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Tackling CVTs

Wednesday, July 1, 2015 - 06:00
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The acronym for “continuously variable transmission” is CVT. Amusingly enough; there are many other titles that use the CVT acronym. For those who have not had a good experience trying to repair a CVT might relate to “Center for Victims of Torture.” Perhaps the bad experience provoked the thought of a career change to become a “Certified Veterinary Technician.” If one isn’t so easily discouraged, “Continuing Vocational Training” may be an option. One thing is for sure, although continuously variable transmissions are the transmission of choice for hybrid vehicles, it doesn’t make it a “Current to Voltage Transmitter” or a “Constant Voltage Transformer.” OK, so I am being a bit of a “Chemical Vapor Transport” (full of hot air that is) for this article’s opening paragraph pertaining to CVT transmissions. No doubt that “clear valid training” is needed to diagnose and repair these transmissions as many shops are now beginning to want to make that choice.

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The M4VA is the first experience I ever had with a CVT used in a Honda Civic HX and to this day the engineering design is like no other. Simply explained, a dual mass flywheel drives the input shaft rather than a torque converter (Figure 1). This input shaft delivers power to the primary pulley via a forward or reverse clutch through a planetary gear set. This is not the unique aspect of the transmission. What makes it unique is the use of a start clutch strategically positioned between the pulley assembly and the differential (Figure 2). This one clutch pack connects or disconnects the pulley ratios to the differential. Quite an ingenious design as it allows the pulleys to stay in rotation should a rapid high to low ratio change be needed with sudden braking maneuvers. Among other things it also is the decouple device when coming to a complete stop preventing engine stall.  What is impressive is that with the excessive release and apply of this clutch, one would think it would fail sooner than when it really does. Good clutch cooling control and clean programming are major factors that keep this start clutch operational as long as it does.

This transmission was an excellent transmission to begin learning CVT diagnostics and repair. Even though it functions differently than others with the use of the start clutch, it provided all the fundamentals. One aspect of this transmission that made it good for learning diagnostics is that it has all the pressure taps you need (Figure 3). All of them are identified with abbreviated embossments in the case next to the tap itself (Figure 4); forward clutch (FWD), reverse clutch (RVS), start clutch (SC), drive pulley (DR), driven pulley (DN) and lube pressure (LUB). 

Checking CVT system pressures are best done using transducers like those from Automotive Test Solutions (Figure 5). CVT pressures can be very high. Some reach 800 psi and greater. Nothing can be more dangerous and unpleasant than to have a pressure line blow out while diagnosing a vehicle on a road test. Not to mention the graph transducers can produce offer keener data for diagnostics. 

Ironically, with each and every pressure tap being available with a Honda CVT, the need to pressure test doesn’t come up too often. This is due to its unique operational characteristics. Consider how it operates. With the start clutch releasing at a stop and applying when it needs to accelerate, the forward or reverse clutch remains applied depending on which range was selected. If there was a delay when Drive was selected and slipped moving forward yet reverse operates perfectly, the forward clutch would be suspect. This symptom would also suggest that the start clutch and the pulley system are working correctly. Conversely, if reverse was the problem yet all was well with forward the reverse clutch would be suspect. But, should there be a problem with forward and reverse engagements and slipped throughout, this opens up many possibilities beginning with the filter to fluid level, pressure control, start clutch or pulley problems. The condition and level of the fluid would be the first check. It wouldn’t take much to drop the pan so it may be the second check to see if it’s loaded with metal. If the pan check doesn’t reveal serious damage, pressure testing of the start clutch and pulleys could then be considered. If only delayed forward and reverse engagements were the symptoms yet drove reasonably well once in gear, the start clutch would be suspect.

Having a basic understanding of how this system works, a general diagnostic approach can be quickly determined based on the operational malfunction. To pressure check this transmission, Honda begins with safety warnings such as paying attention to the rotating front wheels during the test and to make sure lifts, jacks, and safety stands are placed properly.

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