ATF vs PSF for PS

[fdcg27]

Many, maybe most, power steering systems call for ATF.
Some call for PSF, sometimes a very specific PSF, as with Hondas.
What are the differences between these two fluids?
I'm asking more out of curiosity than any desire to ignore manufacture's recommendations.
I'm in the process of doing a turkey baster PSF change on the BMW, which calls for ATF for the PS system. The fluid is incredibly dirty looking and may well be original.
I have a couple of quarts of Dex II laying around, so I do have the recommended fluid.
I just wonder what the chemical or viscosity differences might be between PSF and ATF.

 

[MolaKule]

We have had many discussions on this very topic.

But if your OM says you can use ATF in the PS, then any Dexron type fluid will work.

While DexronII will work, a better fluid would be the Valvoline Maxlife ATF. It has better shear resistance and anti-oxidant characteristics.

Some Honda PS fluids have a special additive system and a higher viscosity.

You may find it will take three or more drains and fills before the fluid clears up.

 

[fdcg27]

Thanks for the advice.
Maxlife it is, then.
I'll just consider the old Dex II I changed in Saturday to be the first part of the flush.
I'm sure that you're right in that it will take a number of drain and fills of the resevoir to make the fluid clean again.

 

[Garak]

Originally Posted By: fdcg27
I'm in the process of doing a turkey baster PSF change on the BMW, which calls for ATF for the PS system.

I'm surprised it didn't call for the ultra expensive Pentosin stuff, as did my even older Audi.

 

[901Memphis]

Use up all the dex ii as the flush then go with max life. Otherwise you may never use the dex ii.

 

[fdcg27]

The Bey Emm Vey calls for no exotic fluids.
Dex II for the PSF and the gearbox, any relatively heavy grade of engine oil (the 10W-40 I'm using is too light for summer, according to the OM) and DOT 3 for the brakes.
Did VWs of the era also call for Pentosin PSF?
I'm asking because I can't believe there'd be much if any mechanical difference between them and the Audis in something like the PS pump and rack.
For that matter, things like PS pumps are typically supplier parts, so I'd think there'd be a lot of mechanical commonality between BMW, DB and VAG parts.

 

[fdcg27]

That's a good idea.
Might as well give it a really good flushing, and I've only got a quart of Dex II left.
I can't think of any other use for it.

 

[antiqueshell]

You'd be surprised about the small changes that can be specified between what might APPEAR to be identical power steering pumps and other similar items.

My guess is that the pump design itself that VW and BMW used during those years are very similar, but the sealing materials and other small items are quite different.

Most VW / Audi vehicles over the past few decades that have power steering have used Pentosin CHF 7.1 mineral based (80s-early 90s) or CHF 11S which is totally synthetic (mid 90s-present)

There are equivalent brands from Febi-Bilstein, and a few others.

 

[fdcg27]

Doubtful.
Any given supplier will use the same gasketing materials on every similar product, since they've found them to work and to be reasonably priced.
The OEMs usually buy these parts off the rack, and they don't have any reason to spec any given gasket material.
They are concerned about cost, delivery ARO and failure rates.
They don't give a hoot about gaskets, as long as they remain leak free for a number of years.

 

[antiqueshell]

Originally Posted By: fdcg27
Doubtful.
Any given supplier will use the same gasketing materials on every similar product, since they've found them to work and to be reasonably priced.
The OEMs usually buy these parts off the rack, and they don't have any reason to spec any given gasket material.
They are concerned about cost, delivery ARO and failure rates.
They don't give a hoot about gaskets, as long as they remain leak free for a number of years.



That doesn't explain why BMW, MB, or VW/Audi would spec very different fluids for their PS pumps though.

I don't think you have a good understanding of how the Germans look at this kind of thing, as opposed to their US and Asian counterparts they can get very particular about such things. Seriously, I know, I work at a Tier 1 company that is a supplier for German car brands.

 

[Garak]

Originally Posted By: fdcg27
Did VWs of the era also call for Pentosin PSF?

I'm not positive, but I suspect they did. Heck, the Audi called for the synthetic CHF 11S, too, not even the older version. And DOT 4 was needed for the brakes. When the steering began to leak, I tell you I replaced O-rings in a hurry. That was much cheaper than the $30/L fluid.

 

[fdcg27]

The point was that there is no reasonable explanation.
You could probably use ATF in any of these PS pumps without problems.

 

[fdcg27]

$30.00/liter?
OMG!

 

[Garak]

Well, I found it for $25 per litre once.

 

[fdcg27]

Such a deal!
I hope you built a stash at that price

 

[Garak]

Not quite a stash, just what I needed to get the leaks in order and have it all topped up. It's good fluid, just a tad expensive. It's a definite incentive to keep leaks under control.