Auto locking diff vs. Limited slip diff?

[Volvo6]

The automatic locking diff is quite common on the 900 series cars and I understand it is a completely locking unit that disengages again automatically above 25mph.

But was there not also an option for a limited slip differential (as opposed to the locking one) that could also be ordered? I know it was an option the 200/700 series cars but did this option continue into the 900 series as well and if so what was Volvo's reasoning behind offering the both the lsd and auto locking as different options?

I mean was the lsd option there to improve general handling as opposed to the locker which was for low speed traction?

As a supplementary question does the auto locking diff on the 900 bear any resemblance to the one on the later 850/V70 AWDs?

Just trying to understand the history of the differential options! I did try the search function but I couldn't find a previous thread that addressed my specific question.

[rogerthechorister]

I am not certain but I think that in the UK the locker was standard only on the 960s and V90s - the whiteblocks.

I am reasonably confident that the LSD but not the locker was an option on the redblock 9s in the UK - but was standard in some other countries, perhaps Skandiwegia and Canada.

[crogthomas]

Have you read this?

http://people.physics.anu.edu.au/~am...volvo_dana.htm

[davebslater(uk)]

the locker was std on 940 GLE too.... , and yes it bears a striking resemblence to the awd rear diff since that is the same as used in the 960/v90 albeit with a different nose arrangement and comes with a 3.3:1 rear CWP...

Dana Trutrac is the cheapest best option for your dana 30 rear end , LSD are available but will be mega money even if you could get one from volvo , sellholm do LSD for about 1000 euros.......yikes

[Volvo6]

Originally Posted by crogthomas

Have you read this?

No I hadn't...thanks.

Although it doesn't explicitly say so it sounds from the timeline in the article that the Dana 30 LSD was discontinued as an option once the new auto locking diff was introduced. So it would appear only on a few early 900 series cars that predated the auto locker and had the Dana 30 installed as an option.

Still curious about the merits of the Dana 30 LSD? What was it's main purpose and why did Volvo drop it in favour something that is just a low speed traction aid?

The one point the article is not very clear on is the relationship with the systems in the 850, saying there as a "FWD version" of the auto locker for the 850 Turbo. My understanding has always been there were no limited slips/lockers in the 850/V70 except the FWD Rs which viscous diffs at the front and the AWDs which had a low speed locker (possibly the same as in the 900?) at the rear. Sorry I know that point is not really to do with the RWD knowledge base.

Sorry for all the questions, I'm kind of in the process of 'rediscovering' the RWD Volvos at the moment and trying to get my knowledge of their history to a similar level that I have of the FWDs...which is far from complete but a lot better than the RWDs at the moment!

[davebslater(uk)]

yeah , the awd just had the eaton locker at the back........ i dont think the FWD lsd bears any resemblence the dana 30 unit ...

you answered your own question with why volvo replaced the lsd , high speed cornering with an open diff is safer with an inexperienced driver ie its more likley to go sideways than spin the inside rear ....

[Neoott]

On the same subject is there any way to know what diff you have without removing it?
Dave

[davebslater(uk)]

axle tag on the LH axle tube , scrape the underseal off carefully itll reveal the ratio and the part number.....

[Volvo6]

If the labels have come off/illegible...

With the older LSD unit if you jack the rears up (and chock the fronts!) and turn one wheel with the car in neutral and the handbrake off the other should turn in the same direction. If they turn in opposite directions then that is a standard open diff. That's a standard test for a RWD car.

The later (c1991? for the 900 series?) auto locking diff might be a bit harder with the above test because what you've got to do is get one wheel spinning (100RPM?) faster than the other in order for the wheels to lock together. A really hard push might do it.

There are quite a few other threads if one searches for "limited slip diff" in the 700/900 series section which might give better advice on this.

[Diesel-do-nicely]

Would it not be easier to think of all the 700/900 diffs as an automatic UNLOCKER ? as to me when ive looked in the cases its just a std multi plate lsd with a centrifugal bob weight that disengages the unit above 25mph. ish.

Since it is active in both forward and reverse directions, the average motorist would find the car understeering into corners in low grip situations, and oversteering out of them , as in , both wheels would break away under power, not just (usually) the inside one.

I have a parts brochure showing the "option" lsd unit for 700/900/240 cars, it seems to be without the unlock bob weight.