How would you know you have one, and what's the advantage of it?

Volvo had a LSD option, but it only engaged at speeds below 25mph -- above that, it was inoperative (according to the accessory brochure).

One reason that would explain this precaution is that a LSD can be dangerous in certain slippery conditions such as ice. While helpful in getting going at a low speed (by sending torque to both rear wheels regardless of whether either of them lack traction), at speed it can cause a severe spinout.

What happens is based on the concept illustrated in traction circle. This is a graph that shows the grip of a tire as a function of the horizontal (acceleration, braking, lateral grip in both directions) forces. For example, a tire can show maximal traction for acceleration, or braking, or steering in a right (or left) hand turn, aka lateral traction; but when you combine two or more forces such as braking and turning at the same time, the graph reveals that the tire suffers reduced traction in both directions. This is why FWD cars have a disadvantage over RWD in racing -- a car with front tires doing both steering and accelerating is at a disadvantage over a car that splits the work between front and rear tires, each doing only one of the two tasks.
Also, the traction circle graph also shows that a spinning tire has no lateral grip at all, an important point.

Anyway, in your Volvo at road speed, you could (given driving uphill where you have to give it more gas pedal) with a LSD find yourself getting a tire slipping on ice and find both tires starting to spin, at which time the rear tires have no lateral traction at all, and your rear fishtails. In contrast, with an open differential, one tire would spin but the other tire would not, maintaining its grip on the road and keeping your rear end in proper line.

I've seen this plenty of times on slippery (from algae) boat ramps: a truck tries to pull their boat up the ramp and one rear tire starts to slip and spins uselessly, but at least the truck stays in position.
Next incident, a similar truck with a limited slip has the same problem, only in this case both rear tires are spinning (locked together by the LSD) and the rear of the truck fishtails violently -- I once saw a truck like that nearly jackknife with the boat trailer so that they were both almost lying across the water's edge. He had to unhitch the trailer, tying it to the ramp (so it wouldn't slip back into the water), to maneuver his truck safely back in line. [of course, these two stories are no credit to their stupid drivers :-)].

Not to be pissy about it but it's just the mood I'm in now after being in the sun all day today. Ken C and kittiesgreyvolvo both describe the locking differential (and I appreciate the information that it was offered on the 240) and both refer to it as a limited slip (lsd)--it's not. There's a ratcheting mechanizm in the locking diff that locks the two sides together within the speed range described. The true lsd diff (as offered in 122, 140 and early 240 cars) in my 144S has a clutch pack to accomplish the goal of having power to both rear wheels and it works at all speeds. That was the point of my post--to differentiate (pun intended) the two systems, as Borkborkbork did not seem to be aware of the differences and if he is intent on having such a system he should know what it is he is getting/looking for and know the terminology. -- Dave

Uncle Bork! Bork! Bork!,

A search on brickboard can reveal the Volvo 240 comes equipped with the Dayna locking differential as a Volvo dealer installed option.

The Volvo OEM dealer installed Dayna locking differential sends power to the (oh I forget - the right rear wheel as you set in the driver seat) like a regular (open) differential. Yet when the right rear wheel slips at speeds of 25 MPH or below, the locking differential send that power to right or both wheels.

Good for snow and slush and mud and slimy gravel. Most any slippery surface when the primary rear (right rear, yes?) drive wheel slips. Not much help on ice with or without studs.

A locking rear differential, if your 240 comes so equipped, or any RWD Volvo, with the factory diff, takes a wee bit of mindfulness. The locking differential can engage when you slip a bit on a turn and you can have some oversteer happenin' and the rear end arcs and slide as momentum carried the weighty chassis.

So there you go.

Other's can add there info here to better illustrate what you can expect.

If installed by the Volvo dealership (or even some at the factory), as you are staring at the rear of the differential 'pumpkin' looking forwards from the rear of the car, a metal tag secured to the left or upper left most securing the differential. The tag, by now well caked with grime and undercoating and a delicious cream cheese frosting (ha!), when cleaned off, display pressed into the metal tag something like 'use special lubricant additive'.

This tag can be lost.

The other method is to raise the rear of the Volvo 240 (or any RWD Volvo). Chock the front wheels, please observe SAFETY when lifting a Volvo, and secure the raise rear axle stands using the rear chassis jack points (same used by the tire jack) or rest on the rear axle using proper support methods and appropriate saddles on the axle (jack) stands.

With the safety (emergency) brake released and the transmission in neutral, by hand, spin one tire:
- If the opposite side wheel spins in the opposite direction from the rear wheel you are turning, this is an open differential and not a locking (limited-slip) differential.
- If the opposite side wheel spins in the same direction from the rear wheel you are turning, this is an open differential and not a locking (limited-slip) differential. You may have to spin that rear wheel pretty fast to get the other side to spin in the same direction.

Does that hepkats?

Are kitties fuzzy?

Is my 1990 Volvo 240 DL wagon in dire need of a front suspension rebuild? Yeah, going rite now.

Questions?

Comments?

Matinee film times?

The 2012 Championships Wimbledon?

Happy Holidays!

Garlic Powder.
--
Lots and lots and lots and lots and lots of GARLIC!!!!!
I really need to exercise.

First you should know that a "locking diff" is not the same as "limited slip diff" (lsd)--sometimes the terms are incorrectly invoked as interchangeable. I do not think the 240 ever had a locking rear--but an "lsd" is a possibility as it was an option through the years. With the lsd rear there is a clutch system which ties the two axles together. If the car is raised in the rear and you spin one tire by hand with a normal open differential the other tire will spin in the opposite direction. With an lsd rear the two tires will spin in unison.
Some 940's had a "locking rear" which would mechanically lock the axles together under certain conditions. The tire spinning check does not work with the locking rear--they will behave as a normal open rear when the tire is hand spun. I found this out when I had a '92 940 Turbo which had been rear ended while parked. When I first got the car I did the hand spin check (not knowing about the locking type) to see if it had lsd. After the impact I decided to spin the l/r wheel up to check that it hadn't been damaged. With the left rear jacked up I started the car, put it in drive and then stepped out to look at the wheel spinning. Much to my surprise the car started doing a little hop with the r/r wheel also turning (slipping on dirt) and the front wheels chocked. That's when I did some research and found the existence of the "locker" which is set to come into play in the 5-30mph range.
The purpose of either system is to get power going to both rear wheels in slippery conditions--especially useful if only one tire is on a slick surface. I've read of modifications that can be made to the "locker" which enables it beyond the 30mph range for performance purposes--ok for drag racing I guess --but on a road course I wouldn't want that system locking and unlocking in the middle of a turn. I've added a used lsd unit to my sometime track car '69 144S and it made a world of difference in certain turns where the inside wheel becomes unloaded and will spin even in the higher gears. -- Dave