Speedometer troubles

[Cody1798]

So I've recently just purchased a 2009 ML 3.2L 5 speed manual. Not long after I picked it up the speedo and odometer both stopped working. I checked the speedo drive gear on the transfer case output and the plastic gears were shredded. I replaced the gear hoping it would be a one time instance. It was not as around 250kms later and after a decent bump in the road the speedo cut out again. Checked the plastic driven gear and yet again it was shredded. I used a mirror and checked inside the housing where the gear is mounted to do a visual inspection of the driving gear on the output shaft, it was all good. I checked for any excessive play in the tailshaft, it was all good. It has a one piece tailshaft which I am unsure if it is standard or if it has been converted from a two piece tailshaft. I greased the uni-joints as it looked like they hadn't been done in a while. I checked the engine, transmission, and transfer case mountings and they all seem okay nothing to be concerned about. I replaced the gear making sure this time I double-checked the alignment of the gear tooth indicator on the housing was correct. Yet again the speedo dropped out, this time after almost 500kms but again it was on a harsh bump in the road which caused the failure. On inspection I found the speedo driven gear shredded yet again. This time I removed the tailshaft and checked the uni-joints properly, they are all good. Removed the extension housing from the transfer case, there was no debris inside they housing which might have been getting caught between the gears. I checked for any excessive movement in the output shaft for both lateral and thrust movement, all seemed ok as I double checked with another gearbox from the same model. The driving gear of the output shaft has no concern. The bearings feel smooth on rotation when rotating by hand as well. I reinstalled everything ensuring everything was installed correctly and changed the gear oil with some new oil making sure both the driving and driven gears were well lubricated. I even went to the extent of getting my wheels balanced to try help remove any vibrations present. All felt like a wasted effort though as after around 450kms the speedo dropped out again. I'm running out of ideas and I am looking for advice on where to look or what to try. The car is bone stock unmodified apart from all-terrain tyres I put on it but they aren't the cause as it had highway tread on it when I bought it when the speedo dropped out the first time. The only other idea I have feels like a stupid idea but I'm thinking that if the tailshaft has been converted to one-piece, it may possibly be too long not allowing enough movement during up and down travel but I have no way of checking that without buying a two-piece tailshaft. I would just use a GPS speedo as everyone else is telling me to do but as I do a lot of highway driving cruise control is nice but that doesn't work obviously when the speedo doesn't work. If anyone has seen, heard, or know of anything remotely similar or even other ideas to check that would be much appreciated

[WUNSIE]

Correct me if I,m wrong, but if its running a speedo cable it sounds like it is binding big time such as having a bad kink and it is loading up the drive way beyond the capabilities of the plastic/nylon drive can handle, and it being the weakest link its simply stripping the teeth.
If it is a cable can you turn the inner cable freely by hand, or hook onto the inner cable with a cordless drill and give it a spin

[RHKTriton]

The speed sensor just converts the gear rotation to electrical pulses, no speedo Bowden cable.

The harsh bump is the key, so you'll have to consider all the things that can happen at this time.

You could measure how far the front of the drive shaft sits off the gearbox compared to someone else's.

Look into the port where the speedo sensor goes and look for movement while someone moves the driveshaft, up and down + in and out.

Is there a shim or spacer missing in the sensor kit?

Long shot - condition of gearbox mounts?

[Cody1798]

The tailshaft is where I'm looking at next as I've checked internal play of transfer, all the mountings, and the sensor itself is good as I've swapped the sensor with a brand new one I had laying around. Assuming the manuals and autos have different length shafts I'll try and take a few measurements and inspections of mine and when I get the chance to check against someone else's I'll do that, I also might go into the dealer and check the genuine part required using the vin if it's supposed to be two piece or single piece

[RHKTriton]

If the internal shaft play is OK, then its unlikely the driveshaft is your problem as other stuff would be getting smashed.

I'd be focusing on the sensor. Is it a genuine part or a flee bay subbie?

If its too long or short it would affect how its cog meshes with the drive train.

I'd go straight to a dealer with your VIN and compare what you've got with what's specified.

Vehicle manufacturers are notorious for changing parts during a production run.

[Cody1798]

I've been to the dealer and asked them to check what tailshaft should be in the vehicle and being it has a super select transfer it is supposed to have the 1 piece tailshaft. All the sensors I've installed have been genuine parts ordered from the VIN, they showed me the parts catalogue and the same part number is used from all models of ML to all models of MN and there hasn't been any updated or redesigned parts listed. Now that the tailshaft has been ruled out I'm out of ideas without just simply throwing parts at it and I don't exactly have that kinda money

[RHKTriton]

A bastard problem you have there.

Have you tried sticking a long screw driver into the speedo port and see if any movement can be produced in the driving gear?

I'm thinking play in the front end of the shaft that drives the sensor.

[Cody1798]

Already tried the screwdriver, even pulled extension housing off the transfer to check the movement from the driving gear and it's all good. When the sensor gets replaced the whole unit gets replaced so the play that the sensor has if any is standard and can't be faulty part because it's had 4 in it now

[DAVO84]

Ok so my speedo and odometer are stuffed the revs, fuel and temp all work fine though.
I’ve changed the speed sensor speed drive put my finger in and felt around in there and it all feels normal.
Checked all the wires to speed drive and sensor and they are fine.
Checked all fuses twice.
Just bought a new cluster and still the same problem.

Now the only thing I can think of is ecu or ecm or is there something else I should try out first?

Thanks

[RHKTriton]

Just thinking a spot you guys could check. There's a commoning plug towards the front of the console (going by son's mn, when we were trying to get a speedo corrector going) that takes the connections from the speedo sender and spits off parallel connections to the speedo, RV module and possibly ecu. If you're not getting signal at the speedo, the jumper plug may have bad contacts.

With Cody's problem, I'm wondering if a short on the circuit may be loading up resistance on the sensor module ( like a dynamic brake ) and then causing stress on the cogs?

Again the jumper plug is a spot you can break the cabling into sections.

Water or rodents can be your enemy.

[DAVO84]

Thanks RHK no luck their think I’m just going to have to change the ECU unfortunately, that’s about the end of the line and only other thing I can think of fixing.