1996 XJS ..... Going 50 mph 20K rpm - take foot off the gas and when the rpm's get down to 14K, the car rpms go to 16K and back and forth ramping up and down (surge)
One mechanic said possible torque converter...another said an idle problem...possibly throttle step motor...
Any way to confirm one way or another ?

 

I will assume a 6cyl car??

That 50MPH (80KPH) is SMACK ON the torque converter "lockup" speed.

My X300 did that, and the same mechanicals if yours is a 6cyl. Changing the trans fluid multiple times, simply drop and refill, made a HUGE difference, so maybe your fluid needs some serious attention.

The XJ40's in my fleet (kids cars) also the same.

 

Thanks - It is a 6 cylinder yes. I just bought the car recently and was told the transmission fluid was just changed. Are you saying to do it again and again? For clarification I said 50 mph because that got the rpm to 20K. The speed at which it surged 14K to 16K was closer to 35 mph...not sure if that makes any difference in your response ...thank you

 

OK, the mud clears a tad, and the add ons in teh coffee are getting serious.

That transmission is a ZF4HP24, with "mechanical" converter lock up sat 50MPH., or very close to it, and in all mine it matters not how hard the gas pedal is pushed, they "lockup" at 50MPH.

I spoke with ZF here in Perth a very long time ago.

The advice was simple:

Drain the trans fluid, via teh large nut on the fill tube.
Refill with Synthetic Trans Fluid, any brand, it matters little.
Drain again after about 100kms, and as many gear changes as possible. So as to "work" the new fluid through the valve body etc.
REPEAT, until what drains out is the same colour as what goes in, or very close to.
DO NOT attach to one of these flush machines, as the new fluid may "shock" the transmission into a dead unit. Introduce the new fluid gradually was his strongest suggestion.

Mine was the ZF4HP22 trans, SAME unit, just NOT th elctrickery version, as it had old style cable kickdown.

After 8 (Eight) of these drop and refills, the trans gear change was near on undetectable, and the lock up, still the same speed, was a simple "bump" noticed mainly by a slight drop in the tacho.

Prior to this, the gearchanges were ROUGH, and the lockup was flaky, and THUMPY.

Same ZF guy explained that this trans is rock solid, BUT, it loves fresh fluid, regularly.

Some of these have a drain plug in the pan, one of my XJ40's did, the rest no such luck.

The filter inside is a Brass or Stainless mesh, and rarely needs attention.

Yours MAY have other issues, but I would NOT be messing with any huge $$ suggestions until the integrity of the fluid is a known item. ONE change is peanuts, its a 15ltr dry capacity, and at 4ltrs per drop and refill it will take 8 or so to purge the system.

The TPS (Throttle Position Sensor) mounted under the throttle body has some input to your trans, and is a known flaky item due to oil contamination from the throttle shaft, BUT, the engine usually runs crap also, so that is not on my radar at the moment, but worth keeping in mind.

Fluid is #1, and even my S Types, and the mighty X Type, all benefited from that drop and refill procedure.

 

coldpad1,

From your description, I wasn't quite sure if you were saying that the car just maintained a surging engine speed of between 1400 rpm and 1600 rpm (even an XJS wont get to 14k rpm!) or if it was trying to accelerate the car when the throttle was released? For example, when the problem happens, will the car slow down to a very slow speed, changing down gear as the speed drops, and then just maintains that 1400-1600 engine speed in 1st gear? Or does it try to maintain the 50mph road speed by holding the throttle at 1400-1600rpm without your foot on the pedal?

There seem to be 2 potential issues here: A transmission problem or a zero-throttle engine speed problem.

4 litre XJSs can be prone to not dropping the revs properly when the throttle pedal is released and the issue can be caused by a number of things. There was a technical bulletin issued by Jaguar early in the car's life where the directed solution was to change the throttle return spring. Other causes can be the MAF or even the ecu needing the idle adaptation resetting.

I would try and eliminate that it's a zero-throttle engine speed issue by trying the following:

If the problem happens when you release the throttle in lock-up top at 1400rpm and the revs then rise to 1600rpm, put the car into Neutral and see if the revs drop right off to idle speed.

If not, you have an engine idle issue. But if it does drop, then I'd start to look at the transmission and start with the fluid.

As Grant has said, you need to change the fluid more than once if the transmission sump is being emptied from the drain plug or dipstick tube. And that's why if someone "has just changed the fluid", well they haven't really!

The ZF4HP24 system in your car has a total capacity of 9 litres. IF someone has just drained the sump and refilled, they will only have pulled about 2.45 litres. In other words you'll still have 73% of the old fluid still in the system!

When I did my last filter change, I did three drain and refills via the dipstick tube with 10 mile runs in-between each change. I then dropped the sump. changed the filter and refilled. I then did three more drains and refills with 10 mile trips in-between. That was the only way that I felt comfortable that I'd eventually changed most of the old fluid. By my calculation, after those 7 drains, I had changed just over 90% of the original dirty fluid.

