Extending the life of a Honda converter clutch

April 28, 2014
Most shops that repair Hondas on a regular basis are well aware that the No. 1 Honda transmission failure is the torque converter clutch.
Most shops that repair Hondas on a regular basis are well aware that the No. 1 Honda transmission failure is the torque converter clutch. If you happen to be new to world of Honda trans, here’s how the story typically develops, so you will recognize it when it happens:

The customer drives down the road and suddenly it neutrals, so he coasts to the side of the road. He tries reverse, then drive again. If he’s creative, he may even pull it to manual low and 2. Nothing. If he’s really clever, he may even check the fluid level. Seeing it is not low, he shuts it off for a couple minutes while trying to figure out what to do. At this point he will typically make one of two choices:

A. A tow truck deposits it at your bay door. You start it up and drive it right in the shop, and the customer says, “Hey! How’d you get it to move?”

B. He waits a while, starts the engine, pulls it to D, and it goes in gear. He takes off down the road and goes a mile or so, then it neutrals again. He repeats the pattern until he finally gets it to your shop.

In both scenarios above, you explain to the customer that his trans has a clogged filter. When he asks if you can service the trans, you then have the distinct honor of informing him that it requires trans removal, disassembly, a complete bath, and several hours of valve body and solenoid work because the unit is contaminated with converter clutch lining. And oh yeah, it also needs a new converter.

Whether you are rebuilding the trans in your own shop, having it rebuilt by a local trans shop, or installing a reman unit from either the dealer or an aftermarket source, there are a couple of very important things that need your attention if you want the converter to have a reasonable lifespan. But it is equally important you understand why these things are important. So let us first discuss lockup clutch function and learn why the converters fail.

There are three operational states for the converter clutch: fully applied, partially applied and fully released. All three states must work correctly, or the converter is in trouble.

Fully applied — The main concern with full lockup is that there is sufficient flow to and pressure in the converter to provide sufficient clamping force to prevent clutch slippage under load. For example, an Odyssey loaded with people on the freeway pulling a long hill relies on good converter charge with sufficient pressure to prevent clutch “creep” from superheating the front cover and lockup clutch lining on the damper plate.

Partially applied — Partial lockup is less active on heavier class vehicles, but on the light stuff, like Accords and TL’s, it can be very active, especially at lower speeds, lower gears and lower rpm. Because partial lock is controlled by duty cycle applied to one of the linear solenoids, which in turn acts on the lockup control valve, the amount, or percentage of lockup can be controlled.

Concerning these first two states: When the whole vehicle is in new condition, the full and partial lock system just barely operates well enough, but as the vehicle ages, many things begin to change, such as:

• The condition and quality of the ATF drops drastically. Even with regular services, all the fluid does not get replaced unless the cooler return line is removed and you keep pouring new ATF into the trans while it runs until it pumps the old ATF out of the converter. Few do this, since they don’t want to pay for 15 quarts of new ATF, some of which will be discarded.

• Linear solenoids don’t regulate as precisely as when new because the internal springs can fade, and valves and bores wear.

• Valves and solenoids begin to stick because of particulates in the ATF.

• The pump gains clearance as it ages and expands from heat and pressure, and pump volume output and efficiency drop.

• The converter clutch lining loses holding capacity as the coefficient of friction changes.

• Overall internal hydraulic system leakage increases through varied amounts of wear here, there and everywhere. It’s not huge at any one place, as much as it is cumulative in effect.

• Resistance in old wiring and connectors rises.

• Engine performance declines with changes in cam and valve timing, as with a host of other things, and the sensor inputs used for lockup control are not at optimum. So the computer does not control the trans and regulate pressures in the same way as when everything was new.

Looking down this list, one might think, “Well, many of these things can be and are replaced when the transmission is rebuilt!” Yes, that certainly is true. But look again at the last three in the list. Transmission repair centers do not sell a new timing belt and valve adjustment with the trans rebuild to get the computer working the way it did when the vehicle was new. And these are significant factors beyond what most people realize. Now add to this some mismatch oddities among dynamic functional characteristics, and programmed operational parameters.

Dynamic functional characteristics

ATF viscosity drops with increased temperature, and internal hydraulic circuit leaks increase with lower viscosity. In principle, this is the same as engine oil pressure dropping when it gets hot. But the Honda transmissions have a low output-capacity pump (small gears). So when it does get hot, leak rate increases, and it requires more pump volume than is available.

