Noises from Power Steering Pump

dave8412  Jun 16 2006 - 1:44pm   

Q: Why does my power steering pump tend to whine/hiss?

A: Dan (burkdani@egr.msu.edu) has some expertise:

As a Co-op student for Saginaw Division for 18 months, I spent nearly all of my time working on the GM built TC, CB, and P model power steering pumps. Since I see some people are bringing up pump whine, I thought I'd try to address the issue.

The 2nd & 3rd generation F car uses a submerged P pump. It is a rather large PS pump that has a housing full of PS fluid which surrounds the pump (hence the term "submerged"). The P pump weighs in at eleven pounds, and is a very reliable unit capable of providing enough hydraulic assist for everything from a Chevette to a 1 ton delivery van. Because the P pump is submerged, it also tends to be quieter than the other models as the oil damps many of the vibrations. In addition, the P pump has been manufactured since the late sixties, and Plant 3 in Saginaw, MI really has their quality control dialed in on that assembly line.

The 4th gen F car uses a CB pump. It is not sumberged, and has a resovoir. It is physically smaller than a P pump, weighing in at about six pounds. It is more suited to passenger cars, as it does not have the obvious excess capacity of a P pump. The CB pump has many similarities to the P pump in that it uses a bushing on the driveshaft, as well as a common flow control valve. All in all, it is quite a reliable unit, aas long as it is manufactured in spec.

A TC pump is dimensionally similar to a CB pump, and can in fact be substituted for a CB pump. The TC is more expensive, and uses a sealed bearing on its driveshaft. Some critics of the TC claim the sealed bearing is noisier than the CB pump because of this. You will find TC pumps on many other GM products, as well as a whole bunch of chryslers, Audi's, VW's, Saab's, and other European manufactured vehicles. I personally like the TC pump. It was kind of like a hot rod, and we'd sometimes spin them up to 9,000 RPM for 100 hours, at 300 degrees F and they'd look fine afterwards. Some say the TC was 'overbuilt', and GM was trying to phase them out, except the customer kept buying them! The TC pump is the lightest of the PS pumps, at just over five pounds. There exist some aluminum pumps from the late 80's Corvettes that weight only four pounds that will bolt into a 4th gen. F car. There is also a new pump called 'direct drive' that runs directly off the cam, but I'll not go into that because I don't know much about it.

Now, the whine you hear when you rev your pump is one of three things. The first is the whine caused by pressure relief. It sounds kind of like a 'Shhhhhhhhh!' If you turn your wheel until it wont turn anymore, the fliud flow to the pump is cut off, and the pump is forced into pressure relief. On the F-car, this means a pressure of between 1200 and 1600 PSI. The pump has to recirculate this high pressure fluid within itself because the fluid has nowhere to go. If left in pressure relief for very long, the fluid can quickly exceed 325 degF, and break down, causing the pump to begin experiencing metal-to-metal contact.

The second noise is caused by cavitation. As you increase pump speed, more and more fluid is forced through the ports per unit time. There comes a velocity where the fluid just doesn't want to move that fast, and the vanes inside the pump will cause a vacuum, which is instantly filled with oil vapor. When the vane reaches the high pressure port, this vacuum cell is opened into a high pressure cell, and oil rushes to fill the void. A little 'sonic boom' results. These individual 'booms' are virtually microscopic, but they add up. What they translate into is a sound that sounds suspiciously like a whine. Now every pump cavitates. It's in the nature of moving fluid. By smoothing the radius of the ports, and relocating supercharge holes (I'm not going into that), engineers can reduce cavitation or push the frequency spectrum of the whine to a less noticable frequency. It is, in fact, where the bulk of pump engineering hours are spent.

Anyway, that funny high pitched whining that Mike Martin is hearing after his autocross run is the third type of noise. This type of whine is from the oil being aerated. When the pump is really screaming, the oil in the resivoir really gets churned up. This allows the occasional air bubble to pass into the pump, where it is totally pulverised into smaller bubbles. Eventually, the oil gets enough of these bubbles in it that you begin to hear it. If you want to really experience aeration, just run your pump low on oil!

None of these three types of noises will cause your pump to die. What kills a pump is heat and oil viscosity breakdown. The pressure relief example will overheat a pump in the extreme case causing galling of the bushing, or wear of the pump ring. The cavitation example, when extreme, like 7000-9000 RPM endurance runs, will cause microfractures to the pump internals and literally blow little flakes of metal off the walls. The extreme aeration example will cause increased heat to the internals, because air does not carry the heat away like oil does. But the common denominator to these is the word EXTREME. Most PS pumps never see this kind of duty, unless the person runs their pump low on oil. But none of us does this, because we all check our PS fluid as often as we check our trans fluid, right?

 
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