Proper use of floor jack and jack stands with a 123-chassis 240D: place where, how?
 

What areas of contact do you use in order to avoid damaging anything in the delicate frame of the car and its components? Do you place any wood as buffer between the lifting devices and the car? Is it wise to use two blocks of hard wood (I already have two sets of 4"-high, secure wooden ramps that I have been using) under the wheels on the side opposite to the one placed on jack stands (with chocks nailed on the blocks of wood to improve stability and safety) in order to keep the car level so that it will be flat on the lifting devices' contact areas and not resting on them precariously at an angle?

Please contribute any useful ideas that come to mind relevant to this topic, and please make it as clear as possible, even at the risk of being redundant (I am using these tools for the first time and, given the potential danger involved in working under a car supported this way, learning how to do it safely is of foremost importance to me...)

TIA,
Rino

I just use the area directly under the holes that you would use the crappy factory jack. I have a rubber pad on my jack but in certain circumstances I will use wood blocks.

I have also placed jackstands under the control arms in the past.

One thing I would not do is jack the car by the differential. Not on an independent suspension car anyways.

I dont think this is the correct answer by a long shot. There are "hardened" point under the car for jacking etc. If you look on the side of the car there is a u shaped channel that hangs down, towards the front tires it has a reinforced end. Towards the rear tires there is a metal plate that is bolted in to the hard point. Both of these are fairly easy to spot. I wish I had a pic to show. The only problem is jacking and then putting stands. Usually I get a nice 2' section of 2x6 and run it under the rest of that U channel. you can lift the whole side of the car this way. Perhaps someone else can chime in on jacking but thosejack stands belong under the hardened sections. Also when jacking always have something softer than metal inbetween. Always chock your tires and always put the car in gear AND use the parking brake. If the parking brake has not real effect then fix that first.

To raise the car, I use the factory jack in the factory holes. Then I put a jack stand under the frame at the jack hole. I use a small piece of plywood as a cushion between the stand and the frame -- see photo (I enhanced the lighting so you can see the jack hole). Because of the way the jack works, it doesn't get in the way of the stand, and vice-versa. I repeat the operation for each of the four locations on the car that I need to have off the ground.

I have also used a floor jack under the jack hole position but then I can't get a jack stand in there and don't want to risk putting it somewhere else. I don't know how strong the rest of the frame is.

Some day :D I'll remember to take a picture of the jack and the jack stand in position together.

Why? That's where the FSM says to lift.

Don't make this to complicated. It's not rocket science.

For the front I use the engine cross member just behind the oil pan to jack the car.
I place the jack stands under the frame box just inside of the jack hole provided by Mercedes. Been doing it this way since I have owned Mercedes, 27 years with no problems.
Look at this post, second page. Those are the 6 ton jacks at Harbor Freight.

http://www.peachparts.com/shopforum/diesel-discussion/122057-617-engine-replacing-oil-separator-check-valve-upper-oil-pan.html?highlight=oil+separator+pan+dave

For the rear I use either the subframe mount area or the differential. I usually place the jacks under the subframe mount area.

Ara T. Don't see why you consider the differential sacred. Ive owned the 300TD for 16 years and have jacked it at this point ever since I got my long reach floor jack about 8 years ago. When I rebuilt the rear suspension. The differencial mount was in fine shape. replace it anyway because I was doing a complete rebuild. But it did not seem overstressed. Remember the differencial is mounted with 4 bolts to the subframe. So the force is on the subframe mounts and the differential mount at the same time. Not just the differential mount alone.

Jeremy5848

Please consider getting another set of floor jacks. I had a set like that quite a few years ago and had the lower straps snap when placing the SL on them. For $25 or so you can get a set of 12 ton jacks at Harbor Freight. I know that HF is not necessarily the best but a set that holds 12 tons should be good for a 2 ton car.

Dave

Sweet I'll start jacking mine that way front and rear (diff in rear).
Thats precisely the area I was talking about.

Make sure and be careful around the jack points if they are rusted, like mine though. Safer to use the frame, IMHO.

x2. I had those same jack stand and have got to say, they are no good. The narrow base combined with the pin-through-hole design made me very nervous when using them.

Interesting advice...
 

My set is at least 30 years old and show no hint of a problem -- maybe they build them cheaper now? But I like the kind you guys are suggesting -- the ratchety feature is nice -- so maybe I'll go shopping.

I use the cross member behind the oil pan as well.

