Hello friends, as some of you know I recently swapped my XJ8's failing transmission with a much newer one that had only 57k miles. After the swap the car kept going into limp mode, which I pinpointed to be a vacuum leak (fixed). After that, the trans worked amazingly well, smooth, quiet shifts. After putting around 250 miles on it however, I have now noticed it's starting to have symptoms like the old one. It clunks sometimes while shifting from reverse to drive too suddenly (under two seconds), and now shifts in gear are hard and slow. By hard I mean the car abruptly slows down for a second or two while it shifts, it almost feels like engine braking. Am I gonna need another transmission again, or does it simply need a fluid and filter change? I doubt it would be that simple, judging on how fragile 5HP24's are, but it is weird that it was working perfectly fine at first. Any thoughts?

 

Did you replace the torque converter too ? Or just transmission

 

I'm assuming the torque too, unless that's not what that blue thing in front of the transmission was.

 

Blue thing?

 

I'm kidding , of course I swapped the torque converter too.

 

Then I believe the shop add correct amount of fluid and clean filter but according to what you're saying the transmission is saying goodbye , back to the shop

 

I think so too. A shame, I barely even got to use the thing. Thinking about it, I probably put maybe 150 miles on it if not less. My last hope is it's some kind of electrical issue, but there's no codes or anything of the sort.

 

New theory: Just drove her again and noticed the car shakes on idle every few seconds. It's an irregular beat but it's there. My first thought is sparks, but maybe the transmission doesn't know when to shift? The engine seems to lose power sometimes as well, but there's no codes whatsoever.

 

Are you in park when it’s at idle and shakes or is it in drive, at a stop light and shakes?

 

It's worse when I'm at a stop light. Huh, hadn't noticed that.

 

And it's not exactly a shake, it's more like the engine shuts off for half a second, then the whole car jolts mildly, then it turns back on instantly (but not actually shutting off).

 

Do you think it’s slipping in and out of gear when you’re stopped at a light?
Do you notice any fluid leaks where you park?

 

That would be concerning; it is definitely possible, but wouldn't I hear a noise coming from under the car like a normal shift or a knock or something at all? It really feels and sounds like it's coming from the engine. No leaks.

 

It’s possible that it could be resonating.

Stepping back a few posts you stated that the engine shuts off a half second then comes back.. Have you checked the bulkhead connector on the passenger side engine bay? Maybe you have a loose, high resistance or burned up power or ground??