Any HGV mechanics here, air brake question please.

[Chris Wilson]

My N reg 1995 Mercedes 18 tonner (Model 1831S a converted artic tractor unit made into a 39 foot rigid box van) has failed its test today on the OSF air brake leaking air. It only leaks when the footbrake is applied, and it's not from the hoses to the air chamber that physically applies the shoes, nor from the chamber itself. One of the main hoses from the chamber goes to a multi ported valve assembly on the chassis rail, the other front brake has a similar unit. It looks like some kind of modulator valve that also has an electrical connection, possibly some sort of ABS modulator valve? Anyway, there's some sort of disc covered aperture underneath these valves and air is escaping pretty heftily from the driver's side one with the footbrake applied hard, but not from the passenger one, which I am sure shouldn't be the case, and the driver's side one is therefore faulty What's it's proper name and is it likely a pattern part will be available please? It looks pretty accessible with the cab tilted and it also looks like the passenger side one has already been changed as it looks a lot newer and is unpainted alloy. Cheers.

[Crouch]

Sounds like the ABS valve to me but i do military vehicles! Meritor are the ABS units most of the stuff i deal with run! if it's accesable then i would remove it and take it down the dealers/auto parts store they tend to be all fairly alike but then i work on GM,Oshtruck frieghtliners and other yank stuff! It could be a quick release valve (deals with brake bias stuff they tend to have 3 air connections and a dump to atmosphere ) but then there would not be one per side! they get porouse diaphrams over time and leak when brakes applied but then it would'nt explain the electrical connection! Hope this helps a little but i'm sure someone will know more about Mercedes stuff than me. (I'm just a contractor for the U.S army)

[pedrosixfour]

It definitely sounds like an ABS valve to me Chris. I had the EXACT same problem on my Scania planttruck and the unit you describe is identical to the unit I replaced on mine, right down to the electrical connection.

 

Relating to that aspect I needed an adaptor loom for my new unit as Scania decided to alter the plug on the new unit so it wasn't a direct fit to the existing loom, needless to say Scania supplies the loom as well.

 

Did I hear someone shout scam?

 

One can only assume that Mercedes isn't behind the door when it comes to keeping the coffers topped up any way they can so do compare new with old before you attempt fitting.

[Chris Wilson]

Yes, I have now been in touch with the local Merc truck place and it is an ABS modulator valve. It's a Bosch part, but a pattern on is available for £93 plus VAT, which isn't as bad as I feared. I took the old one off before going out last night and had to cut the connector wires as the knurled ring holing the connector plug to the unit was seized, but I have managed to free it an am about to remake the loom to it I could have had a new valve last night, if I had opted to go and pick it up from Shrewsbury, as the place is open until 10.30 PM, which was quite impressive. I opted for them delivering it today. Thanks for the reply! Will check the pattern part carefully for differences, cheers.

[Chris Wilson]

The valve arrived, and on examination the connector socket for the three pin ABS cable is different.... You were spot on with the warnings! I temporarily fitted it without the electrical connection and the leak is fixed. They are sending me what is hopefully the right one tomorrow.

 

Meanwhile, despite having a VOSA HGV test yesterday, I find the exhaust downpipe from the turbo is perforated with rust and blowing scorching gasses over a whole nest of plastic high pressure air pipes for the air brake system by the air reservoirs. One is already scorched and it's only done about 60 miles since being laid up. The system was gas tight last time it was used 5 months ago. So I have had to order some exhaust system parts, too. Bloomin' thing. Glad I saw the rust hole though, it WOULD have done some serious damage if left unchecked.

[pedrosixfour]

It's comforting to know that Mercedes are about as well put together as Scania.

 

We paid €50k for our Scania 94D 6x2 plant truck in 2004, the truck chassis was 6 years old at that stage (stretched artic as well) but it had a new 30ft timber body and a year old crane on it.

 

We bought it with 435,000 km on it, the odometer now reads 505,000 km.

 

In those 70,000km I've fitted a steering box, a steering column, a complete clutch kit, rocker cover gaskets, diesel non-return valve, starter motor, frost plugs, ABS sensors, ABS modulation valves, brake drums, brake shoes, brake shoe adjusters, air bags (on the drive axle) parabolic spring (on the front axle), hub seals, several relays, the exhaust silencer box, brake light switch, not to mention the normal consumables like tyres and service kits, oh, and a new 30ft all steel body!

 

Thats just what I can think of off the top of my head.

 

And as for tests, the Scania passed its roadworthiness test one year, the centre fitted new front brake shoes, the next day I had to strip the front brake drums to re-align the ABS sensors so they would pick up the signal that they need to operate!

 

A different year the same test centre gave our transit van a clean bill of health after fitting new front disks, the driver's side hub disintegrated on the drive home from the test centre. The only thing holding the wheel on was the brake caliper. They somehow managed to mangle the bearings in the hub while changing the disks.