ASD question - limited slip or not?
 

Hi everyone,

I have a question about the ASD (hydraulic locking differential) which was optional on a number of 124/140/201 diesels. From the EPC and service manual info, it appears that the ASD diff acts as a normal limited-slip diff until the computer decides to apply hydraulic pressure to the external collars, then allowing 100% lockup. Normal LSD only gets to something like 20-40% lockup, I believe, depending on the tension of the thrust washer pack (adjusts with different thicknesses). Right now there are a number of different claims & speculation that aren't matching up well. I'd hate to buy a used ASD diff, expecting to get cheap LSD by swapping that in, only to discover it doesn't work...!

UPDATE (March 2009) : Yes, ASD has LSD active at all times, as described above. The LSD lockup is 35% when new, but this will decline over time as the clutch pack wears. After 100-150kmi, the lockup can be near zero. Replacing the clutch pack costs $300-$400 in parts, requires a few tools which most DIY'ers may not have in their toolbox, and lots of hours of work. BT, DT. But I did convert an ASD diff to LSD, and install it in my 1987 300D successfully. It's been working great for 2 years now.

:boat:


Best regards,

It's been awhile since I read about ASD, but my recollection is that it is basically an open differential with a mechanism to lock if slip is detected, but the locking only occurs at speeds below 15 MPH, so it will not prevent wheel spin exiting a second gear corner hard (at a speed above 15 MPH) like a conventional limited slip diff will. It's probably an asset for snow traction or drag racing, but not autocross or road racing.

A conventional limited slip diff has preload and can be tested by jacking up one side of the rear and determining the amount of torque to break loose the wheel. Be sure to DOUBLE CHOCK the opposite front wheel so you don't drive the car off the jack. Typically you want to see at least 45 lb-ft prelaod using a torque wrench on a lug bolt.

If ASD is basically and open diff rather than a limited slip, it will not have any preload.

ASD requires some sensors and electronics, and I assume part of the control system is external to the diff, so just bolting in an ASD diff without all the system sensors and controls will probably be the same as an open diff.

Duke

That's what others are saying too, Duke. However the FSM shows the preload torque for ASD diffs to be the same spec as LSD diffs (80-160Nm, or about 60-120 lb-ft). That's twice the spec for an open diff (40-90Nm spec). And the 190E-16 LSD has almost identical internal clutches & side gears when comparing the part numbers to the 124 diesel ASD.


Update: That's because ASD does have a normal LSD ring gear carrier!


:stuart:

They are LSD as well as ASD. Stock amount of lock is 15% (some say it's 35%). This is present without the ASD system operative. I have some Bosch sourced documentation for it. ASD Works under 40 km/h and claims to lock up up to 100% (unverified but don't know why it would lie).

They do seem to axle tramp badly even on brand new cars the press mentioned it.

Check this video I made!!!

http://video.google.co.uk/videoplay?docid=-89451780287524338&hl=en-GB

Oh I did the spin test and the wheels go the same way.

Wow, this is an old post! I edited the original posts to update the information. Anyway:

Yes, ASD has LSD (with 35% lockup at all times) at the core. The hydraulics simply exert pressure on the LSD clutch pack to make the lockup 100%, up to 19mph, at which point the hydrualics release. As the clutch pack wears out, the lockup percentage will reduce. The FSM procedure shows testing it with a torque wrench on a lift, with ASD engaged manually, the torque needed to turn one wheel separate from the other should be at least 100Nm higher than when ASD is not engaged.

I found a factory brochure which details the functionality of both ASD and ASR, I will scan that and upload it to my website. It does a better job of explaining how it all works than I could.

:stuart:

[QUOTE=gsxr;2144191]Wow, this is an old post! I edited the original posts to update the information. Anyway:

Yes, ASD has LSD (with 35% lockup at all times) at the core. The hydraulics simply exert pressure on the LSD clutch pack to make the lockup 100%, up to 19mph, at which point the hydrualics release. As the clutch pack wears out, the lockup percentage will reduce. The FSM procedure shows testing it with a torque wrench on a lift, with ASD engaged manually, the torque needed to turn one wheel separate from the other should be at least 100Nm higher than when ASD is not engaged.

I found a factory brochure which details the functionality of both ASD and ASR, I will scan that and upload it to my website. It does a better job of explaining how it all works than I could.

:stuart:[/QUOTE

Dave, will you post a link when you get it scanned? I would like to read it. Any idea how much work it would be to add ASD to my wagon. I believe the SLS pump is used, so that is in place. I suspect I would need to add a feed line the the differential and the of course change out the differential. I believe in '88 it could be order with it.

Likewise. It's something I wish my 350 had... it was optional in 91.

-J

[/QUOTE

Any idea how much work it would be to add ASD to my wagon.[/QUOTE]

As you said, the ASD diff, hydraulic lines, and connection to pump. There is a valve near the axle that allows the fluid to circulate fore and aft, and then be sent to the diff when called for. The big issue is wiring up the ASD control unit "black box" with it's many inputs needed. Dave can explain why he did what he did to his car, and left it LSD only.

Besides the addition of the computer and wiring, you need a different ABS system as the ASD and ASR cars have 4-channel ABS and 4 sensors (both sides of the differential instead of the nose).

LSD is a great addition, I'm not sure it is worth the extra effort to re-design the wiring and add ASD, Dave might have more insight on this.

Yep, I'll post a link when I get it scanned. It would be possible to add "manual" ASD, triggered by a switch on the dash. Or even a fully manual trigger with a hand hydraulic pump (saw that done by a guy in Europe, I think). But to retrofit the full ASD system, that would most definitely be a challenge (and probably quite expensive). You would need the ASD brain box, wiring harness, speedometer, and all the hydrualics & valving. My '93 300D has factory ASD, btw. However, the ASD still uses the standard ABS system with a single rear speed sensor... only ASR has the dual rear sensors (my V8 models have ASR).

While I really like factoy ASD, I believe it would be too difficult retrofit, unless you had a complete ASD donor vehicle to snag ALL the parts from. Even then, I am not sure what the challenges would be. All I wanted was to eliminate the "one wheel peel" on wet or dirty pavement, and plain LSD has cured that (at least with fresh clutch packs installed). If you buy a used ASD diff, it's almost guaranteed to need new clutch packs.

While converting to plain LSD is not simple, it's much easier than attempting a full ASD retrofit. The cost to convert to LSD is at least $750, plan on more like $1000, including the new clutch packs, seals, fluid, etc. Those numbers do not include labor, that's parts only. I don't know what it would cost to do ASD but I would figure at least $300 more plus a LOT more labor.


:balloon2:

Link to PDF of factory brochure explaining the function of ASD, ASR, and 4MATIC - click here.

:scholar:

That brochure came in my 4matic when new (in '91), in the manual pack. I haven't seen the manual since I sold "Ruby" about 13years ago, thanks for the trip down memory lane Dave.

YIKES! That sure doesn't sound easy. So if I just switch to the LSD and not all the electronics, it would give me 35% lock up, if I rebuilt the clutch packs, is that correct? Would I still need to hook up the hydraulics?

Does my wagon and the 90-93 sedans have the same rear differential?

Does anyone know if a differential from 1992 4 matic will work in an '87 300 TD?