Hey guys Ive a Scania 6x2 rear air suspension with auto level, tag lift, and load transfer, … amongst its continuous minor niggles with calipers and failed chambers, Ive got a small problem I’m not sure where to look first. Basically with a load over the rear axles, the tag axle is braking harder than the drive, meaning its running a little hotter. Not really a problem but will cause excessive wear so I’m looking to fix. I’m guessing theres an airbag pressure controller somewhere thats letting too much force to go to the tag axle brakes, or is there some sort of load balancing valve that equalises the force between the two axles that has gone faulty ? TIA 
Don’t know much about Scania, but what year is it, disk or drum brakes and does it have EBS?
coiler:
Don’t know much about Scania, but what year is it, disk or drum brakes and does it have EBS?
Its 06 . discs all round
There will be a proportional relay valve regulating pressure to the front axle not sure if it has one on the tag. Might be some sort of proportioning issue but i would get some gauges on the test points and measure the pressure in various parts of the system also with varying load. You might find it’s not configured correctly and it’s causing the problems with the chambers and calipers. Has it had a body change as may not be corrected for that. Also the ecu might think the axle is heavier than it really is.
How much hotter though, hot enough you can smell burnt pad?
Own Account Driver:
You might find it’s not configured correctly and it’s causing the problems with the chambers and calipers. Has it had a body change as may not be corrected for that. Also the ecu might think the axle is heavier than it really is.How much hotter though, hot enough you can smell burnt pad?
Was fine until now … no body change … the ECU does think the axle is heavier … front axle is sprung … temperature is only a few degrees warmer than drive axle … not anywhere near burnt pad zone thankfully. tag axle will try to skip a little under heavier braking
What are the plated weights for the two rearmost axles? There will be a difference between the set up for equal plated axles (2x 9t) to the set up for a 11.5 t drive axle and a 7t tag.
Quote: “With a load over the rear axles”. Are you meaning that the load is very significantly biased towards the rear so that the front axle is carrying very little weight?
It would probably be a good idea to weigh the individual axles when the vehicle is loaded.
Bogie hop under braking has been around a long time. With spring suspension it was not unknown for the shackles to turn over, some times disabling the vehicle.
You seem to have the opposite of the normal issue with 11.5t/7t split tag axle 6x2 rigids, which is MOT failure because of insufficient effort from axle 3. So has the vehicle been tested recently?
cav551:
What are the plated weights for the two rearmost axles? There will be a difference between the set up for equal plated axles (2x 9t) to the set up for a 11.5 t drive axle and a 7t tag.Quote: “With a load over the rear axles”. Are you meaning that the load is very significantly biased towards the rear so that the front axle is carrying very little weight?
It would probably be a good idea to weigh the individual axles when the vehicle is loaded.Bogie hop under braking has been around a long time. With spring suspension it was not unknown for the shackles to turn over, some times disabling the vehicle.
You seem to have the opposite of the normal issue with 11.5t/7t split tag axle 6x2 rigids, which is MOT failure because of insufficient effort from axle 3. So has the vehicle been tested recently?
Passed its test fine, both axles braking as they should, just that the tag comes on harder than the drive. The load was placed evenly over both rear axles.
FlatbedFlyer:
Own Account Driver:
You might find it’s not configured correctly and it’s causing the problems with the chambers and calipers. Has it had a body change as may not be corrected for that. Also the ecu might think the axle is heavier than it really is.How much hotter though, hot enough you can smell burnt pad?
Was fine until now … no body change … the ECU does think the axle is heavier … front axle is sprung … temperature is only a few degrees warmer than drive axle … not anywhere near burnt pad zone thankfully. tag axle will try to skip a little under heavier braking
It won’t matter if the front axle is sprung the proportional relay valve will still act on it dependent on load on the air sprung axles.
Could the skipping be the ABS cycling?
I think it’s going be hard to get to the bottom of without measuring some pressures and a plug in.
Is this the vehicle we are talking about and is this how it is loaded typically?
