Tackling CVTs

July 1, 2015
Clear and valid training is needed to diagnose and repair continuously variable transmissions, the transmissions of choice for hybrids.

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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Figure 4

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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Before pressure testing begins, be sure the transmission fluid is filled to the proper level. Then Raise the front of the vehicle, and support it with safety stands. Set the parking brake, and block both rear wheels securely. Allow the front wheels to rotate freely. Connect any adapters needed for the transducer(s) and then connect the transducer(s). Warm up the engine (the radiator fan comes on), then stop and connect a tachometer.

If the TCM has stored a trouble code initiating failsafe, you must use an oil pressure gauge/transducer that measures 711 psi or more when measuring drive pulley pressure and driven pulley pressure.

Pressure specifications:

Forward clutch is checked in the D range at 1,500 rpm:        203-253 psi

Reverse clutch is checked in the R range at 1,500 rpm:          203-253 psi

Drive Pulley is checked in the N range at 1,500 rpm:            28-101psi

Driven Pulley is checked in the N range at 1,500 rpm:          218-334 psi

Start Clutch is checked in the D range at idle:                        35-40 psi

Start Clutch is checked in the D range at stall:                       120-130 psi

Lubrication is checked in the N range at 3,000 rpm:              30 psi

Time has revealed that when considering this CVT as a whole, the start clutch is by far the most problematic aspect to this transmission, especially the earlier years. Honda has produced several versions of this CVT with great improvements to its longevity. But when it does fail, it is a simple and straight forward repair. The biggest problem is laying your hands on a few special tools that make this process a snap (Figure 6).

These tools are listed in the factory manual with part numbers but it seems that Honda does not make them available outside of their dealer network. If you can obtain them, they making pulling and installing the start clutch a simple job (Figure 7).  The special tools shown in figure 6 consist of a reverse brake spring compressor (07TAE-P4V0110), a Start Clutch Puller (07TAE-P4V0120) and installer (07TAE-P4V0130).

The puller is designed to reach underneath the park gear which will pull the secondary drive/park gear out with the start clutch drum as an assembly. If you use a puller to remove the start clutch drum only, you may cause damage to the drum. The secondary drive/park gear will then need to be removed from the Driven Pulley Shaft (Figure 8). The installer is then used to seat the secondary drive/park gear and start clutch drum assembly down onto the Driven Pulley shaft as one assembly. The inner tip of the tool hooks into the inside tip of the Driven Pulley Shaft. The outside part of the tool has a tightening nut which presses the assembly onto the shaft.

One word of caution, the start clutch is held onto the Driven Pulley Shaft with a snap ring. Underneath this snap ring are cotters covered by a cotter retainer. If changing a start clutch assembly for the first time, this cotter retainer can be tight to the drum and spline camouflaging itself as being a part of the drum (Figure 9). Not knowing this, you will have a “continually vicious time” trying to get that start clutch off the shaft. You will destroy the tool and the drum making them and yourself eligible to go visit that “Center for Victims of Torture.”

Figure 9
Figure 10
Figure 11

For this M4VA type CVT, the TCM uses four solenoids to control this transmission; an Inhibitor Solenoid, a Pressure High/Pressure Low Solenoid (PH-PL Solenoid), a Shift Control Linear Solenoid and the Start Clutch Solenoid (Figure 10).

One self inflicted injury that can occur with this M4VA type of CVT is cross connecting the Shift Control Solenoid with the Start Clutch Solenoid. This will disrupt the proper control of the Start Clutch causing stalling in reverse or drive along with a nasty sounding bang. Figures 10 and 11 both show the incorrect connections of these two solenoids which is very easy to do. The good news is this Honda transmission has a pan to gain access to these solenoids should you make this type of cross connect error.

Another aspect to this style of CVT is to perform relearn procedures for the Start Clutch. The procedures varied with year and make but are very easy to do. A few of these procedures will be the topic of one of July’s 2015 weekly Powertrain Pro newsletters as there is not enough room remaining in this article to add them.

This was glimpse of a CVT that was good to learn from in many respects. After understanding how this “snowmobile” type transmission works and operates in a car, it goes a long way in understanding others. For example, The Audi 01J uses a dual mass flywheel to drive and input shaft like the Honda. The difference here is that rather than using a start clutch to decouple the powerflow at stops, it releases the forward or the reverse clutch. This makes one less clutch assembly to consider. If there is a delayed engagement and slip in forward yet reverse operates well, the forward clutch needs to be looked at. Likewise if reverse has issues but forward movement does not, the reverse clutch needs to be considered.

A couple of drawbacks with this transmission is there are no pressure taps to do any reasonable diagnostics and the computer is inside the transmission. This means there will be plenty of occasions of diagnosing the symptom based on operational understanding.

With this little intro to CVT understanding, the JF011E is a transmission beginning to show up more frequently in shops these days and will be the subject of next months article. There are several versions of this transmission as it is used in Nissan, Mitsubishi, Dodge and Jeep vehicles. The article will get into its operation, some diagnostics and repair. The more you know and understand, the more “courageous, victorious and triumphant” you will be with your CVT.

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