Same with some of the Pulsars at Arriva Castleford. Really violent going from 2nd to 3rd (if I remember correctly) sometimes. I'm no mechanic, but it must be something to do with the transmission and engine not getting along too well.
The following explanation is extensive but it should reassure you for those who don’t know of the way modern engines & gearboxes are controlled and how they work together.
The physical mechanics of modern auto transmissions you’ll find in buses from Voith, ZF or Allison isn’t the problem, it’s almost always a sensor, connector, cable or controller that’s faulty if they don’t sync well, not always but mostly.
different gearboxes can have unusual side effects given the engines they are attached to as well, all engines of course have different torque and power graphs and also the weight of the vehicle can have an impact on drivability as well.
naturally as transmissions age they can become alittle lumpier in there shifting but just like engines and even differentials, gearboxes need fluid changes as well to keep them internally healthy, additionally most torque converter autos fitted to buses have integrated retarders, further adding to there complexity, the need for them to be calibrated well with the engine is important to say the least.
Not on all but on most modern buses & coaches you’ll find a TCM & ECM, a “Transmission Control Module” & “Engine Control Module”.
Both these naturally have to communicate with each other and for the most part they should work fine together, done properly, a well-known exception to this is the Wright StreetLite.
It did improve when Wrights introduced the Daimler engined version, however, they still aren’t great, I simply see this problem as a mismatch between the engine and gearbox software not the actual physical mechanicals, for the most part.
After being able to speak to engineers over the years at depots and engineers from various bus builders, it’s because they have been calibrated for all out efficiency and to ensure such vehicles as the StreetLite can gain LECB certification and savings to there lowest costs possible.
As I recall a Voith engineer say to me, when you try and go for all out efficiency a side affect of this is often the gearbox throwing up and down its cogs trying to stay within a very narrow rev band for efficiency and then seemingly becoming Un-talkative with the ECM, as though one or the other has became confused, most often the gearbox.
Any drivers who’ve driven a Cummins/Voith bus, streetlite or otherwise, you can be slowly manoeuvring, the gearbox has made a gear change so you then go to put the foot down again to progress, only for the engine to rev flare whilst you get breathless acceleration then suddenly it’s as though the gearbox has “thumped” into the gear it should have already chosen, Voith units attached to Cummins lumps are notable for it.
Back to Kickdowns, acceleration & shift points, kickdowns can be fitted if you request that, we know that.
Engine Power & Torque is obviously an important part of the way a vehicle can accelerate as is the way in which the gearbox will change gear at a specific RPM.
abit more about the ECM & TCM, the engines “push” is controlled by the ECM which can be increased or decreased, Cummins refers to this as VAM, “vehicle acceleration management”.
Read here:
CUMMINS Inc confirmed its commitment to customers in the heavy-duty truck market with the announcements of fuel-efficiency improvements, a new engine rating, and Connected Diagnostics...
www.fleetowner.com
Even though the engine for example could be rated at say 250hp & 1000nm of peak torque, VAM virtually restricts how hard the engine will push, this is the “exact” reason many newer buses are sluggish, it’s not because these modern engines are under powered, far from it actually.
If the VAM is set appropriately and often it is not, if it were then there is very little need for a forced kickdown switch to be fitted.
The TCM, this is where you‘ll find software to many known as, TopoDyn(ZF), SensoTop(Voith) or FuelSense(Allison).
Essentially this software controls the shift points of the gearbox either dynamically and/or partly dependant on the gradient of the road, older transmissions like the ZF Ecomat or older Voith models had “set” shift points dependant on the “mode” the TCM had, Eco, Normal or Power for example.
It’s obvious what the software does here, it allows each operator to tailor there choice.
Between the TCM & ECM, the variables can be significant between different buses and there operators.
First Glasgows fleet alone is notorious in the way there vehicles perform, there 65-plate E400 models being particular poor on acceleration, pulling away and on the move, put that in contrast to anything diesel that’s been branded for the 38.
