Delay / Hard shift from Reverse to Drive

Symptom: When moving the shift selector from Reverse to Drive there is a 1 second delay before you feel positive forward pressure. If the accelerator is pressed immediately after moving from R to D, without waiting for 1 second, there is a slight lurch with an associated clunk.

Testing: The ZF 6HP 26 transmission shifts normally, up & down, once moving and under any degree of acceleration. The only way to get the problem to repeat is to select R then back to D. The 1 second delay occurs always.

History: the car is a 1999 XK8 convertible, normaly aspirated with 85,000. The transmission filter and oil were changed at 60,000. Oil was Esso ATF LT 71141. No problems at that time. The car now has 85,000 miles. It was borrowed for about 1 year by my brother. He ran over a tire on the interstate and transmission fault codes appeared. He had it serviced by the local Florida dealer who commented upon arrival, “we don’t like to work on these old cars, but since you are here we will do what we can”. After $4,000 worth of work, over several visits, problems continued so I had the car transported from Florida to New Hampshire and started to work on the problem. The first problem of DTC P0121, Throttle Position Signal due to incompatible metals used in a connector and the car defaulting to safe mode was resolved by identifying a loose connector at the throttle body. Tech Bullentin 303-58 had already been worked several years earlier by the dealer in Indianapolis (excellent service BTW) so I knew it could not be the metal. It turned out to be a connector that was loose. This had been completely missed by the Florida dealer. The car now worked perfectly, no codes, except for the R to D shift delay. I changed the fluid and refilled following the refill procedure in the Jaguar Workshop manual for automatic transmissions using Febi bilstein Nr. 14738 which meets all specifications. I replaced the trans fluid in case the dealer used the incorrect fluid.

Next steps: It is my plan now to again replace the transmission fluid and the filter. It is possible the dealer in Florida did not seat the filter properly or used the wrong filter, or didn’t change the filter. My guess is there is something that is allowing the pressure to drop during the shift from R to D but is not impacting the normal shifting pattern or responsiveness.

I am at a loss at this point and guessing. If anyone has any good ideas to try I would love to hear them.

Thank-you for reading my story! :blush:

Your car is fitted with a ZF 5HP24 (NOT a ZF 6HP26).
Fluid level check would be my first step in diagnosis.

Motorcarman, thank-you. :blush: My other Jags have the 6 and I think I got carried away with my typing. Maybe memory problem, hope not. :flushed:Thank-you! :grin: Sometimes small over sites like that can mean a big difference. I have checked the fluid level several times. Start engine, P then R wait 3 seconds, then D wait 3 seconds, then back to P. Check trans oil temp. Leave engine running. Open trans fill plug. Make sure slight trickle of fluid is evident. I have also topped off the transmission just to make sure on the fill level and let it slow to a trickle. Very frustrating. I’m waiting for the filter and gasket to arrive from JagBits to see if it could be a filter problem. In a way, I am very interested in finding out what is causing this mystery. Hope it’s simple. Anyone think it is possible the Florida dealer could have put the wrong program in the ECM? Or am I completely off base with that idea? :thinking: I thought I read something years ago about a “lurch” in an XK that was related to programming. Now that one is a ZF 6 HP 26 I’m trying to find the old thread. Again, thank you for the information.

The ECM should not have anything to do with the TCM configuration.
I don’t think the 5HP24 TCM can be changed without Jaguar Engineering Mode or something similar.

I ‘reflashed’ a few dozen ECMs when I worked at the dealer. It was during RECALLS or SERVICE ACTIONS mostly for Emission or Running Problems.

The Mechatronics (6HP26) reconfigure or CLEAR ADAPTATIONS for the ZF 6HP26 is a different ‘animal’.

I always get the car on the lift and then leave it overnight. (dead cold in the morning)
Get everything ready with fluid and tools. Start the engine and then start working like a ‘madman’ to try to get the fill plug out and make sure the temp stays between 30 and 40 degrees Centigrade. (50 is the absolute max temp that will give you the MINIMUM level)

Texas in the summer will have a morning shop temp of about 30 degrees already so I work fast to keep it in parameters.

A few ounces overfull is probably better than UNDERFILLED.

Hi motorcarman,

Excellent information. I am glad to hear that the ECM programming is outside of the considerations at this point.

I am going to take your process into my garage and place the XK8 on my lift. Texas is a beautiful state. Fortunately, here in New Hampshire the temps don’t reach the highs of Texas, but it is a bit warm at the moment at 84F.

Test complete. Here are the results. Outside air temp 84F/28.9C. Transmission fluid temp 98.6F/37C at the finish of the test. After moving through the gear selection, P - D - R - P process removed the fill plug. Oil moved out of the fill hole at a good slow pace, but slowing pace for 5 seconds. I refastened the fill plug.

At this point I believe the transmission is filled correctly.

This is a very interesting problem, frustrating, but a good challenge. As someone told me, the more frustrating the problem, the greater the satisfaction when solved.

I did find an interesting thread of transmission lurch problems with the S ZF 6HP26 transmission which is not the XK8’s ZF 5H24, but maybe similar enough to have similar problems? The site is I’m reviewing it now as I have a 2007 S.

Keep the ideas coming in . . .

This weekend I was successful in locating 5 interesting documents on the ZF5HP24 transmission. I located a repair manual, parts manual, electronic control description, valve body TSB ( 307-03, Replace Valve Body) and an excellent discussion of service issues written by B A Transmissions. At this point I believe the delayed engagement between P & R and D are being caused by a delay between the solenoid shift valve and the gear clutch. The normal clutch fill time period is supposed to be a gradual increase in pressure covering a period of 250 milliseconds. I am noticing a delay engagement time of 1 full second which generates the hard shift. There appear to be four possible causes for the longer fill times. 1. low voltage in solenoid valves, 2. frictional variations in the fluid values, 3. frictional variations due to aging, 4. impurities in fluid creating greater valve friction. I await the new filter from JagBits and hopefully there will be impurities in the filter causing frictional variations. Wouldn’t it be nice if the solution is a simple one. :grinning: Let’s hope.

This may be a silly question but have you inspected the drive shaft for any slop? The condition you described may be caused by a worn or damaged universaal joint or flex disk.

Hello Bob,

No I have not! That is a very interesting idea. I’m going out to the garage immediately after dinner to check that out.


I checked out the joints, drive-shaft and flex disk. There is less than 3mm of play in the drive-shaft. The flex-disk looks good and solid with no separations. I think I’m back to the transmission. :neutral_face: