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Auto Trans Dyno

The eddy current dynamometer that's not for your engine - it's for your automatic transmission...

By Michael Knowling

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We've all heard of engine and chassis dynos - but a trans dyno; huh, what's that? Well - don't be ashamed - it's not unusual to have never heard of a trans dyno! These things are very thin on the ground - as a matter of fact, this unit (belonging to Rowell and Searle Automatic Transmission) is one of only a relative few in Australia.

First of all, what is a trans dyno?

A trans dyno is - as the name implies - a dynamometer specifically developed for automatic transmissions. Here's how they work:

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First of all, a conventional piston engine (in this case, a Holden 'red' six hidden behind the green mesh screen) is used as the controlled power source. The test automatic transmission - complete with its torque converter - is then linked to the back of the motor using a heavy gauge adapter drive plate. These adapter plates often have an array of bolt holes in them so that they match the bell housings of numerous different transmissions. Heavy-duty metal clamps - like in these photos - can also be used to add strength.

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Heading out from the output end of the transmission, a cut-to-length tailshaft (or an axially flexible shaft as seen here) takes drive into an electric retarder unit. These work just like the retarder on an eddy current chassis dyno. Put simply, the retarder is used to "brake" the tailshaft and increase load through the driveline. An in-built pump (complete with its own filter) circulates the essential transmission fluid.

That's rear wheel drive transmissions - but what about FWD or constant 4WD tranaxles? These too can be installed using another separate retarder unit that couples onto the transmission's second parallel output shaft (ie you need to "brake" two driveshafts instead of one). Note that this hardware has not yet been fitted to this particular dyno. The majority of all wheel-drive transmissions, on the other hand, can tested by simply hooking up the rear tailshaft into the single retarder unit, and letting the front-drive section of the trans "free-wheel".

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The control system for each type of trans dyno will vary slightly from one example to another. However, this Australian-made custom unit uses a carburetted piston engine as its power source, so - of course - it's got controls for ignition, choke and throttle and some bare-bones gauges for water temperature, oil pressure and the like. The other half of the control unit is linked to the electronic retarder. This can be switched entirely on or off, or progressively controlled to increase the load through the driveline.

What can a trans dyno be used for?

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Now a trans dyno won't give you a power and torque reading like an engine or chassis dyno - what it does give is the ability to test transmission operation. Without a doubt, the primary use of a trans dyno is for making diagnoses and pre-delivery checks.

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Used in conjunction with an electronic transmission testing module - such as this TranX 2000 - the trans dyno operator has the ability to fully take control of the transmission. By tapping into the transmission's wiring harness, it's possible to test shift solenoids, pressure control solenoids, force motors, computer system operation and the harness itself. Note that devices such as the TranX 200 can also be used on-road.

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Over and above the functions of the TranX module, the dyno operator has the ability to perform other tests. These are conducted using this board of gauges and by watching the transmission's output shaft. Some typical examples of test that can be done include:

  • The stall rpm of a torque converter can be determined by applying the retarder and revving the engine from idle until the tailshaft starts to spin.
  • Transmission slippage can be identified when the engine is revved and the tailshaft rpm don't rise accordingly (once the torque converter has locked up, of course).
  • The kick-down function of a non-electronically controlled trans can be tested against the engine vacuum gauge.
  • An observant operator can also identify aftermarket modifications that have been aimed at achieving snappier gearshifts.
  • Of course, fluid leaks can also be quickly tracked down as well.

Can you just walk in off the street and have your auto trans tested?

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Rowell and Searle are primarily geared to running up fully re-con'd transmissions that they've built in-house (though, if you ask nicely, they will consider running up your own rebuilt trans). This reluctance to put secondhand or outside transmissions on the dyno comes about because a "foreign" trans can easily block up the dyno's fluid pump and filter system. And, with a new dyno replacement value of approximately AUS$70,000, they - understandably - don't want anything going wrong! However, the job of testing a transmission typically takes around 45 minutes to perform from start to finish (including installation), which means the cost of a test is reasonably minimal.

In summary, a trans dyno is an absolutely bullet-proof way of fault-finding or testing a newly rebuilt trans. It's certainly a lot cheaper than installing a new trans, only to have it later come out with terminal illness...

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Rowell and Searle Auto Transmission Pty Ltd
+61 8 8223 1844

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