Talk Morgan Forums Morgan Sports Cars Morgan Plus Four, Supersport & Plus Six (CX & CXV Generation Vehicles) Plus Fours suspension

I tend to get a clonk when braking coming upto a junction as the gearbox is changing down thru the gears, less so when accelerating. My car is quite smooth in congested traffic, however you are aware with an 8 speed autobox, at lower speeds the box is changing up and down quite often as it tries to find the right gear for the current speed, you usually detect the change in engine note. It's certainly not as smooth or as seemless as say in my Mercedes fitted with 7 or 9 speed autoboxes

When I get a chance over winter. I'll get Matt at Life's to go thru the identified areas and see what that reveals.........I'll mention all the areas of potential concern when it's in for a service at Malvern in October and see what they say.

I've donw somef urther investigation and AI is suggesting the following explanations and potential fix regarding the types of clunk I hear

Slowing down to a junction or switching from forward to reverse isolates the problem specifically to drivetrain torque reversal. When you lift off the accelerator to coast toward a junction, the load on the drivetrain flips from engine pulling the wheels to wheels braking against the engine (overrun). Shifting from forward to reverse reverses this load direction entirely.The single "clonk" or thud you are experiencing during these exact transitions is caused by three specific mechanical culprits:

1. BMW Input Flange Spline Backlash

The most prevalent cause for a directional change clonk is micro-movement in the splines linking the driveshaft to the differential input flange.

The Mechanism:

Over time, factory adhesive on the mating splines can break free. When you reverse direction or transition to deceleration, the input shaft physically rotates a fraction of a millimetre before locking into the opposite side of the spline teeth, echoing a sharp metallic clonk.

The Fix:

BMW has a specific Service Information Bulletin (SIB) for this shared architecture. A mechanic must drop the driveshaft, clean the splines, apply a high-strength thread locker (like Loctite 648), and re-torque the pinion assembly.

2. Differential Backlash (Internal Gear Slack)Every differential requires a minute amount of mechanical clearance—known as gear backlash—between the crown wheel and the pinion gear so the assembly doesn't seize when hot.

The Mechanism:

In a conventional insulated passenger car, you would never hear this play. However, because the Morgan Plus Four features an open cockpit sat directly on top of a rigid, uninsulated aluminum chassis, that tiny physical gap acts like a tuning fork. When the load direction swaps at a junction, the gears slam across that tiny gap, amplifying the clonk.

3. Front Differential Bushing DeflectionThe front mounting bolt takes the brunt of the rotational twisting force whenever torque shifts direction.

The Mechanism:
If the front rubber or polyurethane isolation bushing has slightly elongated, or if the mounting bolt has backed off even a fraction of a millimetre, the entire nose of the BMW differential housing will physically twist up or down during load transitions. When you hit reverse or coast down, the differential nose snaps violently back down against the chassis subframe.⚠️

The Non-Differential Alternative:
Dry Wheel SplinesBefore paying for drivetrain diagnostics, eliminate a classic Morgan quirk: dry center-lock wire-wheel splines.If your Plus Four has wire wheels, a lack of heavy grease between the wheel hub and the car's drive splines will cause the wheel to shift back and forth slightly under braking or reversing, perfectly mimicking a differential clonk.Quick Check: Remove the wheels, scrupulously clean off old residue, pack the splines and the internal conical seating area with high-quality graphite or copper grease, and torque the spinners down tightly with a dead-blow mallet.To help narrow down the next step, let me know:Does your car have wire wheels or alloy wheels?

Is it a 6-speed manual or the 8-speed automatic transmission?I can guide you on how to check the front mounting bolt or point you toward a local specialist.

Knowing the car has the ZF 8-speed automatic transmission narrows the focus directly down to the specific way this automated drivetrain behaves.Because an automatic transmission maintains a much more direct, pressurized connection between the engine and the rear wheels than a manual clutch, the mechanical forces are magnified during certain shifts.

The Core Cause:

Torque Loading and Internal "Backlash"The "clonk" when moving from Forward to Reverse (or vice versa), and when coasting down to a junction, is almost always down to mechanical backlash (free play) within the drivetrain.In a standard modern BMW, this tiny amount of play is masked by thick rubber subframe bushings and hundreds of pounds of acoustic insulation.

