icsmike

on your 928 that is, duh, lol. really though if your were to go and look at a car how will you know> What is the best way?

pmotts

Does it leave 2 black marks when you drop the clutch?
If you mean without driving it the psd unit is behind the drivers side back tire, at least in S4 and later Sharks.

Oh yea, and cookies are much more fun with LSD.

Mrmerlin

to tell if you have a posi, both wheels off the ground , they will turn in the same direction when you turn one side.
In a open diff, one side will spin opposite of the turned wheel.
The PSD modulator is behind the left rear wheelwell block off plate.

icsmike

PSD? LSD? Are you guys saying posi is LSD?

icsmike

I know i laid about a 30 ft. twin blackey from a drop clutch brake hold 3 years a go. They were even to the end. is that LSD?

pmotts

Yes, later cars had PSD, yours has lsd. And lots of us had LSD... different topic.

N/a 944

oh no I'm in the wrong forum... but I'll chime in because I think I know.
Because you said that you laid two marks down that were even, (one didn't stop or start at another time in the burnout) I am almost completely sure you have a limited slip diff(lsd). I don't even know if 928's got a positraction.

Hilton

Jack up the rear of the car, put it on stands, and look at the serial number on the bottom edge at the rear of the transmission.

You might be able to reach far enough using a digital camera in Macro mode to do it without jacking up the car?

pmotts

You're right, 'posi' was not the right verbage.

Vilhuer

Two lines on ground is not absolute evidence of working LSD nor is gearbox number. Lift and rotate method is dead giveaway.

mark kibort

burn rubber by dropping the clutch on some smooth pavement. (as someone suggested. ) thats the easy way! two black marks equal length is limited slip.

mk

fabric

Better is to put one wheel on loose surface, other on road, and see if only the wheel on the loose surface spins. Open diff will cause the wheel with less traction to spin in this scenario. If traction is equal, both tires *can* spin with open diff.

The other option is to look for option code 220, although no guarantee something hasn't been changed.

fabric

http://en.wikipedia.org/wiki/Limited...#Factory_Names


Positraction is just Chevy's name for LSD - it's just the one that stuck, probably cause the other names are stupid. According to same page, Ferdinand Porsche is responsible for the LSD.

hacker-pschorr

My 81 does not have LSD / Posi – it will and has left two solid black marks when taking off.
Yes – same thing.

Positraction is GMs trade name, TrakLok is Dana's and Traction Lock is Ford's – all of them are the same thing: Limited Slip Differential or LSD.

AO

LSD is a mechanical Limited Slip Differential.

PSD offered from 90(?) - 95 is an electronically modulated LSD that is progressive from 0% to 100% in 10% increments IIRC. The PSD uses wheel speed sensors and accelerometers to detect if one wheel should be spinning faster than the other and modulates the PSD accordingly until the two come in line with a predetermined set of parameters. Some have remarked that they don't like the PSD for racing (it's heavy), but for street driving it's quite nice IMO.