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Old 09-28-2018, 04:55 PM
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Squiggle Dog Squiggle Dog is offline
Join Date: Jul 2008
Location: Surprise, AZ, USA
Posts: 3,777
The power steering had been failing and it got to the point where it was extremely difficult to turn the steering wheel at low speed, so I decided to rebuild the power steering pump, but it's a dual-chamber tandem pump.

Why is it dual-chamber? Because Mercedes engineers decided to make an Anti-Slip Differential as complicated as possible--instead of just having clutch packs in the differential, they had hydraulic fluid run through the power steering pump (which also pumps power steering fluid that can't mix with it) and through hoses and tubes going back to a valve unit with an accumulator sphere which then locks the rear axle and then the fluid circulates back into a bottle and filter in the engine compartment.

The main valve is controlled by electrical relays also, which receive input from wheel and brake sensors. To bleed air out of the system, you have to remove a relay, bridge some of the pins with wire, then attach a clear hose to a bleeder valve on the differential while the engine is running. Talk about over-engineering.

But, the rebuild kits for the tandem power steering pumps have been backordered for years and may not again come into production. My roommate and I scoured the internet for places to buy one, only to be told that they were out of stock and couldn't get any more. Finally, my roommate found a place that had two, so he bought what are probably the last two rebuild kits in existence. I still ended up having to anneal some copper washers so I could reuse them.

Apparently very few cars were built with the ASD system and it's a very rare option. Rebuilding the original pump was much more realistic than paying $1,478.99 for a rebuilt one. In order to use a standard power steering pump, the entire anti-slip differential system would need to be removed and the rear axle would need to be replaced with a standard type.

Unfortunately, this didn't seem to solve the power steering failure. Maybe the steering box needs to be rebuilt. I removed the ASD relay in an attempt to bypass the ASD system so the hose wouldn't slap up against the oil pan and the rear axle accumulator sphere would stop hissing, but it didn't work.

The ASD light has been on for a while now, so I guess I'll have to run a diagnostic, replace the hose going back (which is heavily worn) and install a new accumulator sphere.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles
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