I have never used the diff on any car, unless the FSM allows it. I typically use the control arms if I am just raising one wheel off the ground a bit, being that it has to deal with the sprung and unsprung weight.

I have a pair of old cast iron/steel ratcheting stands. Each one holds 6000 lbs. Get rid of those whimpy pin held ones. Ask my father how it felt when one of those gave way in the 70's.

And safe!

FWIW I always leave the jack jacked up just shy of the bottom of the car near where ever I am working. Even jacktands wobble....

Whenever I place a car on stands, I remove the jack from underneath then give the car a good, solid bump at each corner with my hip to detect and test for any instability.

Anal? Nope. I have still vivid, 6 year-old's memories of my dead neighbor's legs and feet sticking out from underneath his car after it fell on him one Saturday afternoon.

Whoa. That's terrible :eek:

I understand using the factory jack lifts the car up, on one side only, of course, and the indy rear will drop to it's travel limit. Using the diff [as I have done on every vehicle I have owned without any known difficulties] I cannot detect any dissimilarities raising entire rear end. I might be missing something. I also use the engine cross member as well in front. Too cheap to get rubber pads, so I use wood cushions, and keep a length of 4X4 in trunk, so if I have a flat, I don't need to wind jack so far, and gives a bit more surface area on ground.

All the more reason to drive a G-wagon......its high enough to lay under when not even on jack stands. :D

I think there is a special tool/attachment that is inserted on the hole that is used when jacking an MB with a non-MB jack. I remember seing it in the MBbraingear website. I can't remember the part # though. Has anyone used or saw it?

I do the same exact thing. In fact, I shake it quite vigorously. If its going to fall, I want it to fall then and not on me.

Such a simple test could save your life. It almost seems stupid not to do it. (not trying to insult anyone here)

Scott

p.s. If you live in California you most definitely ought to do it.

I always wonder what would happen while im under there during a nice earthquake.

Oh well, at least i'd be doing something I like.

I could think of other things I'd rather be doing though... :)

While shaking the car helps to verify it, it is not foolproof, and neither are the ratchet type stands. I do agree they are a bit more solid in design, however I had a close call that tested my faith in them. I have always been somewhat of a believer in overkill with supporting cars-which ended up being my saving grace. I was helping a friend part out an old 70's Ranchero, he wanted the rear end pumpkin out of it, I had the car up, had shaken it quite well, stands under the axle by each wheel on solid concrete. Good to go? I thought so anyway, being my typical self though, I did my usual just in case move of placing the jack back under the diff housing and bringing it up to contact. Crawled under, shaking and banging getting those nuts off the rusty studs, cursing, got a piece of pipe to smack the gasket loose and BANG! the whole car shifted with a good thunk. I flew out and assessed what happened, the jack had caught the car and saved me, one of the stands had failed, there was a roll pin in the handle part that held the ratcheting pawl in place to the handle, that pin had sheared, letting the lever fall down & look like the pawl was fully seated, and it had actually only caught on the tip of it and had finally slipped. That stand was retired from service and its mate was thoroughly inspected (they were not harbor freight cheapies either). Point is, you can never be too careful, and constant inspection of things is your best defense against failures.

I also always keep the jack under the car as a back up safety device. You can never be too careful. Glad you weren't hurt.

Scott

I "third" that very sound advise.

Thank you so much, you all, for sharing your wisdom here...
So, to recap your suggestions:

1) To lift front: floor jack under the engine cross member just behind the oil pan.
To lift rear: floor jack under the differential (or, optionally, under the subframe
mount area). (Are we positively sure the FSM allows for using the differential
this way on a 123-chassis 240D? Could someone please check that...)
Always place buffering in the form of rubber or wood block (2x4) between the
floor jack's saddle and the car. QUESTION: Do you secure the wood to the
jack's saddle in some way of just place it on it? It would seem to me that, if
not secured properly to the saddle, chances are the wood might slip during
the lifting, when things are likely to move a bit around...

2) Place jack stands under the frame box just inside of the jack holes (front set of
holes when lifting front, rear set of holes when lifting rear), buffering the
contact areas with wood (like a 1'-long piece of 2x4), unless you have a
rust corrosion problem in that area. AWAYS test for any instability by shaking
the car hard before getting underneath! ALWAYS keep the floor jack under the
car in light contact with the frame at the lifting point as a back-up safety
device! The manual for the jack stands I bought warns strongly against using
the jack stands to lift just the left or right side of the car, and says to ALWAYS
use the jack stands in pair to lift EITHER the rear OR the front side of the car. Do
you second that? QUESTION: It occurs to me that a car placed on jack stands
this way causes the contact area between the frame of the car and the saddles
of the stands to be at an angle, therefore not firmly seated on them. Wouldn't
it make more sense to place some wood (or use other means of lifting) under
the tires on the opposite side of the car, so that the car now sits level on the
jack stand saddles and not at an angle?

