LSD - question

[magpies]

Hi
I've never had a viscous type lsd but I've just bought one out of a granada cossy - I've performrd the usual check for a plate type diff in that I tuened one output shaft to see which way the other side turns and found it turning in the oposite direction which to a plate diff is a free one and not LSD - on the viscous one how do I check ?

[Rockey]

"a viscous LSD is driven by a viscous fluid. The two halfshafts aren't directly connected, instead they have fan-type things inside this viscous fluid, when the two wheels are spinning the same speed, both the fans and the fluid are all spinning together. When one wheel starts to spin faster than the other, it creates an imbalance in the LSD, the slower wheel wants to speed up and the faster wheel wants to slow down because of the fluid. On a hard launch you won't be able to tell any different between a car with an open differential and one with a viscous lsd until one wheel breaks loose, on the viscous car the power will transfer to the wheel that isn't spinning as fast, and on the open differential that one wheel will just keep on spinning how it wishes."
Taken from elsewhere, Isn't google wonderful

Edited by Rockey on Sunday 1st March 09:22

[magpies]

I guess the only way is to take the back off and see whats inside

it'll either have a normal set od crown wheel and planet gears or I presume (hopefully) a cylinder within the crownwheel which houses the lsd fluid

[TonupS2]

Sorry this is probably a really silly question but I'm going to ask anyway. I was told by the previous owner that he thought the diff is a S3 LSD type. Whilst the diff was off I thought I would pour the oil out for an oil change, I took the fill/drain plug off and tiped the diff up but nothing came out. In my ignorance I thought he must have already emptied it when he took it off. I have not yet refilled it. Without wheels on and on axle stands when I turn one rear drive shaft the other goe's in the opposite direction (thought that meant it isnt LSD?). Am I right in thinking that if it is a viscous diff then I would not need to put oil in? Can I define what type of diff it is by the numbers on the casing?

Edited by TonupS2 on Monday 2nd March 10:00

[ketvrin]

Extract from Sierra Owners site...

And a way to ID a LSD off the car.

Hold the input flange with your hand (the bit where the propshaft bolts on)

With your other hand turn one of the output flanges (The bit where the driveshaft bolts onto, any one will do and it don't matter which way it turns)

Reguardless on whever its an LSD or not the opposite one WILL turn in the opposite direction


Now the test (remember to hold the input shaft still so it don't move), if it is easyish to turn the driveflange then its a normal diff

if it is bloody hard to turn (or even maybe impossible) then its an LSD

[ketvrin]

And another with some ref to the codes... if it has an "AA" ...

hope this helps

K

Heres a diff guide
To add to the list;
3.64:1 7.5" for the RWD Cosworth (with 108mm drive-flanges)
The Granada Cosworth 24v diff is the same as the 2WD Cosworth BUT with Sierra compatible 100mm drive flanges. and the 4x4 Cosworth is the same as the 2.8/9 Sierra's 4x4's and 2.0 DOHC 4x4 is 3.92:1 in a 7" diff
and some 2.0 RWD DOHC's are 3.91:1 in a 7.5" diff and finally some 2.3 diesels have a 3.14:1 7" (hens teeth)

Another way of knowing what diff you have is to look on the diff ID tag (little alloy tag bolted to the rear cover, top left hand side) Shown in the picture circled in red.
There will be some numbers/letters (look at the top row),
EG. 83BG MC 362
The 1st set (83BG) is the case type (7" or 7.5") as there quite a few codes its not worth looking, the esiest way to ID a 7" and 7.5" is to look at the top mounting. The 7" has one long bolt/nut and the 7.5" has 2 bolts.
7" on the left, 7.5" on the right.
Next is the middle code in this case its 'MC' which is an 'OPEN' type diff (NON-LSD) basically ANY diff with 2 or more "A" in the middle code will be an LSD e.g. "AA" or "ADA" any other letters without an "A" in the code IS an open diff.
Finally is the numbers, 362 which is (simply) 3.62:1 ratio.

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AS you can see the 7.5" diff is big begger compared to the smaller 7" diff
Don't be put off by this as the 7" diff is a strong one (used in the 4x4 Cosworth remember) ALL the 7" internals are the same (except the ratio's and LSD bit) so even the basic 1.6 diff is bulitproof

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The following diff are LSD. ALL 4x4 Sierra's and Granada's (2.8/9 and 2.0DOHC and Cosworth)
Most BUT not all 24v Cosworth Granada/Scorpio (upto approx 1994 are LSD, after that they are 'open' type because the Scorpio 24v had traction controll instead) All RWD COsworth Sierra (3dr, Saff, RS500)
Then you might find an LSD fitted as an option to any Sierra or Granada, but unlikely.

[Barkychoc]

You MUST MUST MUST put oil in the diff but its a special LSD oil for the LSD diff.

On my diff turning one of the output shafts resulted in the output shaft on the other side going in the same direction - but there is some resistance it doesn't feel free like an open diff.

Turning the output shaft didn't turn the input shaft even with nothing connected to it.

[TonupS2]

Thanks guys & a fact filled response Kevin.
My tag says something like 5GG HB 364. (no A's in the middle) so it's not a LSD.
Great help.