Technical TPMS on the spare tyre?

[BenL]

My 2021 Ducato Australian Series 7 2021 2.3 160 Multijet XLWB - "Maxi" was delivered with TPMS sensors fitted to only 4 wheels, not the spare.

Does your Ducato have TPMS, and if so, fitted to 5 wheels? Are all 5 working - have you tested all 5? A local company that sell aftermarket sensors incl. for the Ducato have advised that if the vehicle can only support 4 sensors, "having 5 sensors present with only 4 memory locations available may lead to the incorrect wheel being learnt into the wrong location(i.e., spare in place of a wheel that is on the road)."

AlfaOBD reports two front, two rear and an apparently empty spare slot, so I'm guessing I could add the fifth? It'd be handy to know if the Ducato is delivered in some markets with 5 sensors. FYI Australia does not mandate TPMS, would you believe.

Left Front Tire Sensor Altitude Compensated Pressure: 5325.00 mbar
Right Front Tire Sensor Altitude Compensated Pressure: 5200.00 mbar
Right Rear Tire Sensor Altitude Compensated Pressure: 5325.00 mbar
Left Rear Tire Sensor Altitude Compensated Pressure: 5350.00 mbar
Spare Tire Sensor Altitude Compensated Pressure: 0.00 mbar

...

Front left wheel identification: 01EFxxxx
Front right wheel identification: 01EFxxxx
Rear left wheel identification: 01EFxxxx
Rear right wheel identification: 01EFxxxx
Spare wheel identification: 00000000

 

[Steve928]

TPMS in the 4 (or 6) road wheels but not in the spare is standard.
Although the scenario suggested by your local company (spare being learnt in place of a road wheel) is technically possible, in practice it doesn't happen because the spare wheel remains mostly in sleep mode as it isn't being rotated.
I say mostly because the spare may broadcast once upon temperature changes or pressure changes caused by altitude difference, but this is so infrequent that it won't cause the wheel to be 'learnt'.

I have a sensor in my spare and can see it broadcast very, very occasionally but it has never interfered with TPMS operation (and never will).

 

[BenL]

Ah, that's good to know, thanks. I was expecting the spare TPMS to alert to a low pressure and save me from having to check it occasionally. Sounds like even if I did install one, it wouldn't report in properly until I installed it as a driving wheel. i.e. if your spare goes flat, do you get an alert?

Regardless it's still useful to have as I plan on doing 5-wheel tyre rotations to get the most life from the set, and don't want to "go blind" on that spare when it's in service, so definitely good to know the van should support the 5, even if not all at once.

Speaking of tyre rotations, do you know if the TPMS figures out the new tyre positions when they've been rotated? I expect that'd require multiple antennae.

I'll go ahead and order one and see what happens.

 

[Steve928]

The TPMS sensor itself doesn't know the reference/trigger pressure so doesn't know if its pressure is high, low or even flat - so its reporting/broadcasting isn't affected by absolute pressure, just changes. That said I have one set of non-OE sensors that broadcast once per hour irrespective of pressure, but even that frequency of broadcast doesn't interfere with the 4 learnt roadwheel sensors.

When you fit the spare its sensor ID will be learnt within a couple of miles and replace the now-static wheel's sensor ID in the TPMS ECU's table. When doing this I've yet to see a warning light so it happens quickly.

Any new wheel positions are determined using WAL (wireless auto locate) which can determine where the wheel is in relation to the single off-centre receiver unit by assessing signal strength and distance of transmission (or something clever like that..), so it's all automatic.

 

[BenL]

Short version: @Steve928, you were spot on.

I have installed an aftermarket TPMS sensor in the spare, the part is a generic that claims to be compatible with OEM part number 05154876AA. Technically I had to get the local tyre place to do the actual install (AUD$20) - I couldn't crack the bead to DIY. I put the spare on the van rear-left as part of a 5-way tyre rotation, and the van picked up the new sensor just fine. I can see the new ID in place of the old one; the wheel now delegated as the spare has gone to sleep and its ID is no longer showing via AlfaOBD (the spare wheel ID is still 00000000 and is not reporting a pressure reading, so I'll have to keep an eye on it manually - this seems to be a flaw in the overall "monitoring" part of TPMS; still at least now I can rotate tyres with abandon).

I did get a TPMS warning light after about 15 mins of driving but only because the tyre place didn't inflate the spare to the crazy high pressures Fiat recommend; as it happens I have indication of over-inflation on the rear tyres so used AlfaOBD to drop the TPMS warning threshold to 330kPa/48PSI, I'll see how that goes in terms of wear; at present my van is essentially unloaded in the rear so will need to revisit this once the conversion is complete.

So I now have

Front left wheel identification: 01EFxxxx         # (was rear right)
Front right wheel identification: 01EFxxxx        # (was rear left)
Rear left wheel identification: 002Exxxx          # (was spare; w/ new sensor)
Rear right wheel identification: 01EFxxxx         # (was front right)
Spare wheel identification: 00000000              # (was front left 01EFxxxx)

Left Front Tire Sensor Altitude Compensated Pressure: 5125.00 mbar
Right Front Tire Sensor Altitude Compensated Pressure: 5175.00 mbar
Right Rear Tire Sensor Altitude Compensated Pressure: 3425.00 mbar
Left Rear Tire Sensor Altitude Compensated Pressure: 3250.00 mbar
Spare Tire Sensor Altitude Compensated Pressure: 0.00 mbar

Front Axle Minimum Threshold: 3600 mbar
Rear Axle Minimum Threshold: 2720 mbar     # was 4000 mbar

(note the "Rear Axle Minimum Threshold" is below the setting I configured in AlfaOBD's Tyre Pressure Control "Rear Axle Nominal Tyre Pressure Setting", which was 48 PSI / 3.325 Bar, as I recall; presumably some correction factor is applied to the nominal setting to arrive at a minimum threshold.)