Also:

Sure, but when you place the rear on jack stands, putting the car in gear and using the parking brake doesn't do any good... The only thing that can be done in that case is chocking the front tires (I have a set of 4" wooden ramps, with chocks nailed to them, that I plan to use under the front tires when putting the rear end on jack stands (and viceversa). That will also raise the front (or rear) end in such a way as to provide a flat (level) contact area between the saddles of the stands and the car.)

Raising a vehicle is an excellent situation for doing things by the book. EVERY two post lift ever made has instructions exactly like that. You are supposed to raise the car clear of the floor and give it a shake. Same goes for jack stands.

I spent lots of time in my Dad's Independent Garage in the fifties in Killeen. That was when I was from about the age of five to about 10. He made the rules about going near a car that was lifted in the air QUITE clear. As a result of these instructions and him practicing what he preached, we are both still alive with no missing limbs.

It sounds like R Leo is trying to fill in some of the gap of no longer having Deezel, the self appointed Safety Officer. Keep it up!

If your in cali just stick to ramps!

For the jackstands I'll stick with metal on metal.

Rancheros are evil...
 

I also had a bad run in with a Ranchero-1971. I thought I had learned my lesson when I dropped a 1962 crew cab VW truck off a skinny little floor jack onto my nose (wheels were on thank God) when I was 18. Well, 16 years later I had to learn the lesson again. I'll leave the story at trying to pull a transmission out and using non-cool (ok, concrete block, DOH, idiot, I knew better) materials on top of ramps to get a little extra clearance. Luckily I had my craftsman tool box (4 drawer Rally box, all metal) under there. I had pulled the pan on the tranny, disconnected the linkage and I wiggled it-and hit neutral. No parking brake on, car rolled...When the car fell, it landed on my box; leaving just enough room for my head. A neighbor jumped the fence and got the car off me with the floor jack. I still get flashbacks thinking about it. Now I'm back to my paranoid ways. I always slide the unbolted rims under the car at various points. Big huge jack stands. Leaving the jack in place. Parking brake on even if all 4 wheels up. Cell phone in pocket in hopes that I could use it if something did happen. I grab the wheelwell and try to knock the car over before I get under it! Don't get in a hurry like I did and forget everything you know you should do.

p.s.- banged that toolbox out with a hammer and it still works ok as a backup. Nope, won't tell Sears to make a commerical about it- don't want to advertise my stupidity! Rancheros and El Caminos are cool but evil...

~Sugi

just an extra bit of safety tip.
When lifting either end of a vehicle on rough or soft ground that will not allow the jack to roll, DO NOT CHOCK THE WHEELS or set the parking brake until AFTER the vehicle is raised!
Floor jacks need to move as much as 12" when raising, so either the jack needs to move, or the car.
I always try to have the jack on smooth level surface but it is not always possible.
also, I would not use the jack points to hold up the car, I would use the frame itself.
most floor jacks have 4 sharp points that will grip well into wood.
if jackstands are sitting at an angle, the car will slide and fall. try to get the stands so solid metal is in the saddle of the stand keeping anything from moving.
one time I was working at a used car place and in the corner of the shop was a twin post lift with a car 1/2 on the lift. the thing was saftey taped like a police scene waiting for the insurance inspector to come look. nobody went near that thing... rubber pads on metal arms mix in some tranny fluid, wait for disaster.

Don't we all have the hydraulic jack on wheels?