Own Account Driver:
Could the skipping be the ABS cycling?
Quite possibly … I handnt thought of that … I assumed it was a load issue
cav551:
Is this the vehicle we are talking about and is this how it is loaded typically?
Yes … but that was a Demo pic for a promo … why do you ask ?
It has answered the questions about axle configuration and probable rearward load bias.
cav551:
probable rearward load bias.
How do you mean ?
Reason tag or lift axle runs slightly warmer is because it is not in ABS loop.
You cannot put an axle that is some times up and sometimes down into the ABS programme,The lift axle always gets full supply pressure so takes on a higher braking load,
If you look you will see that the lift axle has no ABS sensors.
The picture shows a rearward facing machine weighing about 9 tonnes sitting slightly to the rear of the bogie centre line. There is an attachment sitting on top of the dozer blade. Then there is what is maybe a 2 tonne bole sitting on the beaver tail at what looks like the extreme end of the vehicle’s chassis. Resting on top of that is the folded boom.
Bking:
Reason tag or lift axle runs slightly warmer is because it is not in ABS loop.
You cannot put an axle that is some times up and sometimes down into the ABS programme,The lift axle always gets full supply pressure so takes on a higher braking load,
If you look you will see that the lift axle has no ABS sensors.
It does have ABS sensors on the tag axle
cav551:
The picture shows a rearward facing machine weighing about 9 tonnes sitting slightly to the rear of the bogie centre line. There is an attachment sitting on top of the dozer blade. Then there is what is maybe a 2 tonne bole sitting on the beaver tail at what looks like the extreme end of the vehicle’s chassis. Resting on top of that is the folded boom.
As I said, that was a promo shot, and not the load in place when the problem was observed. I mentioned earlier there was a load evenly distributed over both rear axles at the time, which in fact was 5 tonnes in weight. I’ve never had any problems with the tag axle before under this or any other loading arrangements.
FlatbedFlyer:
Bking:
Reason tag or lift axle runs slightly warmer is because it is not in ABS loop.
You cannot put an axle that is some times up and sometimes down into the ABS programme,The lift axle always gets full supply pressure so takes on a higher braking load,
If you look you will see that the lift axle has no ABS sensors.It does have ABS sensors on the tag axle
So you have a system that relies on wheel speed pulse but all of a sudden you stop sending a pulse from a lift axle because the wheels are not rotating and then what happens?
The ABS then goes into fail safe and shuts down.
WTF you talking about.
Jesus another “expert”
FlatbedFlyer:
Own Account Driver:
Could the skipping be the ABS cycling?Quite possibly … I handnt thought of that … I assumed it was a load issue
It will quite likely be a load sensing issue if the ABS is cycling with no good reason. It will be air at too high pressure being sent to the chamber that causes the wheel to lock and the ABS to cycle. The air will generally be at too high pressure when the ECU thinks the axle is heavier than it really is.
It could also be the drive axle isn’t putting enough brake effort in and working the tag brakes too hard but more likely load sensing if the skipping is indeed the ABS cycling. As I say though without dealer level diagnostics it might be quite tricky but you could measure the air pressures, loaded and empty, in the brake lines and air bags and it would at least give you an idea what’s going on and a roller brake test wouldn’t harm.
If there’s a problem with it not correctly measuring the suspension air pressure, and therefore load, it could be quite pricey. I’d ■■■■ it and see a bit and see if you start wearing pads excessively on the tag. EBS can do all sorts of clever dickery in altering the brake pressures to try and even up pad wear which all depends on the individual system.
Bking:
Reason tag or lift axle runs slightly warmer is because it is not in ABS loop.
You cannot put an axle that is some times up and sometimes down into the ABS programme,The lift axle always gets full supply pressure so takes on a higher braking load,
If you look you will see that the lift axle has no ABS sensors.
ABS has nothing to do with load sensing. Midlifts on tractor units often don’t have an ABS sensor but brake pressure may still be modulated based on load it depends on the setup.