It's been concensus-ed that my '89 740GL wagon has a slipping differential, a terrible thing as of last night.
Loud clanks and jerks coming from the area. Wouldn't start to do it until there was weight enough in the car but then would do it with as little load as we could manage. Seemed to be 'activated' by a sharp turn this morning, but was happy to keep on in straightaways, turns, and anywhere in between once it got going; the turn thing may be a red herring, as we were testing for just the left front end at the time and things rather 'developed'. There's no minimum speed for this problem, and it's so bad as to make me very uncomfortable even trying to move it a mile down the road to a friendly driveway as slow as I can go, by which I mean it's no idle tapping but a rough jerking and banging. Without opening anything up, there's nothing visibly wrong underneath.

So, he's a car person but without much experience in suspensions and drivetrains; I haven't much car experience, but spent a year working as a bowling alley mechanic and he's worked there for a good three or more, on terribly pre-plastic machines. We've worked out that it sounds and behaves as though the teeth of the differential gears are slipping.

Our local yard has only one wagon (does it have to be from a wagon?) and it's not a copy of mine. So I have two questions for the board:
Does this sound like a reasonable diagnosis, or is there something we really need to consider? I've told you what I can think of, but I'm sure a few pointed questions can get more from me.
The question that got me to learn just how to use Brickboard finally:
What vehicles can I pull the back end from that'll be compatible with mine, in the 'day's work' sense, rather than an extensive alteration route?

Thanks a ton.

First, you need to crawl underneath the car and examine the driveshaft, to make sure your problem is not something like a bad U joint.

If, in fact, you need a rear axle, it makes no difference if it comes from a sedan or a wagon. They are identical in every respect, with the exception of sway bar mounts, which the wagon doesn't have, but are easily added (simple bolt-on) if you want to add a rear bar (which sedans normally have as standard equipment)

There are two other things that DO make a difference. First is the axle ratio. Rule 308 is correct in his description of where to find the label that tells you what the ratio is. Be careful cleaning it off. Don't use solvents, as they can dissolve the writing. Gentle scraping seems to work better. You will want to get an axle from a car that has the same type of transmission as yours (manual or automatic) but also check to be sure the ratio is the same as your old one. Manual transmissions usually have a 3.31, which is too tall for an automatic. Automatics will have either a 3.73 or a 4.10. You can use either--I prefer the 3.73 for its better gas mileage and quieter cruising. An axle from an automatic in a manual transmission car will kill your gas mileage, give you fierce acceleration, and be noisy on the highway. 3.54 and 3.91 axles also exist, but seem to be relatively rare, found mostly in older cars.

The year of the donor car is important too. There is a tone ring (toothed wheel) in the differential that sends a signal to the electronic speedometer and ABS (if equipped) There are at least two types of these. Cars up to 88 have a wheel with 96 teeth. 90 and later have 48. I think 89 is the same as the later cars (48 teeth) but I am not 100% sure of this. You must have the correct number of teeth for your car, or the speedometer will be WAY off, and the ABS will not work. Changing the wheel is more trouble than it's worth---better to get an axle with the correct type already installed.

There are also two types of parking brake mechanisms, early and late, but it's not difficult to swap those parts over to the replacement axle, if necessary.

You said that your local junkyard has only one wagon and it's "not a copy of mine" If it's a 240, the axle will not fit your car. If it's a 740 or 940, it will fit, but you need to keep in mind the two points mentioned above. Does the yard have any sedans?

There's 6 or 7 sedans available; if I recall correctly, the wagon is an 850.

As for the driveshaft, aye, we're blocking her up in half an hour or so to give the whole undercarriage a good going-over. By any chance is the number of teeth listed on the plate that Rule 308 has brought up?

Ever since I bought this car I've wondered what the deal was with the '89, as the Haynes manual goes to '88 and then starts again at '90. What you've said nearly implies I need both books... What's the best thing for me in terms of shop manuals for this vehicle? Auto Repair for Dummies isn't quite the in-depth reference.

850 is front wheel drive. Nothing on it is the same as your car. Any 740, you can use, if the other factors are correct, and some 760s and 940s, if they have a solid axle. Some of these cars came with independent rear suspension, which is a completely different setup than what you have.

Number of teeth on the tone ring is not listed on the data plate (which is actually a sticker) You may be able to determine this by removing the speedometer sensor, looking through the hole, and counting the teeth, while you turn it, or you may have to take the cover plate off to be able to see. On the ones I have seen, the 96-tooth has a stamped steel cover plate, and the 48-tooth is cast aluminum, but I can't guarantee this is a sure way of telling which is which.

To be fully informed, you need both the US (covers 83-88) and the British version of the Haynes, or, better yet, the factory shop manual, commonly known as the Green Book.

An important safety precaution when changing a rear axle: Since the axle weighs a considerable amount, there is a possibility that the car will become front-heavy when it is removed, and could become unbalanced and fall off of the lift or jackstands. To avoid this danger, put a couple hundred pounds (sand bags, car parts, whatever) in the back of the car before you raise it up.

It was a rear-wheel drive, so I must not remember the number. Wasn't a 740, though.

Okay, so. We got the differential out; the large planar gear is intact, but the gears in the center are ruined. Driving it to where we worked on it, I stripped it out the rest of the way (and needed a bumper push, quite embarassing). We'll be shopping for a whole replacement differential, a lot of it is intact but we can't budge the hex bolts to get to the ruined parts.

Is the tone ring what my friend is referring to as the planar gear, the large gear around the outside of the whole unit? I want to make sure what I'm counting the teeth on.

Also, now I need detailed advice for replacing my e-brake equipment (pads crumbled as we removed each rotor) and any other systems I may have impared, like the speedometer(?).

Lastly, is the fluid in the differential supposed to smell incredibly bad? I mean, motor oil is one thing but this was motor oil, cat urine, wet dog, old meat, and a bit of alcohol. I mean, impressive.

You're going to get a diff from a 700 or a 900 series car, prefferably a waqon but you can clean the undercoating off of the tag located on the back side of the left axle tube and get the gear ratio off of it. This will enable you to check the other diffs for correct application.

As far as you diagnosis goes, although I have never seen or heard of a V doing what your's is doing it is fairly consistent with having sheared teeth on the ring or pinion. I have seen it happen on full size trucks, Chevys and Fords, but have not seen a Volvo do it before.

Good luck with your swap.

Mark