68oldscutlass

Ok so assuming I'm an idiot (which I am not) can someone tell me a step by step of changing the diff fluid on my 68 Cutlass...posi rear...2.73's. No drain plug. Fluid hasnt been changed in a minimum 15 years. I rebullt the whole car but never messed with differentials. Really more concerned about what I am putting back into it and how. Ive head like whale oil or something is an additive. Ok thanks as always..figured this is the last fluid in car and I am sure MUST be changed.

Tony72Cutlass'S'

MAKE SURE you can remove the fill plug before you break the cover. (fill plug is on the passenger side next to one of the fins on the pumpkin).

Else you're going to have to fill it through the vent hole on the passenger side.

68oldscutlass

Ok...fill it to the top? Recommend certain fluid? Additive?

droptopron

The fill plug is on the right side just above the axle tube. It isn't meant to be filled to the top so fill it to the drain plug. Any kind of gear lube should be fine but I've heard people recomend against synthetic in a diff with miles. You will need a Posi additive that any Parts store should be able to get. I got mine at NAPA.
You will need a gasket for the cover or you could try using just silicone.

TripDeuces

About 3/4 inch below the lip of the fill plug

jaunty75

I was always told you fill it until you can stick your finger in the fill hole, angled down once it's through the hole, and feel the fluid. Once you can feel the fluid on the tip of your finger, it's full enough.

I changed the differential fluid on a '64 Jetstar 88 some years ago. Just bought 90-weight gear old at the local auto parts store with a little pump that attaches to the where the cap on the container goes and pumped it in.

Mak

I'll be changing my diff fluid while my car is down for repairs (I got rear ended.)

<Thread jack>
I ordered that 72 LeSabre fuel sending unit you told me about some time ago... gonna have then drop it in while the tank it being repaired. I'm hoping I ordered the right one (although there doesn't seem to be any options)

Magna86

Yes before you do anything use a 3/8" ratchet and extention to get that fill plug removed from right side of housing. PB blaster is your friend. Also makesure to clean out the plug with a small screw driver as crud builds up there and the extention may not go in deep enough. Makesure to use straight direct pressure while trying to break free as the plug is alittle bigger than 3/8". Go to the parts store and buy 3qts of 75w-90 gear oil (or similar 80w-90) of your choosing and limited slip additive. I believe you can buy the Lucas Gear Oil (white bottle) and the limited slip is not needed. Read the bottle to be sure but I think I'm right. Also pick up a can of brake clean and a tube of Black RTV. (or if you prefer they may have the gasket but makesure its the correct one for your rear before you leave) Go home jackup the car on the frame and set jack stands. Set drain pan under pumpkin and remove the bolts but leave one loosened at the top. Take a flathead screw driver to begin to pry the cover away from the housing near the bottom to get the fluid to start to drain. Finish prying cover and remove the last bolt and let rear just drain into pan. Take a paper towel or rag and clean off as much oil as you can from cover and clean off with brake clean. Scrap off any remains of the gasket and set aside. Scrap the remains of the housing until clean. Before you spray out the housing makesure you have safety glasses on and are in a well ventilated area. If not could get brake clean in the eyes and the housing kicks the fumes back at you making you cough/choke. Put the car in neutral so you can spin the gears as you spray. I like to use a plastic spoon to help pull out the old oil/crud/cleaner that will sit in the bottom of the housing. Once you are satisfied its clean enough then use some papertowels to soak up the left overs that will not scrap out of bottom of housing. Take the RTV and make a 1/4" bead around the cover while rounding the bolt holes to the inside of the cover. Take the cover and gently set it into place centered so you don't smear the RTV while holding inplace install 2 bolts to secure it. One near top and bottom. Proceed by installing all the bolts and tightening. Remove fill plug if using posi additive put in first then follow with gear oil. Fill until gear oil starts to seep out of fill hole. Should be about 2 1/2qts. Install plug, lower car and enjoy.

jaunty75

Yes, I saw that in your other thread. Ouch. You have my sympathies.

You most likely got the right one. Remember that, if you don't have A/C, you'll have to plug the end of the unused line. I just put a rubber cap like the kind you cap a vacuum line with on it with a hose clamp. I've had mine installed for about 3.5 years now, and it's still working fine. Be aware that the gauge will wave at you more as you go around corners and up and down hills as the gas swishes around inside the tank. The damping provided by the can-arrangement on the original unit isn't there. But having a gauge that functions well enough to tell you how much fuel is in the tank is a heck of a lot better than having no gauge at all.


P.S. My car now looks a lot more like your car!

Buick64203

Be careful using the Lucas gear oil. Even though it says the additive is in the fluid, the posi chattered in my GS after putting it in. I put the additive in and it quieted down.

Fill it till it starts to come out of the fill hole and put the plug back in. I use 80w-90. The original API spec was GL-4. In a differential, you can use the upgraded spec of GL-5.

Just as an FYI- dont put the same 80w-90 GL-5 gear oil in any part that has "yellow" metal such as the brass synchros in your Muncie trans. GL-4 only! The GL-5 has additives that are corrosive to brass and copper.

Get yourself a suction gun to put the fluid in. Its available at any auto parts store.

