power steering pump

I own a 1975 Mercedes 450SL which needs a new power steering pump. I purchased one at a junk yard but they do not appear to be the same when placed side-by-side.

First, the original pump pulley is 6" in diameter and the junk yard pump is 5 1/2". Further, the original pump operates on a 2-belt pulley and the junk yard pump operates on only one. I would like to know if the junk yard pump can be installed on my 450 SL without problems. Please advise.

The specs on the junk yard pump are as follows: ZAHNRADFABNK FREDRICHSHAFEN AG.
Type Nr: 7672900404
Fabr. Nr: 542700 1.79
1164601780 65 br.

The specs on the original pump are as follows:
VICKERS TYPE 21
Fabr. Nr. 75 405245 Max Druck 65 bars

[Benzmac]

The pumps were interchanged, but I don't know if they pulleys will interchange. You should take back that pump and specify a Vickers instead of the ZF. They both made pumps for that engine.

The only way it would work is if the pulley will interchange. All else should be the same.

[ejsharp]

Junk yards are notorious for selling pumps that will work on your car but the pullies will be different. They expect you to change/switch the pullies... so I assume the pullies are interchangeable. The pullies are sometimes set with a allen head screw (but not sure about Mercedes).

The new pump pulley diameter is 137mm and is a one belt pulley. My original pump has a pulley 145mm in diameter and is a two belt pulley.

My question is: Why does my original setup have two belts used excusively for my power steering pump. Is this a safety feature in case one belt breaks the car would still have power steering? If I used the replacement pump with the smaller diameter pulley how could this effect things.

Thanks

Brother of The Benz, p Tompkins
The diameter differences of the two pulley would only effect the rotation speed.
Relative to one vs two belt drive many times depends upon the amount of belt contact there is with the pully. With fewer degress of contact, two belts whereas more degrees of contact, one belt. There is no builtin safety feature for the P/S.
Using only one of the two belts, will the original belt align to one of the groves of the used pump. If yes, then just use one grove of the two grove pulley.
If the used pump was a Vickers and the original the same, the swapping of the pullies would solve your concern; likewise with a ZF. It is possible the pulley from one might fit the shaft of the other.
The pulley is held on the shaft by an end nut. The pulley aligns and drives with a woodruff key.
Disassemble the pully section of both pumps to see where you stand.
Is there some help out there?
Happy Trails Beep Beep from The Spiderman in Houston!!!

The original pulley was bent when I tried to remove it. the replacement pump came with a smaller single groove pulley. I have tried to locate an exact replacement for my original double groove pulley but cant seem to find one. the ones I have found are in some way different. I have heard every opinion under the sun and now I have a headache. I now know
pumps are interchangable between years and models. Also a certain amount of belt surface has to make contact with the pulleys of the device the belt is driving. So, lets say I bought the car without a pulley on the pump would it really matter what pulley I installed?

Thanks,

Brother of The Benz, Tompkins
Sorry for so long replying as Joan, our Benz and I were in New Mexico.
It sounds as if you are in a state of ????
Any pump that would physically fit and the belt grooves align will do the job. As for belt contact, if you look at your belts pathe of travel you will see there is minimal circumferencial contat.
There is very small load on the pump except when you turn the wheel to the right and left stops. Then the pump puts out it's max pressure and you may get a belt squeel.
Now for the consequences; if the pully diameter is different the only change would be pump speed.
As speed changes you may have normal pressure at a different engine speed and I can't imagine what difference that would make.
As an old fool I would say if it fits use it.
Happy Trails Beep Beep from The Spiderman in Houston!!!