AH! AH! Guys, you should say these things right away! Now I understand a couple of things I experienced over the past couple of days with my brand new floor jack and stands, and also understand that I got pretty close to screwing up in a big way...
The floor jack I have is small (I wanted it that way) and it is a Craftsman (so I don't think it is a POS), BUT it did NOT roll as you say it should have, and it was instead the car to slide around on it during lift up and lift down... And I did put a nice piece of rubber between the car's metal and the jack's saddle, and yes, the rubber did get oily with diff fluid and engine oil (at differential and engine cross member lift points) and yes, I had the car in reverse gear and parking brake set and even chocks into place while using the floor jack... Yesterday in particular there was a moment the car shifted around while I was lowering it on the jack stands and the saddle contact point slided from center differential to left corner of differential, with part of the saddle protruding out. Had it fallen down, there would certainly had been some damage to the car. After that close call, I've been extremely careful with the procedure, lowering it all the way whenever I detect some shifting and then lifting it again....
I am glad you wrote this last post, which clarified a few important things. So no more rubber material between saddle and car for me from now on... Are you saying that wood is all right? For instance, is it all right to place a 1-foot length of 2x4 between the saddle and the diff or engine cross member?
What do you exactly mean by not using the jack points but the frame itself?
Where exactly do you place your floor jack when lifting rear and front?
When lifting the rear, I place the stands right under the metal thing with a bolt and springs in proximity of the rear jack holes, so the saddle of the stands is grasping the metal in that area, it fits perfectly by the way. But when lifting the front, there's no metal anywhere near the front jack holes, just that frame that looks like plastic... so I place the stands right there, underneath the jack holes, and buffer it by placing in between saddles and frame a 1-foot length of 2x4. Is this all right in your book or am I doing something risky there? I have done it that way a couple of times and it seems to work just fine...

Your post is much appreciated...

Rino

I assume we all do... mine has them as well... but in spite of the wheels did not roll... at all. Rather the car on top of it did... And I am in Cali. All I needed was a nice, little quake at the time of my inspections nearby the car....

So, either I've been doing things wrong (and I assume I did), or the floor jack I got is a POS...

Rino

By the way, vstech, another thing...
I don't seem to be able to use the floor jack longitudinally with the car (if I do, then there's not enough space to pump the handle)... so I've been using the jack (under the diff and ECM) at an angle... There's no other way to do it with this jack...

Rino

I met one guy that survived the fall of the car, but he was a quad.

keep cell phone or other alerting device attached to body

oops, I hate it when a thread I gave info on, had followup questions and I miss them..
sorry rino.
anyway, the jack should be solid on the diff in the rear. if your jack will not reach the dif, you need a longer jack. if you are just changing a tire, using the jack points with wood as a buffer should be fine. but it is inadequate to support the vehicle to do major repairs.
on the front, I jack up with the crossmember solid on the floor jack. and use the lower arm mount to support the jackstands.
to get the jack to slide, you need SMOOTH surface under the jack, or solid wood gripped into the jack surface and wedged solid into frame, if the jack face will not grip solid onto the crossmember/diff, and the vehicle must be free to move as the jack lifts.
John

The pin part is actually very safe... the amount of weight it would take to shear off one of those steel pins is incredible. As for the rest of the design... well, you may have a point.

ATT: Dave Morrison
 

Hi Dave, I have no idea if you get to read this, but I sure hope so.
It was time that I got a new pair of jacks, so I followed your advice and
yesterday bought the 6-ton jacks from Harbor Freight.
What now puzzles me is that these jacks' minimum height is 15-1/8" and
the subframe mount area on my 240D is only about 10" from the ground.
Using the 6-ton stands would therefore position the car at a pretty steep
incline... and it appears the load placed on the jack stands should be as
vertical as possible to the jacks for safety reasons, as prescribed by the
manual that came with them.

Can you give me some feedback on this one? From your picture from
that post http://www.peachparts.com/shopforum/attachments/diesel-discussion/25842d1114794674-617-engine-replacing-oil-separator-check-valve-upper-oil-pan-oil-seperator-fix-6.jpg
it looks like you used an even greater height than 15-1/8", assuming that,
as you said, those are the same 6-ton jacks from Harbor Freight that I have...

(With the 2-1/4 ton Craftsman jacks from Sears that I've been using
so far, I've always placed solid wood under the tires on the opposite
end, so as to lift the car about 3-4" from the ground, and then the jacks
under the subframe, with the car's subframe evenly lifted that
way (back and front ends of the car evenly lifted) at about 13.5" from
the ground. That ensured that the load was completely perpendicular to
the jacks, as I understand that to be an important requirement.)

Also, from your picture I can see you did not place anything between
the vehicle subframe and the posts saddles (I was told to always use
something soft in between, such as a short length of 2x4, so as to
avoid possible slippage). What's the right way to do it, metal to metal
or with something soft in between? A clarification here would be
appreciated.

The big question for me however is, is it safe to work under a car
positioned at such an incline?? (I'll be placing the jack stands under
the subframe box, near the jack holes provided by Mercedes.)
Or should I return the 6-ton jacks and get smaller 3-ton ones with
a lower minimum height?

