Car broke down on the motorway this morning, Gutted!

Was accelerating off the slip road onto the motorway, heard a bang (seemingly from the rear, but I was not expecting it so obviously not listening out) and had no drive. Engine running fine.

Pulled over, tried putting back into gear. Everything felt OK, engine revs dropped a little when the gearbox engaged as normal, engine revved but no traction whatsoever when I tried pulling away (I wasn't expecting it to, but I said a prayer just in case!).

Got towed home and is now sitting on my drive. I have had a crawl under, half shafts feel good, still attached to the diff and UV's still intact. Propshaft doesn't feel loose.

So I am thinking a gearbox problem seems likely, which is not good news at all.

Any advice as to things I could try before putting it into a garage to pay for diagnostics before most likely scrapping it?

 

It could be a problem with the differential. You should try to get someone to push your car and, in the meantime, look underneath to see if the drive shaft is turning.

 

I agree. It feels to me like a diff issue as the gearboxes usually suffer some symptoms before they finally give up the ghost whereas diffs can often fail catastrophically.

I changed the gearbox and diff oil for the first time on my XK8 at 75k miles and whilst the gearbox oil was acceptable, the diff oil was basically black sludge. I know which would have failed first on my car.

Richard

 

With the car on the ground, try putting a large mirror or a video camera under it: start it & shift to drive & - while holding the brakes - give it a little gas. If the driveshaft turns then it's the diff. if the engine revs but the driveshaft doesn't turn, then it's the trans.
As Sleeperdude says: Folks check your rearend - nobody else will do it for you.

 

Are you getting any warnings on the dash? Also, have you tried reverse?

 

Normally, when these cars lose drive due to a transmission failure, you’ll also get a Transmission Fault message and stored fault codes. The absence of that (assuming no warning was displayed) makes a gearbox failure less likely, though not impossible.

Given the loud bang from the rear and the complete loss of drive with the engine and gear engagement behaving normally, this does point more toward a differential failure.

If it does turn out to be the differential, that’s at least better news financially than a transmission replacement—and it also opens up some upgrade options.

• The cheapest and simplest route would be sourcing another 14HU differential from an XK8, XKR, or XJ8, or having your existing unit rebuilt.
• If you wanted to upgrade while you’re in there, this would be a good opportunity to fit a Quaife QDF5W ATB helical differential.
• Another option is fitting a 15HU from an XJ12 or XJR (6-cylinder). This gives you a clutch-type LSD and a stronger unit overall compared to the 14HU. It’s not a direct bolt-in, and not all 15HUs are LSDs, but the swap has been well documented on this forum.
  

An LSD—whether ATB or clutch type—offers clear traction and drivability advantages over the stock open differential, and the 15HU in particular is known to handle higher torque loads better.

Before writing the car off, I’d strongly suggest confirming whether the diff internals have failed or whether it's the transmission —you may find this is far more salvageable than it initially feels.
Here are a few checks you can do to help narrow down gearbox vs diff before spending money or scrapping the car:

1) Engine running, rear wheels off the ground (handbrake OFF):

• Select Drive or Reverse.
  • Propshaft spins but rear wheels don’t → diff failure.
  • Nothing spins → gearbox / torque converter issue.

2) Engine load check:

• When selecting D or R, does the idle drop slightly?
  • Yes → torque converter is coupling, gearbox likely OK.
  • No change at all → gearbox / Torque Converter problem.

3) Propshaft by hand (engine OFF, selector in Park):

• Propshaft should not spin freely.
  • Spins easily → diff internals likely failed.
  • Locked solid → diff may still be intact.

4) Rear wheel rotation (engine OFF, in Park):

• Rotate one rear wheel by hand.
  • Other wheel should rotate opposite direction (open diff behavior).
  • If it just free-spins or clunks badly → broken diff.

5) Diff oil drain:

• Chunks or heavy metallic paste → diff confirmed dead.

6) No Transmission Fault message:

• Sudden loss of drive + bang + no warning message strongly points away from the ZF auto and toward the diff.

If the gearbox were gone, you’d usually still not get a clean bang from the rear, and often you’d see a fault message.

Bottom line:
Propshaft turning = gearbox working.
No drive past the propshaft = diff issue.

One more thing to note: your car has the ZF 5HP24, which is known for A-drum failure. The good news is that this can be rebuilt. According to chatgpt, being in Portsmouth, you’re not in a bad spot cost-wise either—expect roughly £700–£1,500 for a transmission rebuild at an independent specialist.