In the lightweight, uninsulated aluminum CX-chassis of your Morgan Plus Four, that same play is perfectly amplified straight into the cockpit.Here is how the automatic transmission interacts with the BMW differential to cause this:


The Forward-to-Reverse Clonk:

When you shift the ZF 8-speed from D to R, the internal transmission clutches instantly change the direction of the fluid pressure and mechanical rotation. This snaps the driveshaft from one edge of the gear teeth in the differential over to the completely opposite edge. The "clonk" you hear is the physical gap (backlash) closing instantly under hydraulic gear engagement.

The Junction Slow-Down Clonk:

As you brake or coast to a halt, the automatic transmission downshifts through its lower gears (usually 3rd to 2nd, and 2nd to 1st). During these slow downshifts, the car rapidly transitions from "overrun" (the wheels spinning the engine) to "drive" (the engine pulling the car). The sudden change in load causes the internal gears of the open BMW differential to slap across that tiny built-in clearance window.

How to Check the Assembly

Inspect the Front Differential Pinion Bolt: The torque specs on the automatic version are actually higher than the manual. Have a mechanic check that the main mounting bolt and the diff pinion nut haven't slightly backed off from factory settings. If it has any play, the differential housing will rock when shifting between D and R.

Apply the BMW Spline Fix:

If the mounting hardware is perfectly tight, the clonk is coming from the splines connecting the driveshaft to the differential input flange.

The fix requires applying a high-strength retaining compound (like Loctite 648) to the splines to eliminate the minute rotation gap.

A Quick Driving Habit Check

Because you are driving an automatic, try pulling the handbrake tightly before shifting out of Drive or Reverse on an incline. If the car rolls even a few millimetres against the internal transmission park pawl or gear sets, it pre-loads the differential. Shifting it out of that pre-load will cause a loud, alarming snap.To rule out the easiest external factor, do you happen to know if your Plus Four is fitted with wire wheels or alloy wheels? (Wire-wheel splines are notoriously famous for shifting and clonking in exactly this way).

If this is the reason for the thud here is the BMW Sevice Repair bulletin SIB 26 01 18 search on line for it and it shows a video and has pictures

DRIVELINE CLICKING/KNOCKING NOISE ON LOAD REVERSAL

This Service Information bulletin replaces SI B26 01 18 dated August 2018

What’s New:

• More models added

Model
F90 (M5 Sedan)
F80 (M3)
F82, F83 (M4)
F87 (M2)
Situation

A clicking noise is noticed from the area of the rear axle differential, either when:
Transitioning from driving forward to backing up

During load reversal while driving (i.e., engine load to overrun, etc.)
Refer to video attached to this bulletin.

Cause

A slight amount of movement can exist between the mating surfaces of the propeller shaft (aka driveshaft) coupler
notches and the matching splines on the rear differential.

Correction

Apply sealant to the driveshaft/rear differential connection. The sealant will harden and prevent the relative movement
between splines of the two components.
Procedure
Reproduce the noise during a brief road test while the drivetrain is at operating temperature.

Hint:

Select the Sport display in the Central Information Display using the path My Vehicle- Technology in Action- Sport
Displays. If needed, set the units to pound-feet (lb-ft) using the path My Vehicle- iDrive settings- Units- Torque.
The noise occurs at an engine torque between 37 and 73 lb-ft.

With the vehicle stationary, select transmission gear “R” and accelerate slightly with the brake pedal depressed. The noise should now occur once.
Then select “D” and accelerate slightly against the brake. The noise should now occur again.

Apply Loctite 648 sealant (or equivalent) to the connection between the driveshaft and the rear axle. Refer
Repair Instruction 26 11 000 “Completely Removing and Installing the Propeller Shaft”.
Note:
The updated repair instruction is available with ISTA 4.12.3x, as of late July 2018

The driveshaft does not need to be removed from the vehicle, only the rear joint

Apply Loctite 648 (BMW part number 07 58 9067 732) evenly all around the inside
circumference in a bead shape.
Start approx. 10 mm back from the outer edge (1)
The bead depth (2) needs to be approx. 10 mm, and
5mm thick
Note:
The driveshaft (3) must be connected to the
output flange within 12 minutes.
Copyright ©2022 BMW of North America, Inc.


