Basic Tranny/Differential Question

[98 'Bird]

Sometimes it seems like information on the most basic maintainence is hardest to find. Like... When is it necessary to change the rear diff. fluid? Owner's manual says nothing about it. Chilton says change it every 7,500 miles... clearly a misprint. My car has 56k miles. As for the trany, mine is an M5, the book says nothing at all. Chilton recommends that after 100k the tranny fluid should "probably" be changed . I'm nowhere near that, but bear in mind the car is 10 (almost 11) years old. Can someone set this stuff straight for me?

Thanks, all.

Matt

[MN6WS6]

Fluid maintenance intervals, regardless of the type, have several contributing factors. Oil changes are the simplest, as oil is by far the hottest fluid that is in a vehicle. It is nearly part of the combustion process, therefore it's temperature is always high, and is under the most severe continuous duty in the vehicle. Behind that I would say would be the transmission, then rear differential, brake fluid, coolant, and power steering fluid would be last.

I would not go any further than 60,000 miles on a complete transmission flush service, simply because transmission fluid is very susceptible to breakdown when subjected to high temperature. I just read a study on this, and transmission fluid that is just 20 degrees higher than normal operating temperature range wears twice as fast as if it were kept cool. That's why vehicles that tow a lot have a more rigid requirement on fluid exchanges as described in owners manuals. When that fluid breaks down it causes significant wear on transmission components in a short amount of time. Most shops will flush a transmission for around $130 or so, and that's a small price to pay compared to a $3,000 transmission.

I most assuredly would NOT use any solvent based cleaners in this service though, just a straight up remove old fluid and replace with equivalent new fluid. This eliminates using a solvent which could ruin transmission components. However, a drain and fill is not adequate, because all that will do is get 5-7 quarts out of a transmission which could hold as many as 12 quarts. The new fluid will break down more rapidly than normal because it is subjected to such a large amount of existing broken down fluid.

On an f-body rearend, I say 30,000 miles, and change out to a full synthetic 75w-140 fluid, as our rearends notoriously break, and this fluid is more likely to keep components cooler and lubricated even under extreme conditions. All you're doing is delaying the inevitable, but you might as well delay it as long as you can.

While you're at it, change out the brake fluid too. Brake fluid is under extreme pressure in ABS systems, and gets contaminated more quickly in high humidity environments. If the fluid is dark brown or black, it needs to be flushed out. I cannot begin to explain to you how much better the brakes work on my van after I had a brake flush done to it. You would not believe how much brake feel you are losing due to contaminated fluid.

[98 'Bird]

Fluid maintenance intervals, regardless of the type, have several contributing factors. Oil changes are the simplest, as oil is by far the hottest fluid that is in a vehicle. It is nearly part of the combustion process, therefore it's temperature is always high, and is under the most severe continuous duty in the vehicle. Behind that I would say would be the transmission, then rear differential, brake fluid, coolant, and power steering fluid would be last.

I would not go any further than 60,000 miles on a complete transmission flush service, simply because transmission fluid is very susceptible to breakdown when subjected to high temperature. I just read a study on this, and transmission fluid that is just 20 degrees higher than normal operating temperature range wears twice as fast as if it were kept cool. That's why vehicles that tow a lot have a more rigid requirement on fluid exchanges as described in owners manuals. When that fluid breaks down it causes significant wear on transmission components in a short amount of time. Most shops will flush a transmission for around $130 or so, and that's a small price to pay compared to a $3,000 transmission.

I most assuredly would NOT use any solvent based cleaners in this service though, just a straight up remove old fluid and replace with equivalent new fluid. This eliminates using a solvent which could ruin transmission components. However, a drain and fill is not adequate, because all that will do is get 5-7 quarts out of a transmission which could hold as many as 12 quarts. The new fluid will break down more rapidly than normal because it is subjected to such a large amount of existing broken down fluid.

On an f-body rearend, I say 30,000 miles, and change out to a full synthetic 75w-140 fluid, as our rearends notoriously break, and this fluid is more likely to keep components cooler and lubricated even under extreme conditions. All you're doing is delaying the inevitable, but you might as well delay it as long as you can.

While you're at it, change out the brake fluid too. Brake fluid is under extreme pressure in ABS systems, and gets contaminated more quickly in high humidity environments. If the fluid is dark brown or black, it needs to be flushed out. I cannot begin to explain to you how much better the brakes work on my van after I had a brake flush done to it. You would not believe how much brake feel you are losing due to contaminated fluid.

Thanks for all the advice. I do think, however, that what you are saying about the transmission fluid pertains much more to automatics. Although our manual transmission use regular ATF (3.4 quarts for the 5-speed), as I see it, it's not so much about fluid degradation, but rather, contamination from internal transmission wear from normal driving. This is contingent on the fact that fluid in manual transmissions is used ONLY for the purpose of lubrication, whereas in an automatic it is used to transmit power via the torque converter. In short, transmission fluid is subject to much greater pressure and temperature in and automatic than in a manual and will degrade much faster in an auto. I'm sure there's a time factor involved as well.

Also I probably should've mentioned that I don't have a posi rear (which I know requires more frequent changes).

Any thoughts?

[Wild Willy]

Both miles and years- if the manufacturer has a higher service interval for 'severe service' most people should use that- much average driving is really considered severe service- things like short trips without warming up, or stop-and-go driving-

I try to go 3 years or 50,000 miles on axles and auto trannys, and have had very good results. The poster that commented the brake fluid was spot-on, too- for $10 worth of brake fluid and a half hour of twisting wrenches, you can have a firm, new-feeling brake pedal.

[ConElite]

Yeah When I got my car, I went to Flush out the brake fluid. My guess is it had never been flushed b/c that stuff was Green. It was horrible. After it felt a million times more firm. Its been about 1.5 years since I changed it, and about 2 years since I changed the Diff. fluid. Im going to get myself a TA Diff. cover and change that out too while im in there.