Is Anyone Here Familiar With ASD/SLS Systems? I Have Questions And Can't Find Answers - PeachParts Mercedes-Benz Forum

Squiggle Dog · #1 09-23-2017, 01:24 AM

My roommate has a 1991 W126 350SDL with ASD (but not SLS, though it uses some of the same components). The ASD system is ran by a tandem power steering pump. There is a reservoir on the left (driver) side of the engine compartment.

The tandem power steering pump is failing. It is very difficult to turn the steering wheel, especially at low speeds. We searched around and bought the last two rebuild kits in existence (we tried practically every imaginable vendor, including The Classic Center and eBay--all ended up backordered or out of stock with no ETA after paying).

When the engine is idling, at the rear of the vehicle by the ASD valve at the rear axle, every few seconds it makes a hissing noise. The hose that goes from the tandem power steering pump back to the valve lifts up and then slaps against the engine oil pan in sync with the hissing noise.

The vehicle has also had intermittent acceleration problems which MAY have been due to clogged fuel filters.

QUESTIONS:

1. Is the HISS...HISS...HISS... sound by the rear axle normal for ASD vehicles, or does it indicate a problem?

2. Is the hose from the tandem power steering pump back to the rear axle slapping against the oil pan normal for ASD vehicles, or does it indicate a problem?

3. If there is a problem with the ASD, will it cause drag on the tandem power steering pump, possibly causing difficult steering or poor acceleration?

4. Is it possible to put the system into a bypass mode where it will not interfere with drivability?

5. Are there O-rings where the hard lines attach to the cap at the top of the ASD reservoir? EPC shows no O-rings, leading me to believe they are flare fittings, but it looks like the cap is plastic, leading me to believe it should have O-rings.

I have done lots of internet searches on the topic and am not finding much of anything relevant. I looked in the factory service manual and it only shows diagnosing using electronic codes. If my roommate could afford shop rates, I wouldn't be working on it, plus I have a feeling there aren't any shops that would know how to fix it, or would be willing to, anyway.

I have to get the tandem power steering pump on this car rebuilt before my roommate completely loses steering on his daily driver. But, the ASD system is completely unknown to me and is integral with the power steering system so I have to be sure the ASD system isn't causing problems.

Alec300SD · #2 09-23-2017, 09:56 AM

I may be wrong, but:

For testing purposes or perhaps even long term, I believe you can just install a temorary bypass hose, looping the PS pump's ASD output port to the return port on the PS pump.
You should be able to drive the car.

This would keep the ASD portion of the tandem pump lubricated and in use but eliminate any load on the pump from the ASD.

You may want to unplug the ASD relay (wherever it might be) to keep the ASD valve from operating.
To keep fluid in and dirt out of the disconnected ASD feed and return lines, add a separate bypass hose to loop them together.

If easy low speed power steering returns, you've determined the ASD system was creating an excessive load on the PS pump.
Possible ASD failure points could be bad sensor(s), stuck relay, bad accumulator, bad valve, etc.

If it is still hard to steer, than you may have degraded fluid, air in the lines, a bad spool valve in the PS pump, etc.
How does the PS fluid look and smell?
Has the PS filter been replaced (if this car has one)?
Any leaks from the steering box?

ASD activates when there is a measured difference in wheel speed.
Are both rear wheels the same size?
Is there any binding of the calipers or parking brakes?

Hissing, and banging of the ASD line, I believe are not normal.
The hissing may be due to a bad accumulator.

sixto · #3 09-23-2017, 12:04 PM

Can you fit a singular function power steering pump in the meantime so the car is drivable? It might prevent further damage to ASD components.

Sixto
98 E320s sedan and wagon

Maxbumpo · #4 09-25-2017, 08:12 AM

Any of the gasser wagons have a dual-circuit pump that will fit, that would be the best course for a cheap fix.

The ASD is integrated with the ABS. I once asked my local indie about disabling the ASD on my 124 sedan when the special differential seized from lack of oil, he said it could not be done. Worth trying the bypass hose as a troubleshooting method. The ASD computer compares the rear axle speed to the front wheel speeds to detect wheel-spin, and then pressurizes that circuit to engage the hydraulic clutches in the differential to reduce wheel spin (lock up the axle so both wheels get equal power). If the ABS system is on the fritz, it may be causing the ASD system to engage when there is no wheel spin.

Dr. Sternschnuppe · #5 09-25-2017, 03:45 PM

ASD system has separate control unit. So it possible to disable ASD (remove relay) separately from ABS.

Squiggle Dog · #6 09-27-2017, 01:08 AM

Alec300SD, since the ASD reservoir is separate from the power steering pump, I don't think I could just loop a hose from the output to the input port as there would be no way to add fluid, BUT I imagine there could be a way to bypass it by placing a hose from one place to another.