Mak

Wow, your car looks great! (although I'm a little biased to red)

I tested my fuel gauge like you told me and with the wire at the rear bumper disconnected the gauge goes way past full (all the way to hot light!)... I'm hoping that's normal and the new sender will fix it.

jaunty75

That is normal and indicates an open circuit. Remember, the gauge is just a 90-ohm variable resistor with 0 ohms representing empty and 90 ohm representing full. Since an open circuit is infinite resistance, and infinity is way more than 90, the gauge behaves as an ohmmeter should and moves past the "90" mark, represented by the F, to as high as it can go.

The other test you should try is to ground the wire at the rear bumper that comes from the front of the car with the key on. The gauge should go to E. If it does this, then you know there is nothing wrong with the gauge or the wiring to it.

Mak

I did that, and it did go to E. I was just surprised how far past F it went with the circuit open. Does you gauge read fairly accurately with the LeSabre sending unit?

jaunty75

Yes, pretty much, but, like I say, it does wave around a bit. You'll get your best reading when the car is either stationary or has steadied out at a constant speed. I would also verify the actual readings for full and empty by filling the tank and seeing exactly where the gauge settles. Then go 200 miles or something like that, note where the gauge is, fill the tank, and see how many gallons you used.

The tank holds 25 gallons, but you'll never get the last roughly two gallons out as the pickup tube sticks down from the top and stops about 1/2-inch before the bottom. So that means that once the level of gas has dropped to 1/2 inch, the car will run out of gas. I did some math, and given the area of the bottom surface of the tank, a 1/2-inch depth of fluid comes to about 2 gallons in volume.

So you effectively have a 23 gallon tank. If you get, say, 12 miles to the gallon and go 200 miles, you'll have used 16.67 gallons, so you should have just over 1/4 tank left. Hopefully, your gauge will read about 1/4. But whatever it reads, you should just note what it is so you'll know what the gauge readings actually correspond to.

I've had to deal with a situation like this in my '73 wagon. When the tank is full, the gauge goes past F about as far as the distance between 3/4 and full, and when it gets close to empty, the gauge reads about 1/4. So everything is shifted one "unit" to the right. As long as I know that 3/4 on the gauge really means 1/2, 1/2 really means 1/4, and so on, I'm fine.

I do know that, for reasons unknown, the gas tank was replaced on the car back in 1981 when it had about 62,000 miles on it, which is about half what it has now. My guess is that the sending unit was replaced at the same time, and it was probably never "calibrated," if such a thing could be done. My guess is the gauge has been off in its readings like this since then. But hey, at least I have a gauge that I can use to tell me how much gas is in the tank!

Allan R

There is no drain plug on these old diffs.

Changing your diff fluid is stinky work. The gear oil smells baaaaad. Use your oil pan under the pumpkin to catch all the fluid. If you jack up the car to work on it, make sure the front and rear are the same heights to simulate it being on the ground. This is important for later when you're adding the gear oil.

-Loosen all the bolts holding the cover.
-Take out the lower ones first. You might have to use a paint scraper to separate the cover from the gasket. The cover is pretty heavy gage metal. Anyway that will let the gear oil drain into the pan in a more controlled fluid spill.
-Remove all the rest of the bolts when the fluid has pretty much drained - you'll see the stream change to just drips.
-Remove the diff cover.
-Inspect your carrier and gears and look for crud in the bottom of the pumpkin. Clean as required
-Scrape both the diff cover and pumpkin of any gasket material that may be still clinging to it.
-Install new gasket on pumpkin
-Reinstall diff cover and bolts
-Torque cover bolts to 25 ft/lbs
-Clean the web below the bung and spray area down with brake cleaner. This will get rid of crud that might otherwise get into the diff during fill.
-Remove fill bung (3/8" socket extension will do it) Here's a pic for you
fa186313-de28-4a88-81e1-37ce5b30f31c_zps38ad88a7.jpg

-Install 1 pint of 80/90 gear oil, then the limited slip additive. Add remaining gear oil till you can see or feel it oozing out the hole. I like to use my pinky to check when I'm getting close so there's not much oil spillage.
-Reinstall the bung and torque tight.

Clean pumpkin area with brake cleaner and you're done.

Don't use 70/90 in an old carrier. It's way to slippery for the clutches.

The fill plug will come out no problem. Chances are the fluid's been changed more than once.

re: vent tube. LOL! The vent tube has a spring loaded cap to release inside axle pressure. You'd never be able to fill it without busting it or totally removing it....

68oldscutlass

All done, thanks guys. Shudder is gone. Thinking of going up a step or 2 in gears. What's a fair price to expect a shop to charge if I purchase from and have them install?

jaunty75

What!? You can't talk about differential fluid here! We hijacked this thread and started talking about fuel gauges! The nerve!

Glad to hear you got it fixed.

Allan R

Depends a lot on the shop.
I upgraded from an open 2.73 to posi 3.42 last year. Original quote to do that was around 450.00 which I thought was on the high end. Ending up costing over 1100 by the time all said and done, plus they had the darn axle for 3 weeks!

Good guys to ask about this are
monzaz - Jim Mitschke http://www.jdrace.com/
507OLDS - Brian Trick thetrickfamily@yahoo.com