After installing the driveshaft and tightening the recessed nut, allow a minimum sealant hardening time of three hours

Do not move the vehicle during this time period!

Parts Information

Part Number Description Quantity
33 12 7 607 158 Repair nut 1
26 11 7 527 475 Hex bolt 3
26 12 7 536 563 Self-locking nut 3
07 11 9 906 089 Collar nut 2

Bulk Material

Part Number Description Quantity
07 58 9 067 732 Loctite 648 As needed


Warranty Information

Covered under the terms of the BMW New Vehicle Limited Warranty for Passenger Cars and Light Trucks.

Defect Code: 3310013900

Labor Operation: Labor Allowance: Description:
26 11 000 Refer to KSD2/AIR Removing and installing complete propeller shaft
(Man work)
Or:
26 11 507 Refer to KSD2/AIR Removing and installing complete propeller shaft
(Plus work)
And:
26 99 000 1 FRU Work time to apply a bead of Loctite 648 evenly
around the inside spline circumference as
described in this bulletin.

If you are using a Main labor code for another repair, use the Plus code labor operation 26 11 507 instead of 26 11 000.

Refer to KSD2/AIR for the corresponding flat rate unit (FRU) allowances.

Work time labor operation code 26 99 000 is not considered a Main labor operation. Also, since the “work time” FRU
allowance to be claimed is specified, a separate punch time is not required. However, it still requires an explanation on
the repair order and in the claim comments section.

And:

Sublet – Bulk Materials

Sublet Code 4 See the sublet
reimbursement
calculations below
Reimbursement for the repair-related bulk material
(Do not use part number for claim submission)

Sublet reimbursement calculation for claiming the applicable repair-related bulk material (BMW part numbers) is at the dea
net price for the “quantity used” plus your center’s handling.

Enter this material cost in sublet and itemize the amount on the repair order and in claim comment section.

Well MMC checked out my diff and still maintain its the machining tolerance difference between the hub splines and wires, however I noted on the test drive of a new Plus Four automatic fitted with alloys, there was a slight diff clunk when I first moved off, so I'm still a little unconvinced, may need to explore this further especially in the light of the BMW service bullitin.........need to start to just enjoy the car for what it is,warts and all. and consider fitting some noise deadening suppression inside the car to reduce the clonk impact in the cockpit.

Just drive it like you stole it. You will be sorry focussed noises off won't matter

I was just trying to think, how many cars in the current BMW model range have wire wheels....................................................🤷🏻‍♂️😁🙃

Has anyone said 'they all do that sir' yet.....????????
I've had auto BMW's for donkeys years and never had a 'clonk'......(kiss of death)......!

Yes to grease on the splines, but surely not to take up any machining tolerance difference...... that sounds like total BS......🤦🏻‍♂️.......!!!

Looks like another expensive recall for MMC....... they're getting pretty used to them by now so it should go smoothly........

The 'drive it like you stole it' option is probably the way to go....😂

🇬🇧🇬🇧

Julian, your BMW thick sound insulaion probabily masks it, the CX tub just amplifies any noise generated..........so even a little insulation rather than none may help.........but it's get it used, all these fiobles are just a reminder it's a Morgan, Haven't used it much this week been too busy, but last week in the mid 30's temps it performed brillantly whilst we were away in Durham the later part of last week. In the 1000 miles we did once ouside of Malvern we didn't see a single Morgan on the roads..........must have been too hot for the trads or come to that any CX or they were all at the recent gathering of the clans in Malvern Showground and the Midlands. But very unusual........not seeing another Morgan in our travels.

The joy of being an old git is you have hearing aids that can be muted to stop noise

Unfortunately according to Sue I only suffer from selective deafness, in that I only hear what I want to hear.

Don't get me started on selective deafness. MrsW constantly talks to me from other rooms, and from upstairs, and gets shitty when I can't hear her.

She will also ask me to do 'something' tomorrow, and when I tell her to tell me again tomorrow, because i won't remember, she gets shitty.

A friend uses headphones when he watches Yoiu Tube so his wifes irritating voice can't cut through

Happy days