Power is transmitted through an automatic gearbox via a series of planetary gearsets all joined together by clutch packs. If you’ve ever seen a wet multiplate clutch in a motorcycle that’s exactly what they look like. Instead of a cable to operate that clutch there are hydraulic cylinders which clamp and unclamp them. Due to the quite favorable design of the gearsets and the environment they operate in, they usually last a very long time.
The clutch packs wear over time because they slip some while both engaging and disengaging, and that’s happening every time the transmission shifts gear. The hydraulic cylinders that operate the clutches have a series of piston rings and rubber seals to maintain the pressure needed to clamp the clutch pack. Those will also wear over time from sliding up and down the cylinder bores.
It can be pretty hard to diagnose a malfunctioning transmission, especially the newer electronically-controlled ones, but there are some things to look for that you will not see in a healthy unit.
How to check the health of an automatic transmission 101
First off, start the vehicle and get it up to normal temperature. A ten minute test drive should do that. Stop and then cycle the shift lever through each of it’s ranges and be sure that it engages (the vehicle will move if you let off the brakes) within at most 5 seconds. A healthy transmission should give you a smooth but firm engagement within 1 or 2 seconds.
Next, try brake-torquing in ‘D’ by putting your left foot firmly on the brake and press on the gas pedal with your right. The engine rpm should come up to about 2,000 to 2,500rpm and then not climb any higher, regardless of how much more gas you give it. Don’t do this for more than 10 seconds at a time and let it rest a minute between attempts, it makes the transmission very hot. Needless to say you shouldn’t do this while parked right behind some gangster’s Mercedes.
Since the hydraulic pressure used to clamp the clutch plates varies with speed and load (to modulate the feel of the shift) a sign of a failing hydraulic circuit is a soft shift. Drive on a level road using light throttle to the speed where the transmission shifts from 2nd into 3rd. That should be about 60kph or so, just make a note of the speed. Then repeat the same run, but lift your foot slightly out of the gas just before the transmission was about to shift. The shift should be smooth, but quick. The engine revs should just drop quickly as the trans shifts and there should not be any rise in rpm or loss of momentum.
Now check the fluid. Park on level ground and leave the engine running for a few minutes with the shift lever in ‘P’. Pull the dipstick and look at the fluid. ATF should be clear and cherry red. Fresh ATF has a strong chemical smell from all the additives and detergents. It does not smell like motor oil or gasoline. Wipe some off with your finger and check it’s really clear, not frothy and has no floating particles in it. Wipe the stick and put it back to check the level. There will be two marks on the dipstick, low and high.
Froth: If the fluid level is too high it could be that some rotating assemblies are dipping into the fluid in the pan and whipping it up. Maybe draining some will fix this. It could also be that there’s a bad leak in the hydraulic system. You would want to know which it is.
Brown fluid: ATF doesn’t get polluted with carbon like engine oil because it doesn’t come into contact with combustion byproducts, but after a long time it will get contaminated with material from the clutch packs as they wear. It could be that everything is fine and the fluid just needs changing. It’s not a good sign that the owner is looking after the vehicle though.
Strawberry milkshake: Thank the seller for his time and go home. Opaque, milky fluid is contaminated with water (it’s emulsified) and the friction material is fixed to the clutch plates with water-soluble glue. Guess what happens next…
Now, the thing with brown fluid is that it can be completely benign. It’s just old and a little dirty and changing it out will make the trans almost as good as new. ATF has some very powerful detergents and seal-swelling agents in it which are used to keep the hydraulic circuits squeaky clean and the rubber seals plump and soft. As the fluid gets old this breaks down and it no longer does such a stellar job of housekeeping. Sludges and varnishes build up in odd places and start to block the maze of passageways, restricting the flow of fluid and possibly causing problems like slow shifts or lazy shifts. In one other way this may actually be a good thing. As the piston rings and rubber seals get old they wear and also harden. This can be offset with the build-up of sludge on their mating surfaces and the end result is that the seal holds pressure even though it’s basically worn out. What happens when you suddenly swap out the old brown crud for fresh ATF with it’s powerful additives? Well, it can be that in an instant the sludge gets washed away by the agressive new fluid, opening a gap for the pressure to leak through. Simultaneously the rubber seals start to swell from those other additives, but it’s too late for these old and now brittle seals. Instead of swelling to their original dimensions they develop cracks, and more pressure is lost. Next thing you know there is not enough line pressure to clamp the clutch packs properly and the result will be at best a slow, slipping shift (which will overheat the clutch plates and destroy them in short order) or at worst you’ll have no drive ranges available at all.
So, the important thing here is that if you inspect a high-mileage car with brown and dirty ATF, get the owner to agree to take the car for a fluid change before you agree to buy it. If the seller refuses, walk away. The trans could easily be hanging on to its life by a mixture of sludge and luck. If he agrees, try to be around while the fluid gets changed and see the pan come off the bottom of the pan and that the filter either gets properly cleaned or replaced. If you can, grok the filter and the bottom of the pan for shards of metal or bronze colored particles which indicate wear in the gears or bearings. There will be some dark silt in there from the clutch plates. As long as it is not magnetic or shiny it’s okay. Get a time-out of a few days for the fluid do it’s job of cleaning house (check the odo to be sure it’s been driven around), then repeat the driving test again checking for slow or slipping shifts. Any deterioration in shift quality from your previous test drive says the transmission will fail shortly.
A simple auto transmission for an older Japanese econobox should cost anything from 20 to 40k to replace. The most economical and safest route will be a used unit from Japan. You would pay a similar amount to have the transmission rebuilt here, but I’ve heard far more horror stories than otherwise. From bitter hands-on experience I can tell you it is really difficult to service an automatic trans successfully even when you have access to genuine parts and the proper tools. Everything in there has to be perfectly, spotlessly clean and that kind of environment is hard to find in an repair shop here, as I’m sure you’ve noticed. I’ve seen many trannies from supposedly factory-certified legitimate refurbishers fail within a few months, and they seldom give guarantees of more than a few months. Next best bet is to pull one from a smashed car in the scrapyard at maybe 10 to 30k depending how popular the car is/was. You’ll still get no guarantees, but the trans was at least working okay when the car was crashed.
Hope this helps. YMMV, caveat emptor etc. etc.