Nottingham15

I had a check engine light for a P0122 fault. The light has gone but I still have a persistent P0122 reading on the OBD scan, which returns quickly after I clear it.
The reading at closed throttle is 16.5% which is 0.825 of 5 volts - as believe it is supposed to be, so I figure the TPS is working OK.

A couple of weeks ago I did have a couple of low idle stalls but I've got no other symptoms at the moment after giving the TB a good clean.

Any ideas, other than ignore it and stop using the scanner?

DFlintstone

On our (adjustable), Renix TPS we set relative to the actual feed voltage, since it's usually not right on 5 volts. Probably a very small difference though. Anyway verifying you are getting about 5 might make sense.

Nottingham15

Thanks - that's my next step. OBD app doesn't give the actual voltage, just the percentage, and my multimeter is an analogue antique.
Hopefully, the £20 digital version I'll be buying tomorrow will be accurate enough to check the TPS connectors properly.

Also, I guess if one of the connections is a bit wobbly or one of the wires is worn and grounding on something intermittently it would give me the fault code without any obvious symptoms.

Nottingham15

Just realised what you've said there. That's a very good point. I just assumed that there would by 5v across it, as per the manual. Thanks again.

XJwonders

Is yours OBDII?

Check the wiring harness just behind the fuel rail. Look for any wires touching metal. Sometimes it can cause shorts.

Nottingham15

Yes it is OBD2.
I've checked the harness and can't see any worn wires, but it's stretched so ridiculously tight that it's difficult to get a really good look.
The first chance I get, hopefully this weekend if it's not raining here, I need to change the valve cover gasket. That should give me a chance to get a closer look at the wiring.
Are all Jeeps wired like there was some kind of rationing in force? All my wires are so tight they're like guitar strings.

XJwonders

Not supposed to be tight.

Common cause of TPS low voltage code that keep recurring is a short or interference with wiring in that particular area I just mentioned.

Nottingham15

Got my digital multimeter and had another look at it this morning, after giving the TB and all sensors a really good clean.
Without power all connections are good. Resistance measured from the tps is gradual with no spikes or dips.
With power, the tps is getting 5.11v.
Throttle closed it sends 0.835v to the PCM.
WOT it sends 4.8v.

Opening and closing the throttle the output voltage climbs and falls gently and evenly, including when I jiggle the wiring harness around the engine bay and tap the tps.

In short, everything appears to be working exactly as it should...

However, after clearing the P0122 code, getting in the driver's seat and turning the key on (engine off) I immediately get a check engine light - which I wasn't getting before, just the DTC. The P0122 TPS code again.
I took all the TPS readings again. All perfect.

I'm stumped.

XJwonders

Did you CHECK the wiring harness above the fuel rail and look at where it lead to going to the firewall.

In my research and in my past experience, most of people with TPS low voltage code is caused by wires touching hot manifold or rubbing on metal causing shorts.

Nottingham15

I checked it as far as possible. No worn spots at points of contact with any parts of the engine. And I checked the voltage readings while jiggling the various harnesses around without any variation.

XJwonders

Check the clock spring on the steering wheel. I read somewhere that it shares same ground with the TPS. Sometimes when it short out, it cause a domino effect. I forgot where I saw that, not sure if it's legible, though.

Nottingham15

Interesting. Time to check the workshop manual wiring diagrams again.
Thank you for the idea.

Nottingham15

You're right. The clockspring shares a ground with all the engine bay sensors, all of which also share the same 5v input.
Lots of wires to check!

XJwonders

Any luck so far?

Nottingham15

Not exactly. I've cleaned up virtually every ground on the vehicle. I've disconnected and cleaned all the relevant connectors.
And I've rechecked the TPS and it's circuit independently with digital voltmeter/ohmeter and then again with my analogue voltmeter in case there are any really brief drops in voltage.
No abnormal results.

I'm now reasonably confident that it's either a bad connection, broken wire or a short to ground - pretty much what you said.

It's just so intermittent that I can't find it, even with some fairly energetic wire jiggling. It may just be a case of ignoring it until it gets worse, at which point it will hopefully be easier to find.