The "Screw Driver" test

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Styria

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6.9 owners would be familiar with the full hydraulic system that , apart from struts and valves, plus some other components, incorporate five Pressure Cells - four for front and back, plus a Master unit that replenishes the system if there is a drop in the suspension level.

I recently required five units to be fitted to a car I had in my care. Endeavouring to purchase locally, I was unable to locate any units - neither front nor back. I did have three front units on hand which I purchased some two years ago, but I wanted to save those for Gleaming Beauty. Having drawn blanks from local suppliers, I located five front units in Great Britain, but no rears. I just have to back trek a little regarding local availability. Naturally, I let my fingers do the walking and located some front units at a quoted price of $300.00 each for three units. I told the vendor that I would buy four if he was prepared to negotiate on $250.00 each. Much to my pleasure, he agreed and I made arrangements to pick up four pressure cells the next day - I had a grand in my pocket to clinch the deal. Incidentally, the vendor had quoted the owner of the car $220.00 each, but I put this favourable price down to the fact that the owner of the car had done prior business with the vendor.

On arriving next day with the grand in my pocket, the vendor directed me to a work bench with about half a dozen pressure cells on display. Hmmm...."I take it these are new, are they" ? Ah no, they're second hand . Wow, $250.00 for second hand units I said to myself. "So tell me, how do you know that these are fully operational - have you tested them" ? Yes, I insert a screw driver and if the resistance is solid, the unit is okay. So, what happens if a unit is no good that I would have purchased from you - what guarantee do I have - bring it back and I'll supply you with another one. There was no mention however of any compensation for the work involved. In the next post, I shall explain some of the finer points about Pressure Cells. Regards Styria
 
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Styria

Styria

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Front Pressure Cells and the Central Cell are pressure rated at 70 atms - in rough calculations, that translates to say 990 lbs/square inch, whilst the rear units are rated at 60 atms each. For the sake of correctness, each atm correlates to 14.9 sq/in. From memory at the moment, and to realize how critical the pressures should be, Mercedes Benz recommend that front cells should be a minimum of 60 atms, and rears at 40 atms. I could stand to be corrected, but those figures are pretty close. Mercedes also recommends a maximum of 8 atms variation between front and rear pairs. Anything at all that does not meet those specifications, the Company recommends replacement with new units at authorized repair stations.

So, let's get back to this screw driver test that's supposed to be accurate. It's unfortunately, nothing but a furphy. If one can insert a screw driver successfully to push back the membrane, the car would be undrivable as the cell would be full of oil and completely devoid of any damping. The car would be riding like a bogo stick and you wouldn't need to bother finding that screw driver. The units I removed from that 6.9 provided a reading of between 10 to 20 atms for each cell removed. Now, try and push a screwdriver against the membrane in the cell at 10 atms - you're pushing 149 lbs/sq.in. and you will not be able to move that membrane, with an even worse outcome at 20 atms. Forget testing at the recommended maximum or, more appropriately, at the quoted minimums ! No chance.

As a matter of interest, we have the measuring instruments and the capacity to check the pressure of Air Cells on the car, or alternatively if removed from the car charged out at $45.00 per unit - only when removed from car. It saves all the guesswork. Regards Styria

N.B. Needless to say, I couldn't walk away from that deal quickly enough, and in due course purchased five new Front Cells from Great Britain and I managed to find two genuine Mercedes (I believe Lemfoerder) rear units from a wrecker in Melbourne. They were NEW, not second hand.
 
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Michel

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If there was a truer word spoken it has to be .....:(" Caveat Emperor"

"caveat emptor"
the principle that the buyer alone is responsible for checking the quality and suitability of goods before a purchase is made.
 

Oversize

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If they come out of the car they go straight back in if ok or straight in the bin. Although I have kept some for just me on the unlikely chance they could ever be recharged. The screwdriver test is only good to check if the diaphragm is intact. If not, then bin it.

Only ever buy new ones. Correct pressure is critical so it’s not worth using old ones from an unknown source.

It’s not easy to find new cells, so you have to ring around. Then try to find ones with a recent stamp date. Then you need to deal with the transport issue. Forget Aust Post as they’re deemed dangerous pressurised goods. Air freight can only be handled by a few companies.
 
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Styria

Styria

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Mark, to some degree if you're not superfussy, a car will still ride REASONABLY well on units that will only yield about 20 atms for each pressure cell. The car in question that I removed the cells from only exhibited a slightly wallowing experience on the right rear unit. It was down to 10 atms, with the others, once we tested them, giving a reading of 20atms.

The correct reading is particularly important on the Master Cell. If there is insufficient pressure, an excess amount of hydraulic oil will fill the unit and result in much slower response to the rest of the system. Some of you may also be interested in knowing that the rear Pressure Cells on cars fitted with springs at the rear and employing lower spec struts as on W116 and W126s operate on a pressure reading of about 11 atms. Regards Styria
 
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