Wabco Unloader Valve Pictorial

Wabco make several Unloader Valve for all different sort of vehicles.. Mine did start to playing up a couple of days ago, as it wasn’t holding tank pressure any more, and because I couldn’t find any manual or tutorial on how to deal with the problem, I decided to make this pictorial. My particular model is the 975 303 474 0; you should have a little plate on the plastic top chamber with the 10 digit number printed on. This HERE is the Wabco technical sheet for this model..

From the picture above it does look like a complicate piece of equipment but if you have ever opened a carburettor before, you’ll feel right at home. It is actually easier than a carburettor, as one side has no connector and the other two only have an inlet and an outlet. Probably is better to think of it as a cube:

1. Top plastic black (regulating valve chamber) and port 4
2.  Bottom Metal (exhaust port) Port 3
3. Front side (the one with all the brass bits) Ports 22, 23 and 1-2
4. Left side (Inlet) Port 1
5. Right side (outlet) Port 21
6. Back side (nothing)

On the picture above from left to right: Left side & Front, Front & Right side, Back side. The two “see trough” holes are mounting holes and not used if the unit is supported (like in my case) by the inlet & outlet pipes…

From the Wabco Catalogue we can also see that this particular model (shape) does cover sever version numbers..

Now again just to be clear beside the top chamber, the unit has 7 ports (inlet/outlet)

Port 1     = Inlet from Compressor

Port 21   = Outlet to tank or Air Dryer

Port 22  = Auxiliary supply (offset from port 1)

Port 23  = Outlet for control Impulse

Port 4    = Control port

Port 1-2 = Tyre inflator’s connector

Port 3    = Exhaust for compressor idling

You can buy kits to replace most part in it but sometimes all is need it is just a disassemble and a good clean in kerosene…

Starting from the top, remove the 4 big screw that are retaining the plastic chamber. there is a strong spring under there but the screw are long enough that, by the end there is no much pressure at all. It make the job a lot easier if you take the complete unit off the vehicle and do the job on a clean bench.

This is my unit completely disassembled and ready to be clean …

You can see from this pictures, why my unit was not working any more in particularly from the one below… That’s terrible…

The centre hole is where the metal seal is suppose to seat, but as you can see there is a lot of gunk coming from the compressor (and also 20 years of use).

The bottom exhaust is retain in place by a large circlip. Again there is another strong spring under there, but not to create a problem once the circlip is removed.

Then is just a matter of removing all other auxiliary connectors (22, 23, 1-2 and 4)

You should be able to keep all the part together for an easy reassemble, but make sure that none of the O-rings are perished and that the main membrane (on the top chamber) has no damage..

If the spring have collapse or no longer retaining the correct pressure, there is a factory calibrate(with red paint) screw on the top chamber that can be adjusted, by using the pressure gauge inside the vehicle, just turn the grub screw clockwise a bit by bit until you get the desired pressure in the tanks again..

That’s pretty much it…

NOTE:

9/02/2018

I was able to find on Internet a Wabco 975 303 474 0 (Chinese version) for around $150.00 including delivery, to keep in the vehicle as a spare.

Just a word of warning here…

The Chinese unit may (and should) work as good the the OEM but in the one that I ordered, the quality of the alloy cast finish was pretty average. If you are intending to use one of those unit, my suggestion would be to take it apart and check the internal finish of the cast especially where it really matter, like around the O-ring seal… Buyer beware…  ;)