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I have looked left and right, far and near, for what seems to be the first step in addressing P0121 throttle position sensor issues: to replace tin with gold pins on the connector. I downloaded the bulletin with instructions, I memorized, I've been a very good boy.
But, it boggles the mind: whence does one procure oneself of them replacement pins? are they generic? if so, i would have no idea how to ask for them, and from whom. If not, are they a Jag part, ordered online or through the dealer? Because nothing is coming up of the part on the internet.
I may be new in the game, and would gladly keep everyone here abreast of developments, but it seems a common enough issue not to take up anyone's time with it. I just can't seem to be able to source the part I need. Any thoughts?
It would help others provide the best answers if you could let us know the year, model and engine in your Jag. It's a great idea to add that info to your signature line using your User Control Panel so the info will show up in all your posts.
I don't know if you've looked at the actual Jaguar service documentation, but, for example, you can download the Jaguar Diagnostic Trouble Code (DTC) Summaries for the 2000 model year here:
The first items listed under Possible Causes have to do with breaks, high resistance or shorts in the wiring between the TPS and ECM. It would be worth first inspecting for corrosion or oil contamination of electrical connectors and corrosion on the critical ground points related to the TPS/ECM circuits.
According to the TSB, new terminal leads with gold-plated terminals are to be spliced in to the existing harness. The link kit part number given in the TSB is LNG 3956AA. Searching that part number at jaguarmerriamparts.com shows the kit available at $70.47:
Personally, before I went to the trouble of changing the terminals, I would try cleaning the connectors at the TPS and ECM, as well as the grounds used by the ECM, which you can see in Figure 4.1 of the X308 Electrical Guide. Also inspect the wires for breaks, chafed insulation and shorts. You can download the correct guide for your model year from jagrepair.com. For example, here's the link to the 2000 MY guide:
I'm truly overwhelmed and humbled by the extent of your response - how can I thank you? I hope I can be useful to the community myself.
Also, thank you for the resources: I knew not of either jagrepair, or Merriam Parts.
Based on the suggested troubleshooting:
1. Check and clean all connections between the throttle body and the ECM.
2. Clean all grounds, especially those related to the ECM.
3. Take it from there re. pins and throttle.
From a first inspection, everything under the hood looks exceptionally clean from oil, dust or other contamination. The connectors surrounding the throttle body, on both sides, are secured with zip ties - must check for any breakages on the locks of the sockets.
I will proceed with methodical rigor and keep you updated.
One thing I seem to have understood is that I shouldn't worry about speed sensors throwing the 0121 code.
Finally, I will invest in a decent voltometer. I've never before had to delve into electrical issues and my knowledge and tool collection reflects that. Mechanical problems have been much more my province and this car is the first featuring such sophisticated electronics.
Secured with zip ties? That is not factory installation. The locks on the connectors have been broken and they may be loose. That's somebody's patch job.
Further to my detailed inspection today, I have the following to report:
The zip ties, of course, are not stock. They seem to be holding the TPS connector tight in place as the clip was broken. I guess, a cheap and effective fix?
Regarding everything else, and again thank you Don, I downloaded schematics and followed the procedure:
1. cleaned the TSP connection with electronic cleaner spray
2. refastened it using a zip tie
3. checked the wire until it met the big harness, no breaks or tears
4. unfastened and cleaned all grounds: the one on the firewall, the front right one, and three on the driver's side. Not much corrosion, but significantly better after the brushing.
5. checked and brushed battery terminals
Following that, I drove the car for 40 miles. That is the longer it's gone without the 0121 fault appearing. It would usually come on after only a few blocks or once the engine warned up. I was ecstatic, you can imagine.
After stopping for about 2 hours, on the way back it threw the fault again, and that's where we're at now!
In short, it seemed to make a temporary improvement but not solve the problem.
I guess the next step is connector tips replacement (they are not the gold version on the wire end)?
Many thanks for your help and for the happiest 40 miles of continuous driving I've enjoyed in long.
Alex, not to pour cold water on the pin connector fix, but if your car has mileage over 100K then the TPS is suspect, unfortunately these cannot be bought seperately and a rebuilt throttlebody or low miles s/h unit is the only known fix. If you search this forum for that particular fault code you'll see the measures people have taken and the temp nature of those fixes. There are companies mentioned in the throttle threads that carry out the rebuilds.
The issue lies with the duel tracks the ECU gets info from within the TPS, once worn the signal breaks and the car goes into limp mode, sometimes a hard reset will clear it and you can get home.
I battled with a p122 which is the TPPS on the other side of the throttlebody, same problem, same symptoms slightly different function as it monitors throttle pedal position. A replacement throttle cured it......for now.
Thanks Jim:
That pretty much answers the question of the circuitry involved. It sure looks to me like two variable resistors, not pots, with one giving rising and one falling resistance with rotation.
Alex, not to pour cold water on the pin connector fix, but if your car has mileage over 100K then the TPS is suspect, unfortunately these cannot be bought seperately and a rebuilt throttlebody or low miles s/h unit is the only known fix. If you search this forum for that particular fault code you'll see the measures people have taken and the temp nature of those fixes. There are companies mentioned in the throttle threads that carry out the rebuilds.
