Greetings Jag forum denizens,

A strange issue has plagued my XJR since I swapped in my 4.2L short block that has been puzzling me.

Occasionally, when I start the car, a "Transmission Fault" will present itself on the dash. I will cycle the key and it's gone. Unfortunately, when this happens, the throttle will appear to forget what it is supposed to be doing as it will only go to a maximum of 23% on the OBD scanner live data even with the foot to the floor. Sometimes, I can do a throttle relearn and it will go back to normal. I'd also like to mention that the cruise control does not appear to be affected, as I can make the throttle percent go higher when accelerating the cruise. I assume this indicates a faulty/failing throttle position sensor on the driver's side of the throttle body, but before I take it apart I would like to get a second opinion...

I have not been able to find any similar issues using the search function, leading me to believe I may have a unique issue!

I would appreciate any insights you guys have!

 

Yes, I am one of the denizens here, naturalized in this region like an animal or plant...

Two definite things, what to do first:
1. Use a code scanner to read out the failure code.
2. Check, if your ATF level is too low.
This post of mine also shows you, where the check plug is, and if I remember correctly, the level needs to be checked while the engine is running and the car is raised on stands and levelled:
https://www.jaguarforums.com/forum/x...-flush-265900/

> I have not been able to find any similar issues using the search function
Which search function did you use? The search function on this forum does not work. My browser tells me the following:

Occasional Transmission Fault Messages in Jaguar X308 Models

The "Transmission Fault" warning light appearing intermittently in your Jaguar X308 (1997–2003 XJ8/XJR) is a common issue with several well-documented causes, ranging from electrical glitches to mechanical wear.

• Electrical & Wiring Issues:
  
  • CAN Bus Communication Faults (P1796–P1799): These codes often stem from poor electrical connections. A shortened transmission wiring loom (noted as "Ford’s penny-pinching") can cause the connector to pull loose over time due to engine/transmission mount wear. Cleaning the contacts and securing the connection with dielectric grease and cable ties may resolve the issue.
    
  • Turbine Speed Sensor (P0715): This is a frequent culprit. A faulty or intermittent input turbine speed sensor can trigger the fault light and cause harsh shifts, especially from 1st to 2nd gear under light acceleration.
    
  • Poor Grounding: A rusty or corroded earth strap can lead to intermittent electrical faults, causing multiple warnings and even engine shutdowns. Check and clean all grounding points.
    
• Mechanical Failures:
  
  • Forward Drum Snap Ring Failure: A known weak point in the ZF 5HP24/6HP26 transmissions used in X308 models. If the forward drum’s snap ring breaks, it causes clutch pack misalignment, leading to harsh shifts or failure to engage. This often results in P1722 (stall speed fault) and may require a full transmission rebuild.
    
  • Internal Clutch Wear (1st & 3rd Clutch): Early signs include a firm or thudding shift into 2nd gear and occasional limp mode. This is a progressive issue that can lead to full transmission failure if ignored.
    
• Other Contributing Factors:
  
  • Incorrect Transmission Fluid: Using non-OEM fluid (e.g., Castrol ATF instead of Esso LT71141) can cause internal damage and trigger faults.
    
  • ABS Sensor Interference: A faulty rear wheel ABS sensor can send erroneous signals, causing the ECU to disengage the transmission as a safety measure.
    

Recommended Actions:

1. Scan for fault codes using a compatible diagnostic tool (e.g., iCarsoft LR V2.0) to identify specific codes like P0715, P1797, or P1722.
   
2. Inspect the transmission connector (EM46/EM44) on the driver’s side for corrosion, wear, or loose pins.
   
3. Check grounding straps and ensure proper earth connections.
   
4. Verify transmission fluid type and level—use only Esso LT71141.
   
5. If codes point to internal failure, consult a specialist for a rebuild rather than accepting a dealer’s "new transmission" recommendation.
   

Note: While the issue may seem intermittent and non-progressive, it can escalate quickly—especially if the forward drum fails. Addressing it early can prevent costly repairs.

 

Hi Peter,

I will go ahead and check the transmission fluid level, when I had the motor out I did empty out the torque converter and filled back up but I may still be short a quart. I have an XJR so it should be the Mercedes transmission, I hope that snap ring doesn't apply to me...

I did see some transmission fault related messages and I figured the root cause of that might be the conductor plate as I have heard those tend to go bad on the 722.6. What I couldn't find was any information related to any changes to throttle position reading based on that. I wasn't able to find any codes in the computer when I scanned it, only the P1000 that you get when you change the battery.

I will also go ahead and check all the grounds since they all got disconnected and (I think!) i reconnected them when the new engine went in.

 

What year/series is your car? This forum is for the X308, covering XJ MY's '98 to '03.

 

She's a 1999 XJR, all the top end electronics and intake work is stock for 99, the engine block is an 03 4.2.

 

That's what I thought when I first read thee post, but then had second thoughts.

Yea, you have the MB 722.6, model W5A580; easier to work on from what I've read. Do you have the "dip stick" for checking the fluid? It's not really a stick; more like a drain cleaning cable to snake around the bends in the tube. I've attached the checking procedure. If you don't have one of those, you can drain the fluid; there should be close to five quarts in there. Doing the conductor plate is pretty easy, but a good cleaning of the valve body and fitting the upgrades should be done, especially if it has never been done. That's somewhat tedious and you need to keep your full attention to what you're doing, but it makes a difference. Let me know and I'll send the pages out of the manual on how to do it.

For the TB, the senser on the left side is the Pedal Position Sensor (PPS); the Throttle Position Sensor (TPS) is on the right. I don't have much experience on the AJ26S TB since mine is an AJ27S and there's a big difference between the two. Here are couple of threads that discuss what you're describing, thread 2 is a link to one of the posts in thread 1 that wasn't linked. Thread 1 Thread 2

 

I will go ahead and check the fluid level when warm, I do in fact have the dipstick, bought one a long time ago for a mercedes. I'd rather not do the conductor plate if I don't have to so I will report back on the fluid level. It's been behaving for the last two days and if it doesn't go into "transmission fault" the throttle doesn't act up...

 

That last sentence indicates that the "transmission fault" message is putting the ECU into Fail Safe mode. That will sometimes clear with the key cycle without doing a hard reset. How's your voltage levels? They should be at 12.6 before startup and 13.5 to 14.5 with the engine running. If not within those numbers, you're more than likely getting a voltage drop when you hit the starter. If the voltage goes below the threshold for the ECU and TCM to stay in sync, you get a logic lock and then a default to Fail Safe mode.

I hear you on the conductor plate. I'm not a transmission guy, but I wanted to put the AMG blue tops in, so I did some reading and ended up getting the whole manual on the 722.6. That helped my confidence level to take on the project.