RetroGuy

I just did a differential fluid change on my '96 C4 automatic. I drained (as much as I could siphon out) and replaced my differential fluid with "Mobil 1 synthetic gear lube LS 75-90W." It states it has the limited-slip additive needed for our C4s. The old fluid looked rather dark. Here's a few tips for the DIY'er: If you don't have a full lift, jack up each rear side and place a jack stand under the right and left rear supports near wheels. I only raised it enough to access comfortably while on my back. The fill / drain plug is on the right rear side of the differential. Nothing has to be removed to access it. I used a long handle 3/8" allen wrench to remove the plug. It was really tight, but a rubber mallet on the allen wrench helped. As soon as the plug was taken out, some fluid started to flow out. I used a "Multi-use transfer pump" purchased for around $7 at Harbor Freight, to siphon out as much of the old fluid as possible. Looked like I could only get approx 1 quart out. I wiggled the siphon tube until I got it to snake its way down to the bottom of the differential case. One can "step-down" the diameter size of the siphon tubing and it would make insertion into case easier. After extracting the old fluid, I pumped in the new Mobil 1 until it overflowed out of the fill / drain hole. I continued to pump the new fluid in until I was purposely overflowing the differential case to hopefully create a flushing effect. I used two 1 qt bottles to achieve this. Yes, I know I was wasting good fluid. After overflowing the new fluid, I waited for the outgoing flow to reached the same slow flow rate that it initially had after initially removing the drain / fill plug, (educated visual guess). I replaced the plug and wiped down everything. So far so good!

Matt81

Same process i used on rejuvenating the 35th Anniversary with a complete fluid change .It was hard to tell when the siphon Bose was all the way at the bottom. As you said, had to really shake it around. Those fluid pumps sure make it a lot easier!

Whaleman

I also used the Harbor freight transfer pump to get most of the fluid out. This may be extreme but I then taped a 1/4" 4 foot section of clear tubing to the hose of my old shop vac. I then proceeded to spend the next half hour sucking the rest of the fluid out. The 1/4" tube is easier to move around than the transfer pump hose. With the clear tube you can see if fluid is being sucked out or not. I am retired so time does not matter to me. Dan

drcook

I used the same pump. My driveway has a slight incline down from the road, so I pull the car up on ramps. Fortunate for me, the angle is such that it keeps the car level. It is perfect for changing oil, diff oil, etc.

RetroGuy

Hope your shop vac didn't suck out any vital gear parts I'm sure you were able to get more old fluid out than I did.

jim2527

Drain and refill again after driving a few miles. You'll be amazed at how dirty the fluid is.

Whaleman

It just amazes me that Chevy did not put in a drain plug. Seriously, how much could it have cost? Dan

Sleazy Rider

one of the first things i did was, when i pulled the rear out i broke it apart and drilled and tapped the lower portion for a 1/4 npt plug. now changing fluid is a lot easier and can be done more frequently.

Zrxmax

I think Mid America makes a drain plug kit. They show you the precise place to drill the rear cover with a 1/4 hole I think. Then install a tapered allen screw with teflon tape on it and your good to go. On a Dana 44 the lowest part of the diff is just to the left of center.

If you really get **** you can drill the end of the bottom bolt hole inside the case and make the bottom center bolt a drain plug at the same time.

Bomvette

belairbrian

I guess I'm lazy. I have a 12v fluid pump that uses 1/4" tubing. Easy to get to the bottom of the differential and all I have to do is turn it on.

RetroGuy

NOT lazy...efficient.

Zrxmax

A 1/4 hole isn't going to compromise the strength of the rear cover.

2LZ

Glad I ran across this. Thanks RetroGuy! I too have a '96 LT1 A4.
I just had my 3.54's installed about 400 miles ago and at 500 after a well-mannered break-in, I'm supposed to change the fluid. All my questions are now answered! :-)

Shock96

I have been using a 60cc catheter tip syringe for jobs like this. Works like a champ!

ctmccloskey

I use a vacuum and pull the fluid out of the differential. I saw the guys at Jiffy Lube changing the differential fluid in a SUV and that is what they used, not that Jiffy Lube does things "right" all the time.
Learning to make vacuum molded parts in my garage so I have a large vacuum pump that pulls the excess epoxy out of the molds. This system uses a large container for the excess epoxy or the thick and nasty differential fluid. The container I use is a old gallon paint can with two fittings on the top. The vacuum is on one of the fittings and the other fitting is for the hose coming from the mold or in this case the differential. I have a shut-off on the "in" side of the can, this way I can shut off the vacuum and then move the pickup hose to where I want it and then turn on the vacuum. The oil is gone very quickly this way and the tools are used a variety of applications in my garage. This system with the container in between makes it very easy to drain the oil and is fairly neat at the same time.

BelairBryan is on the right track, why drill and tap a hole when there are already ways to do it effectively and completely? I would certainly use the electric pump idea before drilling holes in my differential. "Keep it Simple,Silly" They make 12 Vdc oil transfer pumps to allow you to change the oil on Boats where the engine is not always in the best location. They are available on Amazon from $40 and readily available and very easy to use.

Before you even start with this mess be sure to have the Posi-additive ready as well as "quality" differential oil for when you re-fill the differential. If you keep it clean while doing the job your car won't wreak of 90 weight for weeks to come!

Best Wishes for a Complete and Neat oil change in your Differential!

turbobuick33

Does royal purple diff fluid have the additive in it?

RetroGuy

I know that "Mobil 1 synthetic gear lube LS 75-90W" has the additive already in it. I bought it at Pep Boys.

turbobuick33

How many quarts does it take to fill up the diff?

RetroGuy

Your owners manual states the capacity level for rear differential fluid / oil. It depends on how much you're able to drain/pump out. I was able to pump in a little over a quart before it started to overflow out the drain/fill hole. So, I bought two 1-qt bottles to make certain I had enough...and also did my "overflow flushing technique" as stated in my first post.