-

Check engine light set, pull codes, come back with a P0121 (Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance Problem, basically the TPS values aren't lining up with the rest of what the PCM is seeing) P0123 (Throttle/Pedal Position Sensor/Switch "A" Circuit High Input, basically the PCM detected high voltage on the TPS signal wire) and a P0420 (not surprising, my cat/exhaust system is shot, looking at re-doing it all here soon.)

Now here's where it gets really interesting... got out my voltmeter to start tracing down the P0123 and I come back with about 0.39 volts on the signal wire with the connector disconnected and the engine off. Like, with the key PHYSICALLY IN MY POCKET. Doesn't matter if I'm measuring to the signal return wire or the battery ground. I can do the exact same thing with the 5 volt reference at the TPS, CKP, and MAP and get about 0.40 volts, same thing key in my pocket, connector disconnected, signal return or battery ground, doesn't make a difference. 5-volt reference at the TPS with the key on is about 5.1 volts. Thoughts on this? I've already replaced the TPS with a new Delphi unit (mostly just because the connector latch on it was broken and you could disconnect it just by lightly pulling on it.) My first thought was a short to voltage somewhere on the signal wire, but with these weird voltage values I'm getting when I should be getting NOTHING, I'm starting to suspect the PCM might be internally shorted. I'm debating de-pinning the TPS signal wire at the PCM connector, plugging the rest of the connector back in and re-checking for voltage, but I'll need to buy or borrow a special tool for that. Already checked all the grounds I could get to in the engine bay on the recommendation of another guy, no more than a 0.3 volt drop on any of them from the battery negative terminal, but I might just go ahead and clean a few up anyways. Any thoughts? I seem to recall somebody saying that the dealership has a special tool that can determine if your PCM is internally fried, but I really don't feel like dealing with stealership wait times or labor rates.

First, you need to discover if what you are seeing is normal, or not. PCM failure is extremely rare, and should only be considered after a logical and complete diagnostics of the more common and likely faults. Test all the OBD sensors, visually inspect and test all the OBD sensor wire harnesses for shorts/opens and chafed/melted wire insulation. Make sure none of the PDC relays are sticky. Make sure the ignition switch is not the cause, disconnect and re-test. Disconnect the keyless entry module and re-test. Inspect the driver door wire harness for shorts, disconnect it and re-test. Pull the horn relay to determine if the clock spring has shorted horn voltage to the TPS ground.





P0121 P0123 JEEP - Throttle Position Sensor

Possible causes

- Faulty throttle position sensor
- Throttle position sensor harness is open or shorted
- Throttle position sensor circuit poor electrical connection

I guess I should also mention that my rig is a '96 4.0 Sport lol. Didn't get around to all of the diagnosis today (wanted to adjust my drums and check out my blower switch- yep, it had another thermal event. *roll eyes*) Here's what I did get done though-

Removed ignition switch connector- no change of voltage at TPS connector signal wire, still 0.39 volts or so.
Removed clock spring connector- still no change.
No wireless entry module, so nothing to test there.
No real door harnesses to speak of except the speaker wires, so nothing to really test there.
Tested all PDC relays. Fan relay would click, but had no continuity when engaged, replaced that, no change in voltage readings.
Removed PCM to test TPS signal pin & wire. All okay, no short to ground, about 1 ohm of resistance from connector pin to connector pin. And that's when it got interesting...

Just for shits and giggles, I check for voltage on the PCM connector pin next to the TPS signal pin, which is fused battery voltage. Get 0.3-something volts between it and the battery ground (had the cable connected so I could check for shorts) What the... pull the 30-amp fuse that supplies it and it goes to nothing, 0 volts. Reinstall the fuse and it goes back to 0.30 or so. Fuse also supplies power to some relays, check all of those and there's about the same voltage at the 30 pins. WITH THE BATTERY POSITIVE CABLE DISCONNECTED. Pretty much all the PDC fuses showed the same voltage. Move battery cable as far away from battery as possible, thinking that was the problem. Same thing. Okay, now I'm really confused, turn multimeter off and quickly back on to 20 VDC. Voltage at the PCM fused connector pin and the fuses is down to like 0.10 to 0.12 volts. Okay weird, but more what I'd like to see... at this point, I starting to think my el-cheapo Crapsman multimeter I've been using for all my measurements is suspect. That, and I suppose it's worth mentioning that this thing had the remnants of a stereo system when I bought it. I've pulled some of the wiring out, but there's still some left and, at one point, I found what looked like a capacitor that was about the size of a roll of Kodak film under the dash... I'm thinking about doing a stereo system hunt just to make sure there isn't anything left that could screw with the electronics.

Update- Verified that my DMM is in fact okay using a regulated voltage supply that you would use in a school environment to build simple circuits and such. Backprobed the TPS signal and signal return wires (after verifying that that the signal retrun was okay), came back with something like 0.72 volts on the signal wire with the throttle body closed and 3.8 volts or so wide-open. Verified that these were the voltages the PCM was seeing with a scan tool. Wiggle-tested and did not see a change in voltage, voltage sweeped using my DMM and the voltage changes okay from 0.72 volts to 3.8 volts. The voltages seem a little off to me, but I flogged the thing afterwards just trying to set a CEL or a code, couldn't do it. Seems to me like the voltages are a little off, but like I said, no CEL or codes. Seems that if you disconnect the TPS with the key on or engine running, it will set codes. Might try disconnecting my clock-spring anyways to see if that changes my voltage readings, but so far no issues.

The TPS is toast. The TPS should show .25 volts at minimum throttle opening (idle), and 4.8 volts at WOT wide open throttle.

Yeah, that was kinda my thought as well... Ideally, I'd like to see somewhere in the neighborhood of 0.4 or 0.5 to 4.5 volts. 5VREF is okay, about 5.1 volts running into the TPS. CEL is back, haven't run codes yet, but my gut feeling says P0420. I'll check Mitchell, but I can't remember seeing a voltage spec anywhere...Also, with me just having put in a new Delphi TPS to the tune of $50 or so, any suggestions on a new TPS? I know everybody says Mopar, but... 1. Prices. ($8 for a bolt? Seriously?!) and 2. My local dealership says they don't even make fuel pump assemblies for my rig anymore lol.

My time is worth money. I know a genuine Jeep TPS or other engine sensor will work right, and last many, many years.

Many Dealerships have websites and the prices are discounted 30% from the walk in retail prices. I have also bought Dealership parts on Amazon.