No info found on this
My XK8 seems to slip and throws a gearbox fault code when I kickdown from 6-5. However, all upshifts and other downshifts are smooth. Also, if 6-5 downshift is performed by J-gate manually or decreased speed, it shifts fine
Any thoughts?

 

Have you checked the fluid level?

 

I would start with a fluid change on the transmission and filter change. Also replace the sleeves and connector block to the mechantronix unit. Normally that will fix most problems. Like this GENUINE ZF 6 SPEED 6HP26 6HP28 6HP32 6HP34 SLEEVE TUBES AND ADAPTER SEAL KIT OEM | eBay

 

Thanks for the lead.
Guess I’ll start from there.

 

Many of us have used Ford's much less costly Mercon SP ATF for a decade now with no issues. It is essentially ZF's Lifeguard 6 ATF colored red instead of amber....

 

It goes without saying, but I will say it anyway. There will be no good money thrown at this problem until you read the actual codes from the TCM.

If the codes suggest anything wrong with any of the solenoids valves, you might need/want to replace them. There are kits available. The solenoid valves are on the valve body, (easily?) reachable from the bottom. Some go further and procure a replacement rebuilt valve body, including the valves.

Best of luck, keep us posted.

 

AND, it's a Crap Shoot if you develop a "SQUAWK". The initial change on mine was great. Filter & fluid, all was good. Next time, same procedure and we had the SQUAWK. . Downshift from 6th to 3rd or 4th and was very irritating. Jaguar discontinued the Additive separately but it is now in LG-6. Since the Ford 6R80 is a ZF licensed design of the 6HP26, Ford had a similar problem and has a kit,
Motorcraft XL16. We used this and now there is no Squawk on kick down. Use the suggested amount. XL16 cost us $75 and required google search. Hope this does not deter you from the service.

 

A few questions from a squawker. I did a search on Motorcraft XL16 sales and got two types of hits. One type shows a single syringe of additive being sold with a label of "6R ATF ADDITIVE T" . The second type shows a two syringes, one of additive with a label of "6R ATF ADDITIVE T", and a second syringe of additive with a label of "6R ATF ADDITIVE F", with the twin syringe package labelled "Motorcraft 6R ATF Additive Kit XL16".

* Did you use the single "T" syringe or the twin "T and F" syringe kit?
* Where did you find the suggested amount?

Later edit, found a useful video on the XL16 Kit: search YouTube for "VsPetsHaKcU"

A related question: Say one has the gear box correctly filled to the "full" level so that the transmission fluid barely weeps out the fill aperture (while the engine is running, with the transmission fluid in the required temperature range) . Now, if one then has the same gearbox COLD/engine off and removes the transmission fill plug, will the transmission fluid leak out the fill aperture or not?

 

If you take the fill plug out while the transmission is cold, yes you will lose a lot of fluid that you need. Don't ask ...

 

Thanks!!
Just what I suspected.

 

After some further research, I think that I have answered my own questions. The referenced Ford Parts YouTube video "VsPetsHaKcU" was VERY informative. The XL16 Kit from Ford does include both the "T" and "F" syringes. The "T" is just a tracer additive and is not to be used. Just throw it away - all the vendors selling just the "T" syringe should be ignored. The "F" syringe is what is needed to deal with the squawk issue and does not seem to be sold by itself. Only 9-10 ml of the "F" should be added to the 6R80 transmission (the analog to our ZF gearboxes). The 6R80 transmission is "famous" for its squawking for which Ford issued TSB 14-0-0100, which has the instructions for the "F" addition. The XL16 Kit can be found on the internet for a wide range of prices. Also, the fill plug should be removed only with the engine running (ahem).

 

I assume this is really about finding a way to inject this additive into the box. For that, check the 2 lines to the cooler on the cold side of the radiator. These are fairly high and can be disconnected without much of a leak. I would start with the top line and see if the syringe with Mercon can push the fluid in there effectively. If it works, then proceed with the expensive additive.

Best of luck, keep us posted.

 

Finally borrowed a diagnostic computer, scan results show 5th gear ratio fault code. No solenoid codes. Will continue with oil, filter, and seals change first.
Also, I did hear sqwak noise occasionally. I’ll try the additive, too.
Thanks for the useful intel everyone

 

Few ratio faults are cured by fluid/filter changes. You should take this opportunity to change the sleeves that lie between the mechatronics (aka valve body) and the trans housing. These harden over the years and can often cause weird behavior. Do it now and avoid paying for more fluid later after you find that the fluid change didn't help.