Xjr gearbox issue

Hi, My 1996 xjr (GM gearbox) will change into and out of overdrive when climbing or descending moderate hills. Before you dismiss this as normal behaviour, it’s happening consistently. Selecting 3 on the quadrant is a workaround that solves the problem. When i get to a flattish road and go into D again…no problems.
Around town there are no issues, smooth quiet changes. There are no error codes detected by my OBD2 reader.
Has anybody had this experience?

Are you sure its GM, they were normally ZF in the X300 XJR, at what speed is this happening ?

Just to clarify, are you you you sure you’re getting a change back-n-forth from 3rd and 4th gears? You would see, oh, a 300-400 rpm difference on your tachometer at highway-ish speeds.

Or might you be feeling the torque converter locking and unlocking? This is generally subtle…100 rpm difference or so.

The latter can happen with the slightest provocation…just a slight increase in engine load or even a tiny tap on the brake. It’ll feel just like a gear change but usually less perceptible…unless it hunting crazily. Then it become impossible to miss. A mis-adjusted brake light switch, among many other things, can cause the torque converter to lock-unlock inappropriately.

Weak transmission mounts can cause excessive movement of the transmission…damaging to wiring at the connector. Common-ish on these cars. Something to look for; can give erratic transmission operation.

The two speed sensors and the A-B shift solenoids are problematic on the 4L80E and can give weird symptoms…although I don’t recall if excessive ‘hunting’ is one of them. The sensors and solenoids are DIY replaceable if you don’t mind getting slathered with dripping transmission oil.

Should be a routine matter for a decent trans shop. I think every possible failure on a 4L80E has been discovered and ironed out.

Others will come along with more ideas, no doubt


The XJR/6 and the X300/V12 both used the GM 4L80E transmission.

ZF was used in the normally aspirated 6-cylinder X300s


Doug, many thanks for your thoughts and quick reply
It is a subtle thing, so maybe your torque converter theory holds water. Would the holding in 3rd workaround stop it?
Hmmm, i did change the brake switch a while ago and adjusted it up to contact with small brake movement. Should I back it off?
We tested the leads going back to the ecu near the glovebox with good results. The big donut mount looks fine.
I will go for a drive tomorrow and watch the thrashometer carefully.

Thanks for the clarification Doug. That was my thought regarding the lock up feature as well hence the question of what speed it happened.

The lockup occurs at ~ 52 mph and if you select 3 in the J gate obviously you stay in 3rd. gear no shift through to 4th and then to lockup. As Doug says that transition is a subtle nudge as it shifts up and down at something of about 2000 to 2500 revs. I can’t recall the exact without looking.

Many thanks for your information Neil. Yesterday we were climbing long hills with me consciously maintaining 100kph and the transmission would “change” from 2000 to 2500 and then repeat the process again at 5 second intervals. On flat road or downhill everything is fine by changing into top and staying there at 2000 revs at 100k. Would you say that is normal behavior?

Doesn’t sound normal off-hand but it IS possible to have a sweet spot…er, sour spot…where the transmission can’t decide what gear to be in. Engine speed, engine load, road speed, throttle opening all at an unfortunate nexus.

What happens if if increase/decrease road speed by a couple 2-3 KHM?


Thanks Doug, yes that’s a good plan. I am hoping that there is nothing evil lurking. Otherwise the transmission, like the rest of the car at the moment, is performing faultlessly. Happy Christmas and thanks for the assistance.

when MINE ACTED UP LIKE THAT A 1996 XJR I sometimes had to remove the harness plug and clean the connections.
That car was bullet proof but primitive as the compuetrs were still in infant stage, hence the gm box/metric.
Good luck

That’s a good point, IIRC there have been posts over the years where folk have spliced in complete plugs or individual wires to repair breaks. Certainly on the X308 and I’m pretty sure on the X300.

Thanks GT Joey, I will do that. Having been behind that glove-box so many times now I am thinking of putting it on wingnuts! Cheers Charlie

Neil, that’s a terrifying thought! I am considering doing some preventative work like that on the boot hinge wiring though, which is an issue that gets some airplay. I am sniffing around for a second hand transmission control module to swap in just to eliminate one variable at at time. That strategy worked for the wipers…so maybe. Thanks Charlie

Also put a ground wire from the case to the frame, I think I did that too, even if its temporary.
I thought that used to pull/bend or crack as others have said if the kookie tranny mount was broken or loose.
Its years ago, but its all related.
Get it up on a lift, My OTHER xjr 2006 had a bad tranny mount which made that thud like it wasn’t shifting but the whole unit was rocking around making pressure on the shaft.
Hard to explain but you get the idea.
Good luck guys, I put 50,000 miles in ONE YEAR on my 2006 XJR , It was fun, ONLY thing I didn’t care for was the large tires that would tram in the grooves
Today probably Michelins would solve that?