TIA,
Rino

Rino

A couple of opinions. I have always placed the jack under the frame box on the W123 and W107. I have not used a 2X4 or any other "pad" between the jack and frame, the wood can split and cause the car to shift. A hard rubber pad would be better. Harbor freight makes them for the 6 ton jack. http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95952
I have not bought them because I am comfortable with what I am doing, And I will spend money at Harbor Freight ( can't get out of that store without dropping $100 ) Make sure the frame box is centered over the jacks stand pad. I make contact on both sides of the pad at the 2 edges of the boxframe. I jack the car up raise the jack to just below the boxframe and slide it towards the rear allowing it to make contact with the boxframe at the two pad spots. Do the same think on the other side but make sure the 2nd jackstand is raised the same amount of clicks as the other one. This way your car should be level ( left to right ). Lower the car on the jackstands and give it a shake so you can see it's stability on the stands. If it does not seem stable , move the jackstands to get it more stable.

I do make sure that the transmission parking pawl has engaged when I stop the car, engage the parking brake and I park on a flat, hard surface.
the car will sit at a angle on the jackstands. The amount shown in the picture allows me to work under the car easily but I am not uncomfortable with the angle the car is sitting at. I am just working on the engine. If I have to work under the complete car I use 6 ton jacks ( I edited this to change the size to 6 ton jacks )and jack the car up pretty high in 2 stages. I also have the HF long body jack that can raise the car 33 inches. As I said I do this in 2 stages.

If your uncomfortable with just the 2 jacks. I sometimes leave the raising jack under the engine crossframe instead of removing it after jacking. Drop the car down on the jack stands and allow the raising jack to just kiss the frame. Now your supporting the car in 3 places. Or place another 2 jacks near the 2 installed but just barely touching the frame, a sort of backup.

Always make sure you chock your front wheels when jacking just the rear of the car. Good chocks, not 2X4's.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=96479
It will roll on you when jacking and be unstable when on the jack stands.

Good luck and be careful.

Dave

Well, if you say you've always done that and that the inclination thus achieved is no big deal as far as stability goes, provided that good chocks are used, I'll go ahead and give it a try the way you do it. I was also a bit concerned the direct contact of the jacks's saddles with the frame box might damage the latter, but if you've been doing it all along with no ill effects... then I'll do the same thing myself. I won't even have to raise the jack's posts at all, since 15-1/8" (the height of the 6-ton jacks with the posts all the way down) is more height than I have ever used and all that I need. 12-ton jacks as you say are definitely very safe, but their minimum height is 19-1/2" and I don't see how you can raise an end of the car that high without also raising the opposite end (right?), and that quite complicates things a lot, IMHO.

Yep, the same thing happens to me too each and every time when shopping at Harbor Freight. I went up there yesterday with the intention of getting just the 6-ton jack stands (they are currently on sale at $19.95) and perhaps a couple of additional low-cost items. I ended up buying a whole bunch of different things that I have no immediate need for for a total of slightly over $120. There's something *hypnotic* about HF that causes you to buy, buy, buy. The only good thing about it, is that I'd been HF-free for over 6 months...

Thanks a lot for all your tips and advice. I'll keep the 6-ton ones and give the things a try when my oil change becomes due at the beginning of February. These jack stands are H-U-G-E compared to the Craftsman 2-1/4 ton ones, and look at least 10 times as sturdy. I am glad you suggested 'em and that I finally got 'em.

Just please confirm this once more for me: on the w123, when, for instance, you raise the front end, you place the HF 6-ton jacks (which have a minimum height of 15-1/8") under the frame box right where the MB jack holes are with the rear wheels of the car on the floor. Is this correct?

Thanks again,

Rino

I edited my post to correct the size jacks I have.
The picture your looking at on the post showing my car jacked up are 3 ton jacks.

When I really want the car high I use the 6 ton jacks. After reading your dimensions on you last post I measured mine and checked them. I have the 6 ton jacks not the 12 ton.
I do jack the car up and place them on the 6 ton jacks and allow it to sit at the low position angle without any problems. If I want the 6 ton jacks higher I do the lifting in a 2 stage lift. Front up to the lowest jack stand position. Then the rear to the height I want and then do the front to the same height as the rear. Check for stability and then get to work.

Sorry for the confusion. Had 3 real long days of flying so I was pretty tired.

