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Jaguar Repair/86 xj6 automatic shifting problem and thermostat replacement


Shifting from 2nd to 3rd will not occur until ths accelorator is lifted. Then it shifts properly. What causes this.?
What is the difficulty in replacing this vehicles thetmostat?
I have no experence with jaguar.

Hi Bruce,

A late or delayed shift from 2nd to 3rd is a common fault on the Borg Warner 66 transmission in the Jaguar series III cars. I have had several and worked on a lot of them, but I am not an automatic transmission tech, so I can only guess as the two Jags I now have both do it.

At first I suspected a band adjustment but had that done on one of them and it still did it until it warms up which suggests to me to be a problem in the valve body. I have already adjusted the kick down cable so that is not the problem which means it is inside and an automatic transmission shop is needed. This is an American transmission so most automatic transmission shops can work on it.

Try this, slowly accelerate up to crusis speed and note at what speed it finally shifts and then do it again after at least 30 minutes of driving to heat the oil up and see if it shifts better after heated up.

Either way there is nothing you can do from the outside.

As for the thermostat replacing, first get a Jag thermostat as it is different then a domestic thermostat. There is a balker disk on the  end of it that blocks off a large bypass port in the thermostat housing (at the end of the upper radiator hose)

The series III Jag has the best thermostat system of any car I have worked on in my 50 years of working on cars. If the car does not heat up (on the gauge) very quick, then the thermostat is the first thing to check. But if the car is over heating you need to check that and check for CO in the coolant with the dye available from most auto parts stores.

When every thing is right the engine temperature is very stable winter and summer and does it from cold start to within several blocks of driving.


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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