This MAY not be the cause of your problem. But if you don't know exactly the state of your transmission fluid, you should change it properly anyway and then see if the problem still persists. We'll be here waiting for you!

Good luck

Paul

 

I'll try to explain more clearly. The car shifts through all gears fine. When I get the car to approximately 50 mph and take my foot off the pedal and leave it off...the car slows down to about 30-35 mph and as the tech reaches 1400 rpm it then goes to 1600 rpm...back and forth between the 2. It doesn't "feel" like the transmission is changing back and forth between gears when this is happening but it could be I guess. It feels more like the cruise control is on and its adjusting the speed...The cruise control is not on.

I'd like to also question if there is any danger in using synthetic fluid as suggested by Grant. A local Jaguar mechanic said " I would not use synthetic fluid"... just asking why or why not.

 

Just took her out for a quick spin and did as you suggested. When I put the car in neutral the problem at 1400 -1600 rpms went away...Thanks

 

so...I should try the tranny fluid...please confirm
1. Should I or should I not use synthetic?
2. Drain by drain plug and refill every 10 miles 3 times.
3. Drain and remove pan, change filter and refill
4. Drain by drain plug and refill every 10 miles 3 more times.
Is this correct?

 

I will not get into fluids, but all min ehave Synthetic, even the old style BW65/66.
If you are not comfortable, use Mineral, the car will still luv you.

Drain whats in there, you sound ypu have a drain plug, goodo.
Refill with fresh fluid.
Drive it, my drop and fill timing was based on home/work/home = 40kms. The more gearchanges it does is more important than freeway style, thus working the new fluid through the valve body.
Drop again, and refill.
Keep at this until the colour coming OUT is about the same as whats going IN
FINAL TIME.
Drop the pan, do the filter, refill, and put that trans on the "annual" list of servicing.

It really is that simple. The filter inside is a "mesh" medium, unlike the V12, which has a "fabric" style filter. The "mesh: can be simply washed out in Solvent, the "fabric" no.

 

Thank you

 

Update... 3rd drain and refill and still a distinguishable difference in color...

 

It would have been better to flush the transmission with a transmission flush machine. Ready in one action.

 

I was told by numerous people never ever to use a tranny flush...are they incorrect

 

I refuse to use a flush machine, BUT. I am very OLD school.

Also, ZF here is Perth made it loud and clear that flush machines, CAN, SOMETIMES, shock the trans internals, and cause failure.

I will leave it at that, as it will cause a "he said, she said" thread, which is silly.

At #3 you are barely half way. Mine took 8 to 9 drop and refills, ALL of them, from the old Series 2's, right up to the current 2010 X Type. The fluid in those cars was BLACK and STINKY, and I dont regret the procedure I used.

 

There is a procedure on the X Type forum that I used and it seemed to give a good change. Drain as much as possible, them refill with the same amount of new fluid. Start the car and let it run for 14 seconds. Shut off the car. Drain fluid and refill. Repeat 3 more times.

The idea is that by running for only 14 seconds the fluid is picked up from the pan and starts to circulate. Essentially the 14 seconds is the time needed to suck the new fluid from the pan and start it circulating through the transmission. By only running for 14 seconds, it minimizes the mixing of the old and new fluids, so gives a more complete fluid change. If you do one change and then drive the car for a while, there is mixing of the old and new, and when you drain the pan you will be draining some new fluid out. Obviously the goal is to mimimize draining new fluid, but change as much of the old fluid as possible. I don't know if 14 seconds works for the ZF trans, but the amount of fluid in the X Type per drain was 2.7 litres, so should be similar.

I see no reason not to use synthetic of the appropriate type. I do not like the multi spec fluids, I prefer to use the proper spec fluid. The ZF is Dexron III I believe, so that's an easy one to match.

 

Dexron 'anything' is merely a standard, lubricants have improved massively over the years, Dexron vi is compatible with all previous, GM recommendatons (it is their spec), they advise to replace all but not strictly necessary. There were problems with early synthetics and older trannies (and engines) but these have long since been resolved - alas the urban myths survive. There is a part of a GM document here GM Extract (I'm trying to find the original), it warns why flushing is problematic (because many that flush boxes use solvents), and how flushing 'should' be done.

I had my supercharged XF done by a ZF approved transmission shop, it wasn't cheap (almost £600) but they showed me the crud that came out which was impressive, I've never looked back and have put 20k on the box since, the change in box performance was noticable.

There are no dinosaur based fluids left in mine.

 

First investigate what's going on with the transmission and then decide whether to flush or not. My Jaguar specialist (One of the largest in the Netherlands) does a lot of them without any problems.