Old school mech needs help!!
1999 xk8. When starting out at times will let rpms rise and slam in to gear. Seems to happen more often when warm. Have replaced the "pedal switch". Seems to get better when i do a hard reset.(connect batt cables..) Really could use some advice. Thanks Joe

 

Sounds exactly like fractured 'A' drum in transmission. See if this fits: when accelerating, even modestly, from a full stop the engine revs up and the car barely moves then wham! If so, what is happening (at least it describes exactly what mine did) is the A drum has fractured and cannot obtain adequate pressure on the clutch pack. When the computer reads the difference between the input and output transmission speed sensors as too far beyond expected it defaults to fifth gear. The already revved engine instanly applies power to fifth gear and that's when you get the wham. Normally this would be accompanied by a 'Gearbox Fault' message in the display.

I suppose it could also be low ATF, a near-plugged trans' filter or both. As the engine runs long enough for the fluid to stir up debris in the trans' pan the filter cannot initially pass enough fluid to build adequate pressure.

 

My '99 does the same thing. I don't think it is an A drum problem. After it slams on take-off, it shifts normally, and in the correct gear. My feeling is that it is an electrical issue, like a solenoid is not allowing oil flow to a clutch or band, until pressure perhaps overrides the solenoid as though it were stuck.
I am not a trans tech by any means, but have a vague understanding of how they work. I would appreciate any insight into this problem as well. Mine has been doing this sporadically for nearly a year.

 

More info. Only will do it when "using brake pedle" if you use the ebrake to stop it will not do it. Have been on this goose chase for a while. It is intermittent..Have been told there is some "connection" with the trans, throttle body,ABS and trac control. Once again a hard reset will last maybe a day. Also not sue on how to correctly adj peddle switch (where the cable attaches).Trans fluid is full. Trans seems to not be the issue.

Thanks for replying.....Hope Someone helps get the goose...LOL

 

My first thought was low transmission fluid, although if that was the case I'd think it would be worse when cold and get better as the fluid warms up and expands. I think the first, easiest, and cheapest thing to do would be to just check the fluid level. Then change the fluid and filter if it isn't low.

 

Although it may sound strange, you might first try cleaning the throttle body and mass airflow sensor. And checking to see that your air filter is clean also. Dirty TBs/MAFs/filters can cause the same symptoms you have described when starting off from a dead stop. Happened to me with my 99 XK8. (If you are having shift problems in other gears, this may not be the solution, however.)

 

+1 I had the failed "A" drum and it did not act that way. Check and change the fluid!

 

Cleaned Mass sensor,tb changed filter. No help...but worth a try,thanks. Again do not think it is trans, If you hold the rpms at 1500 while stoping,when you have the brake on you can feel it "unload the trans,then load again" really think it is in abs>trac control> tb...Car runs and shifts perfect, no codes.....

 

What makes you think it is the ABS / Traction control? You say no codes, are you testing with an OBD reader to read Trans & OBD codes? This situation seems new and not sure what to think, Trans, Eng and Electrical. This is a 1999 xk8 and it has problems with the TB recall (expired) & TB Plugs and a few other things like the transmission A & B drums. I think we need additional information tell us more about what it is doing.