Dave

Thanks for clarifying, Dave. Those could not have been 6-ton jacks with the posts raised as shown in the picture and with the other end of the car resting on its wheels...
One important thing to keep in mind (and which I did not fully realize last year after getting my first jack stands) is that the max. capacity given by the manufacturers always refers to that weight evenly distributed PER PAIR of jack stands - therefore, each "3-ton" jack has a max. capacity of only 1.5 tons; and 6-ton ones, are really good for only 3 tons each, and so on.

I'm curious, when you do the two-stage lift and have the front end of the car sitting on a pair of 6-ton jacks at the lowest post position, how much do you feel the other end of the car can be safely raised above that level (with the car eventually sitting on four jack stand at an incline, though only temporarily)?

Rino

When I do the two stage lift I usually raise the 6 ton jacks about 2 clicks and then jack the car up to settle on that height, for the first stage. for the second stage I jack the rear on the differential about 25 inches. Install the rear jacks allowing the car to settle down to the closest jack stand height that will accommodate the height of the car on the lifting jack. Then I raise the front, adjust the jack stands to the exact height of the rear jacks stands and settle the car onto all 4 jack stands. I notice how the car is sitting on the jack stand pad and if I'm not happy with it I will reposition the car on the jack stand. Actually I jack up the car and move the stand so that it will allow the car to better sit on the jack.

Now be advised I own the Harbor Freight long body jack. A standard jack will not fit under the rear of the car all the way to the differential.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=42820

I was just under my daughters 240D replacing the speedometer cable ( it was broken) and had the 6 ton jacks on click #2. To tight working under the rear of the tranny so I jacked it up some more raised the jack stands another 2 clicks and was able to finish the job. It sat at that angel without any problems.

Dave

Dave

When I'm rotating the tires (every oil change) - admittedly, I'm not UNDER the car for a particularly long time except for the drain plug - I jack up one side or the other at the front, with a floor jack and a loose wood block (2x4) right under the factory jacking hole - where the rubber jack pads would be if they hadn't fallen off or crumbled. Then, I put a jack stand under the front... well idk what it is, just inside from the wheel, maybe the spring perch? Something stable in that general area. Nice flat surface in the same place on both sides. Then I go around, put the other side of the front up the same way. Then, I jack the back by the differential (NO block of wood here, it fits perfectly on the little jack plate anyway) and generally leave it up just like that while I rotate the tires since i'm not actually under the car during that process anyway.

It's never even tried to let go that way.

**EDIT: It probably isn't the spring perch. But it's whatever that nice flat surface that fits the metal jack stands so nicely is - the jack stand probably sits three or four inches towards the center of the car from the wheel's inside rim.

Thanks for the explanation!

So in the case of the 240D you had the saddles (6-ton jacks) raised at about 18" (4 clicks) off the ground, with the front wheels on the floor... And that with the jacks saddles right under the frame box (near the MB jack holes), correct? Wow, that way the car is positioned at a pretty steep inclination...
Even if you place a set of good chocks under the front wheels, I don't know how safe I'd feel working under a car with the rear or front end raised that high. But then, again, I don't have much experience with this type of scenario, and I assume that once you start playing around with it then confidence follows.

I'll give my new 6-ton jacks a try in a week or so... I can't wait...

Rino

Rear wheels on the ground, with transmission pawl in place, parking brake on and wheel chocks. It's really not unsafe.

Dave

Wait a minute... As far as I understand it, the only things holding the car steady
in the instance you described (while working on the speedometer cable with the
rear wheels off the ground) are the chocks, and the jacks' saddles gripping the
frame box at the points of contact.
Both the parking brake and the transmission act upon the rear wheels only, and
when the latter are off the ground... only the chocks and jack stands prevent the
car from moving forward.

Rino

Rear wheels are ON the ground, front wheels are off the ground

Dave

I read the whole thing again and, yes, YOU'RE RIGHT!

But suppose I go about changing the differential fluid and place
the 6-ton jacks, at the 2-click level (16-1/2" height), under the
frame box, with the back wheels up and the front wheels on the
ground, which should give me enough space under the rear end
to do the job...
Do you think it safe, in this case, to rely on just the chocks (and
and the jacks' saddles at points of contact) for stability?

Soon, I'll have to do that job too, and I'd definitely appreciate your
expert advice in that regard.

Thank you, Dave, you have been tremendously helpful.

Rino

http://i3.photobucket.com/albums/y74.../Photo0141.jpg

safe.. I was under it while doing the engine mounts.... do it all the time

buy a set of Mercedes wheel chocks

http://i3.photobucket.com/albums/y74...eelChock-C.jpg
http://i3.photobucket.com/albums/y74...eelChock-A.jpg