2000 Jaguar S-Type 4th to 5th Gear Jumping Back and Forth,

the brake switch fault could be the cause of the transmission shifting error .. repalce the switch and repost with your results .. check for any engine vacuum line leaks for the idleing fault !!

Just swapped the brake switch and the speed control switch right above it. Still idles really crappy, and is super jumpy in the shifting around 45-55 mph. Any other suggestions are greatly appreciated.

Did you delete the fault codes from the cars computer ?? .. they need to be deleted and the car go through a relearn drivecyle .. if you did delete the codes is the check engine light back on ?? is there any new/same codes ?? did you check for vacuum line leaks ??

Note : not all generic handheld fault code readers wil supply transmission codes .. have the vehicle plugged into a professional code reader .. (autozone/aamco test free usually)

Well, I got the new brake switch and speed control switch installed, and went to clear the codes. To my surprise, there was a new code. (I'm sure there was only the one about the brake switch before). The new one is P0306 and the old one was P1572. As far as I can tell, P0306 is "Cylinder 6 Misfire"

Could that be causing the poor idle and shifting problems? Upon closer attention, it looks like it's jumpy when shifting from 3-4 and 4-5.

I cleared the codes, and neither of them have come back yet. I've only driven the car about 30-40 miles though.

The only ideas I've seen online are to change the transmission fluid, and reflash the PCM, but I'm doubtful either of those will work.

THANKS SO MUCH for your help up until now. Any future ideas are greatly appreciated.

So I changed the cylinder 6 spark plug and coil, no improvement. There was a little oil down around the spark plug...but with a brand new plug and coil you'd think it would at least fix it temporarily. I was receiving such good help...please don't give up on me!

Sorry I did not get back to you sooner work/family commitments etc. A cylinder misfire will not generally cause erratic shifting between 4 & 5 ... did the codes come back ? is it still shifting erratic or harsh between any gears ?? .. there are numerous PCM (computer) reflash updates for this vehicle it could be it has not had them all ? I would not advise a transmission flush on this vehicle !! Let me know

No codes have come back, and I've put on plenty of miles and the engine must have cycled enough times by now to spit more codes out. All I can say is that once it warms up, the idle is lousy and it jumps back and forth between the upper gears from about 45-60 mph.

Any other ideas would be great. Is jag the only ones that can do the reflashes?

Yes, i'm afraid only dealers in the U.S. have the software to reflash the computer ... I have been thinking about your idle problem and remebered this TSB ! POOR IDLE/CHECK ENGINE MIL - V6 ENGINES - IMPROVED BREATHER ELBOW - SERVICE ACTION S158 TECHNICAL SERVICE BULLETIN Reference Number(s): S303-S158, Date of Issue: July 2003 JAGUAR:2000-01 MY, S-TYPE 3.0L VIN:L00001-L86901 SECTION:Engine System Related Ref Number(s): S303-S158 ARTICLE BEGINNING Issue S-TYPE 3.0 liter V6 vehicles, within the above VIN range, are equipped with a 3/8-inch bore 90 ° rubber elbow as part of the engine breather system. This elbow is manufactured from a type of rubber that is subject to degradation from the gas and vapor that pass through it, resulting, after a period of time, in air being allowed to enter the intake system of the engine. Vehicles within the above VIN range may exhibit poor idle and a 'check engine' MIL may be displayed on the instrument cluster. Action On 2000-01 S-TYPE 3.0L vehicles within the above VIN range, install a revised breather elbow, at the next available opportunity. Also remembered there was an updated valve body introduced for inside the transmission on this year vehicle 5-SPEED TRANSMISSION - ERRATIC OR HARSH SHIFTS - VALVE BODY KIT AVAILABLE - REPAIR PROCEDURE TECHNICAL SERVICE BULLETIN Reference Number(s): S307-19, Date of Issue: June 2005 JAGUAR:2000-02 MY S-TYPE VIN:L00001-M44997 SECTION:Transmission System Related Ref Number(s): S307-19 ARTICLE BEGINNING Issue Some 2000-02 MY S-TYPE vehicles within the above VIN range may exhibit erratic or harsh transmission shifts. This can be caused by valve body or control solenoid failure. Action NOTE:Check for connector/wiring and Powertrain Control Module (PCM) faults, ensure the transmission fluid is clean and there is no debris in the sump. Refer to Global Technical Reference GTR Workshop Manual, section: 307-01 - Diagnosis and Testing. Any faults other than the valve body kit replacement, are to be carried out as a separate Warranty Claim. When a fault has been confirmed with the valve body or control solenoid, a new valve body kit must be installed. Follow the Service Instruction. check/replace the elbow connector and test vehicle .. although i'm not sure both faults are connected .. check to see if both are still occuring ?

let me know

That is a lot of good information, and I looked for the elbow but the picture doesn't look the same as my car. All that stuff was for the 3.0 V6 I think, and mine is the the 4.0 V8. I just don't know what to do anymore.