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Rebuilding a transmission is one of the most expensive things that can be done on a car and I've been told it's one of the main reasons people dump cars other than timing belt issues. I've read conflicting reports of "when a transmission is going bad" and was wondering what one should look for / avoid when buying a high milage car with an automatic transmission.

I'm interested in two cases:

  • No vehicle history
  • An intentional attempt to hide the problem
  • How can you tell if a transmission is good? Coming out of a pull it yourself scrape yard? Or is it anyway you can test it to find out how good it is? – Adrian Dotson Jan 16 '17 at 18:46
  • @AdrianDotson The intention is when you are purchasing a whole vehicle to drive, not when looking for one in a junk yard. – Robert S. Barnes Jan 17 '17 at 11:22
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Obvious things to look for:

  • Dirty/burnt transmission fluid
  • Slippage when driving (can be hard to detect in an automatic, unless it's very pronounced)
  • Very clean transmission fluid (re: are they trying to hide something)
  • Check to see if pan/pan gasket looks new (re: are they trying to hide something). If no real pan gasket and has a drain plug, does this area look cleaner than the rest of the engine area?
  • While at a stand still and foot firmly on the brake, run the transmission (slowly) between reverse, neutral, and drive several times ... does it seem to shift easily (not by the gear shift, but by feeling what's going on)? Does it seem to be solid when it goes into gear? If it feels like it just sloppily goes into gear (long duration to get into gear), there may be an issue. This is a listen and feel procedure ... what are your senses telling you.

Sometimes the tranny fluid just needs to be changed. Just because the fluid is very clean doesn't mean there's an issue, it's just something to be wary of is all.

  • One thing I've read is that sometimes old dirty "ineffective" ( but not burnt ) ATF can cause slippage and that changing it and the filter can remedy the problem. What is the dividing line between probably a minor issue fixable with new ATF/Filter/+Addative and stay the heck away? – Robert S. Barnes Jul 7 '15 at 16:45
  • Weird whining noises mostly. I know precious little about automatics, but whining noises mean gears are doing things they're not supposed to. – Captain Kenpachi Jul 7 '15 at 17:31
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    @RobertS.Barnes Yes, sometimes bad/old ATF can cause some slippage, but in general if your ATF has gone that bad there's already internal damage from the slippage. Other suggestions would be for an automatic is full throttle acceleration. This will make slipping even more pronounced and cars in the last 10-15 years should detect the WOT and produce firm shifts. If it feels like a slush box it probably is, Additionally, engine break from about 40mph to around 10-15mph and then go full throttle. If you're looking at a car and they won't let you do these tests... walk away. – elmerfud Jul 7 '15 at 20:58
  • @elmerfud What should I look for when doing the engine break? – Robert S. Barnes Jan 3 '16 at 14:06
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    @RobertS.Barnes - I think the same thing ... firmness of shift and how the tranny reacts. You can imagine downshifting to get an engine (compression) brake going, then once the engine has slowed things down to 10-15mph nailing the throttle ... it should be very pronounced. Should almost give you whiplash. If it doesn't, or if you get strange noises, there's probably something going on with the tranny. – Pᴀᴜʟsᴛᴇʀ2 Jan 3 '16 at 14:10

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