SHOP TALK
Those Are the Breaks: Mercedes 722.9 Pump Failure
really paid attention to all the details and took your time on the rebuild. You’ve checked the ATRA bulletins and you’re confident it’s a winner. It’s a high-dollar rebuild and you can’t afford mess it up. After the installation and a few
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preliminary checks on the lift, you’re ready for the road test. You take it out on the road and it works great. Your plan is to run it by the dealer and make sure it has all the computer software updates and calibrations; you want it to be perfect. As you’re headed over to the deal-
er you notice a faint whining noise. “Hmm, that wasn’t there a moment ago.” As you keep driving, it gets a bit louder, and then louder yet. At first you think it might be a clogged filter. You turn around and head back to the shop. Now it’s screaming! And then all of a sudden — bang! — you’re in neutral, looking for a place to coast over to the side of the road. Once you get it back to the shop
and get the trans out and apart you find the problem: a spun pump bushing and broken inner pump gear (figure 1). You don’t see a cause for the failure so you think it’s just a fluke. You get another pump, go through the unit, and try again. You head back out for the road test with a little trepidation. You decide to
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ere’s the scenario: You’ve just rebuilt your first 722.9 Mercedes transmission. You
go for a good long drive. After about 20 minutes you begin to hear it… that faint whining sound. Damn! You turn around and hope you can get back to the shop before it blows. Unfortunately, this is
an all-too-familiar problem. The Mercedes 722.9 is a 7-speed automatic transmission, introduced in 2004 for the SL500. It was later made available to other models and now it’s pretty much mainstream for the Mercedes line.
Figure 1 As common a scenario as this is,
there are a few measures you can take to avoid it. Naturally it’s important
by Dennis Madden
members.atra.com www.atra.com
Figure 2 GEARS August 2012
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