Hello,
I was driving our 99 XJR last Friday when I noticed she was revving a little higher than usual once I hit the expressway. Shortly after that observation I saw a "transmission fault" displayed on the message center.
I was in the middle of a timing chain replacement on our E39 beemer so I just parked the old girl for the weekend. I did manage to do some research and watched a guy called "mercedessource" on you tube discuss how he revived a 722.6 trans in his 2000 Merc wagon.
He tracked the trouble to a bad conductor plate and some leaking O-rings in a connector housing to that conductor plate. His video was pretty easy to follow even though you have to be patient with him, he repeats a lot of stuff (IMHO).
I was going to do a filter and fluid change as the XJR was taking a second to go from drive to reverse in the mornings lately and I'm wondering if it's just worn fluid but I wanted to see if anyone had had any run ins with this conductor plate and or connector housing. I have 83k on the clock and no idea if the fluid was ever changed.
Let me know what you think. One interesting thing, even though you have to wade thru the whole 6 part series to get there is he points out that his "fix" only really fixed things after he went to someone with a code clearing device that could clear the transmission code.

 

The conductor plate replacement is a fairly common operation, often due to a speed sensor failure. You may be able to read a stored code which will help clarify the problem. The parts stores will read them, but the reader may not capture transmission codes.

I don't think the fluid would trigger a limp mode unless it's low, so odds are it's the conductor plate. They are available on ebay or amazon and as you saw are easy to replace. Definitely do the plug, also.

 

I have changed out the electrical conductor plate on my XJR and Mercedes C280. (They both have the same 722.6 tranny.) My thoughts are that the speed sensor on the conductor plate went bad on your car, which is a common failure and causes the transmission control module to be confused, hence the transmission fault and/or limp home mode. You have to realize this plastic part is submerged in hot tranny fluid all these years and cracks develop. I got mine from Amazon.com. It is a really easy swap.

If after you did this and you still have problems, your tranny maybe slipping(?) Use Mercedes ATF134 fluid only for fluid replacement, even though it costs $23 per liter. You will need 7.5 liters for a full change. Do not use the compatible Shell ATF134, Valvoline Maxlife or anything else. I did and I think it caused slippage in my C280, 4K miles later after using Maxlife. I had to rebuild the tranny afterwards, and my friction clutches still looked new. That told me the fluid did not had the correct anti-slip formula in it. Ugh!

 

I used the Shell fluid in a XKR two years and about 20K mi ago with no problems, but that's just one car.

 

Thanks for the replies so far. After RJ237's I started looking for the conductor plate on ebay. Are all the 722.6's the same or are there variations depending on model? I entered my model info in several ads and it came back with a "not being compatible with my vehicle" message.
I asked the question directly of one of the sellers but haven't received an answer yet. There are a bunch of people who sell the plate and coupler housing so this must be a common issue.

 

They are the same. several times in the past this issue has been brought up. I just bought a transmission oil seal that fits a BMW but not a XK8, even though it's the same 5HP24.

 

If your transmission had a slippage problem, the friction clutches should have looked worn out, not new (excessive slippage causes the friction material to wear off at a greatly increased rate).