For comparison:

• LSD upgrade (Quaife or 15HU swap): roughly £900–£2,000, depending on parts and labour.
• Diff rebuild or good used replacement: usually £300–£700.

So even in the worst case, this is very likely repairable without completely breaking the bank.

 

^^^^^ THIS ! ^^^^^

Upon purchase, The first maintenance I performed on my 2002 XKR was to change all the fluids. Including the rear end / differential fluid. At 116,000 mileage on the car, I was expecting the worst, and that’s what I got. From the looks of it, never had been changed.

Aside from the looks, differential fluid never smells good even when new, and it’s downhill from there. Unsolicited advice: don’t get any of it on your clothes or skin. If anything is worse smelling, I don’t know what it is. I changed it again 40,000 miles later. Better, but still was showing the effects of the previous poor maintenance by the way it looked. Finally, the last time I changed it, after another 40,000 miles, it was starting to look like normal used rear end lube.


Although the OP’s issue is probably not the transmission, it doesn’t hurt to show it some love and start a regular drain and re-fill schedule.

Z

 

Thanks for a comprehensive reply. Certainly helpful.

When it immediately happened there was no warning code. Then in the time between thinking "wtf was that" and coasting off of the motorway There was an engine light and "gearbox fault". By the time I was stopped the engine warning had cleared, and there was no code to read from the gearbox according to ICarSoft. "gearbox fault" text remained on the dash, and would not clear through iCarSoft, but cleared by stopping and starting the engine. My thoughts at the time were that probably the issue was engine revs not corresponding to road speed changes, but I cannot say for sure. I will start the car and try pulling away and see if that brings any codes back though.

Much as I would love to fit a LSD I am afraid the car has already stretched my budget to breaking point, so a good used item is probably the most likely if it does turn out that the diff is the culprit. I will change the fluid before fitting and cross my fingers!

This test occurred to me while ruminating on it in the pub earlier.
Yes, the idle does drop slightly when the gearbox engages. You sound more confident that this is conclusive than I am!

I will certainly try these checks.


My fingers are firmly crossed on this one!

 

If the worst has happened and you do decide to scrap it, give me a shout as i may be interested in it as a parts car.

 

Lol

 

Isn't the speedometer drive coming from the tail shaft housing of the transmission? No need to jack up the car to look at the prop shaft turning. It will be turning at the same time as the speedo, as long as you are not in reverse. What does the speedo say when you have it in D?

 

There is no speedo cable or mechanical drive on the 5HP24. The speedo is electronic.

 

Would that not be something like a Hall effect sender on the transmission? I don't think it's a GPS. Except now I found that the TCM vehicle speed input comes from the ABS unit. That sucks.

 

There are internal sensors on the gearbox to check that input speed matches output speed - in theory the OP should have a fault code if the gearbox has failed mechanically. There may well be a sanity check with the ABS speed sensors also.

 

Update now that I have had a chance to look further.

with the car running but in park, you can see the engine output turning through the inspection hole under the bell housing.

When you put the car in gear the engine note changes momentarily, but the revs goes UP slightly then drops, where I am sure it used to drop and then recover,

The propshaft is not turning with the car in gear and engine running.

So looks very gear-boxy

But there are some other things that are strange and I do not understand -

iCarSoft says the gearbox input speed is zero, even when I can physically see the input of the gearbox turning through the inspection hole, I can understand that perhaps the sensor for this is not immediately at the input and also that possibly the torque converter is at fault, but I would then expect a sensor error if the engine is idling and the gearbox input is not? No codes are being thrown at present.

 

iCarSoft has input and output speed as zero with the car running and in gear, although I can see the immediate input from the engine turning through the inspection hole.

 

Can you see the torque converter turning? It does sound like the TQ bolts stripped or it just failed internally.

 

You sure its got fluid?

 

I'm assuming that both the flex plate and the torque converter are spinning?

Definitely check the fluid level whilst the engine is running and the fluid temp between 30 and 50 degrees.

Richard

 

That's 2 people suggesting that fluid may be the issue. I haven't checked it, but I will do so.

But given the sudden nature of the failure and no warning, I can't imagine that is the issue. Certainly there is no sign of catastrophic leakage, and the car has not left any drips on my driveway, either before or after, I think fluid is not likely to be the immediate problem, although conceivably might be the root cause of the failure, I would still expect some sort of noise in the lead up to it.