At your leisure, can you post a picture? Thanks.

 

Sorry hard reset does not clear the codes.

 

UPDATE: new shop says the half shaft U joints are the cause of the surging. Told me that the U jonits are very noisy due to all the play in them.


I never heard a single sound when driving the car. I had to ask them if they have me confused with another car...they said no, that the U-joints are the problem. $2400 to replace since the rear has to all come apart.


I never heard of such a thing!!
I am going over to the shop in the morning and see this for myself.


U-joints????

 

There are 2 half shafts, one on each side, going from the side of the differential to the rear wheel. Each shaft end has to stay parallel to its mating surface, so there are 2 universal joints per half shaft, 4 U-joints total. And yes, they are U-joints, not sealed CV joints.

Each joint has a grease fitting, and is a often-neglected maintenance item. They are supposed to be pumped with grease every few thousand miles, like at every oil change.

The logic I can think of is that for a car travelling at a constant speed, the play in the U-joints makes the ABS reluctor ring on the wheel hub pick up a speed that seem to vary, so the computer constantly changes gear if it is around a shift point. If you accelerate a bit, the play is gone, so the transmission stays in gear. If you slow down, the play is pegged the other way, and the transmission stays in gear again. I suppose the same logic would apply going at constant speed uphill or downhill (it would not shift). I guess this theory is possibly testable as I believe the vehicle speed is part of the CAN/OBD messages that a good reader would show. Might even be possible to see it on the speedometer I suppose. Might be your root cause right there. As I said, there is a logic to it, but I lack the direct experience to be able to say for sure that fixing the U-joints will fix the original problem. I think this is a very good find. Best of luck, keep us posted.

 

Will not cure everything, but it does clear the P-codes that can be read over standard OBD-II readers, does it not?

 

I have never greased the half shaft U joints...I had assumed they were the non-greasable type....my bad. And I can agree with the logic to a point, however having a problem with why the P1797 TCM to ECU CAN Bus error is consistent if the problem is the U-joints. Unless the error code is set because the ABS speed does not agree with the TCM speed. The ECU must be listening to the TCM because engine RPM vaires without moving the right foot.


The lady from this shop tells me that there are no control or trailing arms and that the U-joints support the weight of the rear of the car. I told her that her information is incorrect as there are lower control arms and struts. I still think they have me confused with another Jag in their shop.


Like I said going to the shop in the morning to talk to the "tech".


I think I can change-out all four U-joints for alot less than $2400.....was told $363 for each U-joint. I think not.

 

Lanny, no cluck at any time?
They are a pain to do, but that price seems high. I don't recall anything other than removing the axle from the rear hub and the four bolts at the rear end. Might have to knock the lower shock bolt out. Not the whole rear end. They need to pay attention to the shims at the rear end side of the axle.


Wayne

 

After reading this thread I am not entirely sure just what the issue is.


If the main problem is surging at a constant speed (especially when cold) I would be very surprised it is any of the issues so far discussed due to he car having such relatively low miles. Unless the car had been driven extremely hard or tracked by a previous owner, which makes little sense for an XK8.


I would seriously consider this route as a relatively low-cost option, especially if you can DIY.


Read this thread https://www.jaguarforums.com/forum/x...03-xkr-147016/ for results on the Mercon SP fluid change.


Hope this helps. Personally, I would not go throwing money (seemingly big money too), at "unconfirmed" diagnostics by this shop or that shop.


This transmission has been used in many other car makes such as Ford, BMW, etc. and shows many of the same issues.

 

No it will not the DBDII codes will only be cleared by a code reader.

 

Please understand that this is not your dads Buick! This transmission is controlled electrically by readings from the Shifter position, Shifter Control Switch, BCM, TCM and all engine conditions are read and transfer information to the BCM to the TCM on demand of the TPS and PPS to telling it what to do. If in fact the car is surging at a steady speed I would be looking at the ECM, TCM and the valve body. The torque converter was brought up and not having a good strong background on the torque converter it is hard for me to say it is part of the problem but with the codes you have I would be looking at the TCM and or valve body you have no other codes to move you into another direction the “U” joints did not provide that code.

As for the “U” joints they have been overlooked for many years and have caused problems for many including me. However, I am hard pressed to say that is the problem. They do not throw transmission codes.

Edit Added Code

 

I just checked out the price of U joints and most run in the $30 to $40 range for the XK8. I was able to find a couple in the $250 range. There is always somebody that wants to screw you.


Wayne

 

Now that was a true statement.

 

First thing I will do is shoot grease into each U joint and go from there.


The shop gave us a Smart Car as a loaner....for some reason wife does not want to trade her XK8 for one of these....wonder why....LOL......

 

This U-joint issue is more than likely independent from the code issue. All I am saying is that there is some consistency between the surging and slack in the drivetrain. The issue is that there is no independent TCM speed. The speed of the vehicle is measured by the ABS sensor (and reluctor ring). I don't think there is consensus yet on the P1797 code being a symptom for the surging for sure.

For this, I believe the vehicle speed is constant, and the transmission shifts, so the rpm has to change. Let me ask you, do you have this surging at any and all speed, sort of on-demand, or is there surging only around the shift points (say, 30 Mph and 50 Mph)?

For the slack in the U-joints, I guess you could test it. With the car sitting still, engine running, and the foot on the brakes, gently go from D to R, then R to D. Check if there is a strong jerk. I assume the stronger the jerk, the bigger the slack.

 

Just returned from the shop and talking with the tech, he had me place my fingers on the flange that the inner U joint bolts to as he twisted each tire back and forth. I could feel a very slight movement which to me was gear lash but tech says was U joint. I got him to add grease to all four U joints and he said, "oh yeah...that reduced the movement some". We went for a drive and he kept asking if I feel that vibration...he feels it in the seat...I never did. I got him to hold the speed at 45mph and he saw the tach move 100rpm or so. He also mentioned that he heard a clicking in the transmission when another tech was in the driver's seat, engine running, trans in Drive and on the lift. Said it sounded like one of the solenoids clicking in and out.


He checked and there were no transmission error codes. I told him about the P1797 and his comment was that could be anything.


He also said that the U joint slop could be causing the solenoid to be clicking in and out.


He was going to reset the adaptives and I said fine and that after that he was done; will pick the car up.


This is where I am at on this.


I will do the D to R, R to D with foot on brake while I listed for any clunk...but have never heard anything when driving.


Surging only noticeable when at a constant throttle 40 - 50 mph. Have not tried driving at higher speeds...will try this weekend.

 

Hey GUS....what part number of U joints did you use? Did you use parts from Jaguar or aftermarket ones?

 

I used aftermarket "U" joints. If I told you the number of aftermarket items I used you would go into shock. A few things I will not use aftermarket but the "U" joints did not upset me at all.

 

Also you have two speed sensors in the transmission one is at the turbine (forward) and the other is output shaft (aft) and they are part of the valvebody wiring harness that reports to the TCM & ECM...

 

Does not seem like a terribly good test to me. If there is play in the U-joints between the diff and the wheel, a better test would be to hold the diff flange steady, and check if there is any movement at the wheel when you try to rotate it. There should be hardly any.

For your test at the diff flange, was the tech moving the wheel quite a bit? That would be the problem...

 

fmertz: yes I agree...that would have been a much better test. I think he was trying to convince me that the gear lash was U joint wear.


The test at the diff flange: tech was just rocking the wheel back and forth a small amount...less than an inch of travel in either direction.


GUS: I see that JagBits has aftermarket U joints listed; $43 each...much better than $250 each for "Jag" U joints.


The saga continues......................................