XJS 4.0 transmission limp-home

XJ-S
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Not a question for once, but just adding to the archives.

Our limp-home fault was cured by changing the trans oil - twice! First time, the garage guy saw nice clean oil coming out and thought the excercise was a waste of time. Refilled it and went down the road a few miles, on return the oil was filthy and smelly! Must have all been lurking in the torque convertor?

Another change saw it all nice & clean, with the limp-home almost but not quite disappearing. Turning off and on would reset it, then after a few miles it settled down and went away altogether, and it’s been that way now for 300 miles so I think we’ve cracked it. Might just do a final change to be on the safe side.

If you haven’t encountered limp-home, it manifests itself by giving you the trans warning light, third & reverse gears only, and very jerky when engaging said gears. You’ll definitely notice it!

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… You 'll definitely notice if you’re having ZF 4HP24 electronic tranny and if you’re true ‘JagLover’…

Otherwise, changin ATF fluid several times/dynamically before The Anniversary of Death - would be highly recommended.

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What you can also do, which I’ve done with my XJS V12 TH400 as well as other cars, is bleed it all.

Disconnect the transmission cooler line to radiator (make sure to get outward one!) connect tight fitting hose to it and put other end into bucket or gallon container. Idle car, going through the gears, until about 1/2 gallon comes out. Stop engine, and add 1/2 gallon new ATF through dipstick fill tube. Do this several times until only nice pink atf comes out. You will have bled entire transmission and torque converter using your transmission pump! Do NOT pump out too much without replacing, or pump could run dry.

Only downside, it can get messy, takes time and you need a lot of atf! My TH400 took 5 gallons if I remember correctly. My front wheel drive cars took 3 gallons. But you’ll have 99.999% new atf.

And this is not a power flush. Its a gentle flush.

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Thanks, Greg and Janusz, for the further info. All still well but I’ll definitely change the fluid again before too long.

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Sounds like what happened with Superblue a couple of years ago … She would go into that mode shortly after starting (although sometimes not until she had gone down the road a ways). If you shut the engine off and back on, that would clear it, but usually only temporarily. I took her to several shops and none could diagnose what was going on. :confused: One shop suggested changing her trans fluid and filter, and that action had no effect on the problem. Later on another shop (actually, a guy at his house that works on cars) claimed there was a wire loose in the harness going into the shifter from underneath, or some such, and supposedly took care of it. Even after I had Superbule back from his place she would still act up from time to time. :open_mouth: I finally just left her abandoned (yes, I was THAT upset at the time, frustrated over her unreliability :rage: ) in a large grocery store parking lot. Eventually she got stickered for tow off, after a few weeks. Before towing her off though, apparently the (very considerate :angel: ) tow driver left a personal note on her, pretty much making it clear that he really was about to tow her off for good. I remember finding her that way with one low tire, and, after airing the tire up and deciding to give her one last chance, cranked her over (I think her battery was low though, and had to use a jump box on her). Surprisingly, she started right up and no limp home light/mode. From that time on I never had the issue return. Strange, even for a Jag! :crazy_face: I really don’t know how just sitting that long fixed the issue, but apparently it did. Maybe the fresh trans fluid in her finally dissolved a bad “clot” somewhere in her system, and that did the trick? :confused: