I've read that volvo 740s had locking diffs as an option, how do you tell if you have one? I did a burnout on gravel and both wheels spun together until like 20 mph. do I have a locking diff?

 

Volvo had a limited slip in their parts catalog, back in the 80's - I found a new one at my local Volvo junkyard and put it in a 78 240 I had - with it's 103 hp - rev it up and dump the clutch on a wet road turning slowly - if the back end of the car comes around - you have a limited slip.

Some of the later 960's had a locking diff that disengaged at a low speed (over 15 mph?)

 

I don't have a manual transmission... but if I can do a 2 wheel burnout wouldn't that mean the rear axle was locked?

 

Then power brake and try to do a donut

 

the locking diff used on select 700/900's will only lock at speeds below about 25-30 MPH, and if you spin a tire on a sharp turn on a dirt lot, you can feel it lurch as it locks.

re: straight line burnouts, both tires can spin without a locker, and probalbly will if A) you dump enough torque on it, and B) the traction is roughly equal side to side. the part number on the differential will say what it is, but I'll be darned if I remember the codes or numbers. Do note, you can't readily swap rear axles between a 240 and a 7/940, as the suspension and strut mountings are quite different. ABS cars have different speedometer tone rings than non-ABS, and there are a few different final drive ratios depending on what the car is its out of.

supposedly you can take the locking parts out of a locker diff, and add them to another, as they are all Dana 30 differentials.