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On WABCO Vacuum pumps


Boydie

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I was reading a thread the other day regarding a method to repair to oil leaks in the front cover of the Wabco Vac pump which the author rectified by replacing the 6 rivets with 5mm bolts & nylock nuts.

As my unit was having the same oil leak problem I followed his example and carried out the rectification, oil leak solved, however......................

The WABCO vacuum pump is at best a fairly simple beast (WABCO = Westinghouse American Brake Company) never were ones for high tech products and originated making brake componants for the US railways. In a previous life I worked for a now defunct English Company, Davies & Metcalfe who made a superior product for steam engines, as it was Wabco purchased D&M and both were eventully bought out by a superior technological european railway engineering company, AB Boveri who were/are also into DC traction motors, anyway I digress.

The Wabco pump fitted to the 200 & 300 TDi is a simple pump consisting of a strong spring, a shaft which is therefore pressed against a lobe on the cam shaft and a piston fitted inside a cylinder connected to the other end of the plunger shaft, the spring being in the centre as it were. The piston on its "in" stroke -- that is when the spring pushes the plunger towards the flat of the cam the piston travels away from the external valve plate and air is sucked from the brake servo through a simple spring loaded non-return poppet valve, on the "out" stroke, when the lobe on the camshaft pushes the plunger and compresses the spring the piston moves towards the external valve plate and the air is pumped through a second spring loaded poppet valve and into the crank case.

This being the case and on investigation and very close scrutiny I noted that a tiny section of the lip of the piston had broken away, this was/had allowed, over a period of time, engine oil to pass past the piston and into the discharge chamber of the pistons cylinder, that is the portion of the cylinder capped by the valve plate that was leaking. No doubt the fact that oil resists compression is one of the reasons for the valve plate leaking as the piston would have been hammering the oil against the valve plate and past the "O" ring.

It follows therfore that if the valve plate is leaking oil past the "O" ring then oil is getting past the piston and possibly further into the brake servo and contamination of the rubber diaphragm. The only mechanical thing preventing this is the suction poppet valve and the fact that the oil would be held retained in the cylinder by the suction of the piston.

WABCO, ever the boys with an eye for profits (the cost of an interconnecting vacuum hose used on railway carriges would make you faint!) naturally do not sell spares for these pumps and I'd guess that being as reliable as they are there simply isnt enough failures to warrant a Chinese company from tooling up so that only repairs able to be carried out are the replacement of the rivets as per the thread I mentioned and welsh plugs that retain the two poppet valves, but as I said from my observations tha only way for oil to get into the suction chamber of the pump is for the piston to be leaking.

For my part I'm on my way to my LR parts dealer to buy a new pump :(

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On the RRC with ABS the pump is only there to power the EGR system. I'm not sure but I think the Disco needs the vacuum for the brakes with or without ABS?

Anyhoo, if I could get a blanking plate the vac pump could go in the bin, from what you say it would make a good doorstop.

If you wanted to bother with a mod, the alternator off something else (Cavalier/Isuzu TDi, Transit etc) would have a vac pump on the back of the alternator, ther is perhaps just enough room for it on the 300TDi.

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  • 3 weeks later...

Here in South Africa we seem to be experiencing numerious WABCO vacuum pump failurs on the 300 Tdi's. They tend to pop the bottom welch plug, and the dreaded head oil leak. We do the rivit replacement, and one owner is investigating the manufacturing of a better quality welch plug.

I know there are other manufacturers like Bearmach, but I have no record of their reliability.

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If the piston isnt worn or damaged then oil should'nt get into the bottom poppet valve gland which is the discharge into the engine sump..... ergo there shouldnt be any leakage from around the welsh plug, however if there is and then a new welsh plug inserted with a reputable sealant (Rolls Royce Sealant) should cure the problem. Knowing Wabco the size of the welsh plug will be an oddity, but if the end plate is removed then having it honed out to a standard size, say 20mm shouldnt be a problem as I recall there is considerable land around the plug.

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