I'll consider unplugging the ASD relay if that in effect prevents any load from being placed on the system. I hope it's that easy!

The power steering fluid was very dark. I did a power steering flush and replaced the filter, but that did not correct the issue. The new fluid went from crystal clear to muddy red within a few days from contamination that remained.

The steering box leaks profusely out of the Pitman arm shaft seal (it loses about a quart per month). We ordered a new seal and I will replace it when I rebuild the power steering pump.

The rear wheels are of the same size, but the left rear tire is worn down further than the right rear because it was once on the front and the front left tires wear abnormally fast, and the front end is so worn out that the alignment shop won't touch it.

The entire brake system is brand new and functions properly.

Sixto, I don't know if I can use a regular power steering pump in place of the tandem one. That's a question I wouldn't mind an answer to.

Maxbumpo, thanks for the tip. I have a rebuild kit, so I'm going to rebuild the original pump.

Alec300SD · #7 09-27-2017, 04:32 AM

Sorry I wasn't clear in my description.

As I understand it, the tandem pump has two separate circuits, one for the power steering and one for the ASD.

Each circuit has its own reservoir, fluid and lines.
I don't believe they normally interact, unless there is an internal breech in the tandem pump.

What I meant for you to try was to loop the high output ASD line at the tandem pump back to the low pressure ASD return line at the tandem pump.
Effectively cutting the hydraulic fluid to the back half of the ASD circuit controlling the differential, but still keeping the ASD portion of the tandem pump lubricated and operating under a no load condition.

You might need a temporary reservoir to bleed the bypass loop.

Same idea as mini tank under the hood to bypass the fuel tank.

The tandem pump appears to be part number 126 460 15 09 which is NLA, but it has been replaced by part number 126 460 21 09.
It also appears NLA.

A leak in the steering box wil definitely cause steering difficulty and perhaps may affect the tandem pump's ability to pressurize the ASD circuit.
Here's a good pictorial on rebuilding a tandem pump:
Tandem Power Steering Pump Rebuild

Squiggle Dog · #8 10-21-2017, 10:44 PM

I removed the ASD control module to try to bypass the system. It didn't do a thing. The rear of the vehicle still goes HISS... HISS... HISS... and the hose going from the tandem pump to the rear of the car keeps slapping against the engine's oil pan.

I suppose it's going to need a new $100 accumulator or something. For a free car, it's sure become a money pit.

Maxbumpo · #9 10-23-2017, 03:31 PM

I am pretty sure there are no accumulators with the ASD system, just the special pump, the special differential, and the ASD control circuitry. Regardless, it will probably cost something to fix it!

Squiggle Dog · #10 10-23-2017, 03:34 PM

Quote:

Originally Posted by Maxbumpo

I am pretty sure there are no accumulators with the ASD system, just the special pump, the special differential, and the ASD control circuitry. Regardless, it will probably cost something to fix it!

I meant whatever this sphere thing is.

Maxbumpo · #11 10-23-2017, 03:35 PM

I think the next move is to intercept the high pressure supply coming off the tandem pump and route it directly into the reservoir. This should stop the hissing and slapping, and take any load from the ASD off the tandem pump so you can assess the condition of the power steering circuit.

Maxbumpo · #12 10-23-2017, 03:36 PM

Quote:

Originally Posted by Squiggle Dog

I meant whatever this sphere thing is.

I missed the sphere thing. Did you put up a picture of it?

Maxbumpo · #13 10-23-2017, 03:37 PM

Quote:

Originally Posted by Dr. Sternschnuppe

ASD system has separate control unit. So it possible to disable ASD (remove relay) separately from ABS.

Well, it was worth a try, but perhaps my mechanic is correct and the systems are too integrated to make it that easy.

_Or_

Squiggle Dog pulled out the wrong relay? I can't read any numbers on that, are you sure that is it?

Squiggle Dog · #14 10-23-2017, 03:48 PM

I guess I'll have to figure out with what materials I am going to bypass the ASD system to see if it's causing drag. But, I guess I could just hook it all up after the power steering pump is rebuilt and the steering box has a new lower shaft seal on it and see if it suddenly steers easier.

Either way, some of these options are such a headache. And whenever these systems need repair, it never seems to be a good time financially. I was a bit worried that a W126 would be too complicated to work on when my roommate got the car. Even my W116 is too complicated for my liking.

I posted a picture I found on the internet of the sphere that could possibly be the cause the the hissing and slapping. It screws into the valve body at the rear of the car.

Squiggle Dog · #15 10-23-2017, 04:02 PM

I think the part number on the control unit I removed was 008 545 24 32. It looks like it was superseded by 013 545 01 32. It also had the little diaphragm on the side, so I think what I removed is the ASD control unit.