The issue lies with the duel tracks the ECU gets info from within the TPS, once worn the signal breaks and the car goes into limp mode, sometimes a hard reset will clear it and you can get home.
I battled with a p122 which is the TPPS on the other side of the throttlebody, same problem, same symptoms slightly different function as it monitors throttle pedal position. A replacement throttle cured it......for now.
Hello Sean,
Thank you for that. Understood.
It was worth giving it a try, and it should be an improvement anyway, having connections cleaned.
I'll proceed with a rebuilt unit, and post updates. Meanwhile, I have a 7 mile daily drive to and fro work. Since it invariably flags the code at some point (usually close to the destination), I end up driving the car on limp mode for a few blocks at a time, sometimes though in heavy traffic. Is that sustainable for the engine for, say, a few more weeks until I get to the project or is the limp home mode harmful to the engine/transmission?
That pretty much answers the question of the circuitry involved. It sure looks to me like two variable resistors, not pots, with one giving rising and one falling resistance with rotation.
Ross,
When you get a chance, please take a look at the X308 Electrical Guide, Figure 4.1 (Engine Management). For convenience, the 2000 MY guide is available at the link below, and Figure 4.1 is on pdf page 101:
My understanding has been that the TPS is a dual potentiometer (with conductive plastic traces, judging by Jim's photos), and that both traces are wired as voltage dividers, with V+ on one end, Gnd on the other, and the wiper giving a voltage between 0V and V+.
If they were wired as variable resistors, my understanding is that only two terminals per trace would be used, the wiper and one end of each trace. It appears that the reason the TPS has only 4 terminals is that V+ and Gnd are common to both traces, and only the wipers are discrete. The Pedal Position Sensors use the same V+/Gnd connections to the ECM.
You are probably right that as the resistance of one trace rises, the other falls.
Please correct me if I'm wrong - I'd hate repeat my wrongness to others and mislead anyone!
Nonfinito - has your car got Cruise Control? I found that it overrides the PPS so could drive my UK car on CC without it dropping into Limp Mode. Mind you - it's not easy on crowded roads!
When you get a chance, please take a look at the X308 Electrical Guide, Figure 4.1 (Engine Management). For convenience, the 2000 MY guide is available at the link below, and Figure 4.1 is on pdf page 101:
My understanding has been that the TPS is a dual potentiometer (with conductive plastic traces, judging by Jim's photos), and that both traces are wired as voltage dividers, with V+ on one end, Gnd on the other, and the wiper giving a voltage between 0V and V+.
If they were wired as variable resistors, my understanding is that only two terminals per trace would be used, the wiper and one end of each trace. It appears that the reason the TPS has only 4 terminals is that V+ and Gnd are common to both traces, and only the wipers are discrete. The Pedal Position Sensors use the same V+/Gnd connections to the ECM.
You are probably right that as the resistance of one trace rises, the other falls.
Please correct me if I'm wrong - I'd hate repeat my wrongness to others and mislead anyone!
Cheers,
Don
Don:
You are absolutely right about the wiring diagram. of course. Looking at the traces along with the wipers though, I cannot see the current path for a pot. There must be some connections that are not obvious. Notice the wipers apparently only jump between the two traces (and move in an arc about 90 degrees). What am I missing? I will try and spend a few minutes with an ohmmeter tonight!
Last edited by sparkenzap; 11-02-2015 at 01:36 PM.
As planned, the rebuilt TPS arrived yesterday morning from ASI. The excitement of getting the Jag back was only matched by the anxiety that something might not actually go according to plan - a state of mind only known to that rare species, the Jag owner.
And so the installation began:
I cleaned the mating surfaces, introduced a new gasket and placed the TB in position; reconnected all electronic equipment; reattached the coolant hoses; torqued the TB to18-24Nm; added the air filter element. To the best of my ability, everything was reinstalled and connected properly.
I proceeded to to a ‘hard reset’ per ASI instructions: held + and - wires together for about 3-4 minutes, then connected them to the battery. Switched the car to ‘ON’. I lit up like a Christmas tree: ASL/TRAC/FAILSAFE routine came on. I let it be for about 1 minute. The warnings didn’t go away. Started the engine and, to one and none's surprise, it behaved exactly as bad as when it’s on limp mode with the unwelcome addition of an unstable idle.
Well, here we are. I admit inaptitude with electronics, but this seemed like a straightforward repair. Did the TPS not get fixed? Did I not manage to do a hard reset? Or…?
As a side note, I should say, I connect my OBDII and got some beautiful extra codes I didn’t have before: P1122, P1222. OH! And I got the Gearbox Failure caution.
I have until Sunday (this Sunday!) to make this work and move it from the driveway. These orders come from the powers that be. Non. Negotiable. Ha!
Ideas, helps, suggestions, all are appreciated and will eagerly be implemented!
You may need to calibrate the throttle body!!
Note how it has two screws that you can loosen and adjust the sensor position. I can't remember the exact thread, but in the XK forum someone took resistance readings between different pins at closed and open. You might need to loosen the screws and turn the sensor until it gets back to the correct range.
If you're lucky I'll be able to find the thread and post it up here...
09-19-2015, 01:04 PM
P0121 Throttle Cure First Step
