There's an old axiom that a transmission only performs as well as the engine does. Prudent service personnel should check for engine-related trouble whenever an automatic transmission behaves strangely.
My field experience has been that service managers and technicians may not instinctively think of the engine when a transmission isn't working normally. However, savvy trans specialists will attest that problems in the basic engine itself—not to mention its control system—could cause an automatic transmission to act up.
For instance, problems on the engine side of the powertrain may cause the transmission to shift erratically, shift harshly and/or shift abnormally late.
I used to hear stories about the engine-transmission "relationship" when I was turning wrenches, but I didn't learn the facts until the late 1970s when I began attending the transmission specialists' technical conferences. In particular, the members of the Automatic Transmission Rebuilders Association (ATRA) were—and continue to be—invaluable sources.
Indeed, the most common engines of that era were not yet computer-controlled. Nonetheless, fairly basic engine trouble such as low manifold vacuum or a restricted exhaust system could cause serious trans performance symptoms.
What's more, something I learned nearly 40 years ago still applies today—the engine's impact isn't the same across various makes and models.
Perhaps the reason many techs don't suspect engine trouble first is that they just don't encounter the condition every day. But when a technician least expects it, some kind of engine-related trouble may be the cause of abnormal trans operation. It's easy to be fooled.
Another reason some service personnel may be fooled is sweeping generalizations they have made about today's powertrain technology. Namely, these service writers and/or technicians believe that onboard computers faithfully set trouble codes whenever something is amiss in the vehicle.
Worse yet, they've convinced themselves that whatever trouble codes are stored will be both valid as well as crystal clear. (Surely, the trouble code topic is one for another time.)
But in many instances, an onboard computer doesn't set a trouble code until a severe, "hard" failure occurs.
For instance, the traditional format is that an automatic transmission shifts later—as well as firmer—when the engine's producing a great deal of power. Suppose you floor the gas pedal in order to accelerate quickly for some reason. Mashing the gas pedal opens the throttle, feeding the engine a huge gulp of air.
Meanwhile, sensors such as the mass air flow (MAF) sensor should recognize the increased air flow. A throttle sensor (TPS, APP) also should detect the rapid throttle opening. This kind of information should mean "heavy engine load" to a control computer. In turn, that computer should command much firmer, later shifts from the trans.
But suppose that critical engine sensors report inaccurate information to the engine and transmission computers.
For example, suppose the computer erroneously believes that the engine's under heavy load—but it is not. If so, then that oh-so-sophisticated transmission may shift noticeably later and harsher during normal driving—a very objectionable sensation for the driver.
What's more, errant signals from failing engine sensors may be abnormal enough to upset the transmission but not skewed enough to trigger appropriate trouble codes.
Key sensors such as the MAF, TPS, APP, etc., may affect automatic transmission operation as well as engine performance.
Consequently, techs should evaluate engine sensor signals before blaming the trans or its control computer for abnormal shifting. This also includes "fail-safe" situations where the trans computer temporarily limits trans operation to reverse and second gears.
When all is said and done, a solid-running engine is still the first priority for a smooth-operating vehicle.