Consider how the main pressure regulator system is calibrated to regulate a static or fixed value of 120 to 125 psi at idle in P or N. When in gear and torque is applied, the stator arm transfers force from the converter stator assembly to the pressure regulator boost sleeve to add compression to the pressure regulator springs (see Figures 1 and 2). In other words, Hondas regulate a static minimum of 125, and with mechanical boost at max torque push this to about 210 to 220 psi.

Here is where the dynamics come into play: Pump volume output is proportional to crankshaft speed. When the trans and fluid are cold, and the engine runs at high idle, the pump has no problem satisfying the pressure regulator (PR) springs to maintain 125+ psi. But as it begins to warm and kick off the fast idle, the pump slows, ATF thins, pump output drops, and line pressure begins to drop below the 120 psi minimum spec when at idle.

At this point, the PR valve closes and bottoms in the bore, effectively shutting off converter feed and lube circuits (see Figures 3, 4 and 5). Now line pressure is determined by available pump output as applied to the total mainline system and its leaks. When the fluid and trans are very hot, pressure may drop as low as 65 psi while idling in gear at a stop. If you move the shifter to N, the idle speed picks up and it may run 85 to 95 psi. In gear, when accelerating, the pressure will rise proportional to rpm until the PR valve begins to open again (typically above 1,000 to 1,100 rpm, which is why Honda tells you to take psi readings at 1,200 rpm). Then the converter and lube circuits open and it regulates 125+ (depending on torque load). But anytime the line pressure is below 118 psi, the PR valve is bottomed in the bore and converter feed is effectively and practically shut off. The only supply it receives is what leaks past the PR valve via bore clearance. This is the approximate equivalent of two .032-.037 inch holes!

Programmed operational parameters

Here is where this whole scenario gets real interesting. While doing research on these transmissions, we road-tested several Acura 3.2TLs with pressure gauges attached to line pressure and cooler line, and also a gpm flow meter on the cooler. We discovered that there are times when, at low rpm in third gear between 32 and 34 mph, the computer will command LOCKUP ON (often with PARTIAL lock activated). Yet unfortunately for the transmission, the low pump speed (at about 950 rpm) was insufficient to make 125 psi. We were in traffic, low speed cruise, with only 110 to 115 psi on the pressure gauge. This meant that the PR valve was closed, and converter feed was cut off. This was not a good situation. With lockup commanded ON, but converter feed shut OFF (insufficient pressure/flow to the converter), the only thing holding the damper plate against the front cover is the centrifugal weight of the ATF as the converter spins.

But when we would toe into the gas pedal ever so lightly (so as not to kick off lockup electronically), we could get the converter to slip until finally released. When we observed this, it became all too clear why these TLs and several other models suffer repeat converter failure: It was the result of discord between the dynamic functional capabilities of the system, and the programmed operational parameters. The OE system was no longer adequate. New technology had to be created to bridge this gap.

Fortunately for transmission rebuilders everywhere, I was able to reengineer the whole PR system, and our new Sure-Cool® PR valve (patent-pending) extends the range of active converter feed down to 95 psi so that any time lockup is commanded on, the converter receives a steady and reliable supply of charge. But now this brings us to the third operational state:

Fully released — One might think, “That’s easy enough. Lockup is off. That’s no work at all!” But such is not the case. Interestingly, one of the times the lockup clutch can take a hit is when stopped at a traffic light in D. Remember that when these things get real hot, and idling in D at a stoplight, line can drop as low as 65 psi. Of course at this point, even the Sure-Cool PR valve must be closed. Yet there is a special built-in metering system to provide as much converter feed as possible without adversely affecting hot idle pressures.

Of course, if one were to steal too much flow from the mainline, the pressure could drop out the bottom and engagements can suffer. Therefore, there is a lower limit beyond which we cannot go. But as in the above scenarios, this is where some other dynamics come into play. Here is why:

Any time lockup is commanded OFF, there must be sufficient converter charge to the cover side of the damper plate. Typically in non-lockup mode (and this applies to all lockup transmissions), fluid is directed down the input shaft and delivered in front of the damper, where it flows between the cover and clutch lining to hold the clutch released. Pressure on the release (front) side must always be greater than on the apply (rear) side, for if it is equalized, rear side fluid can centrifugally drag the clutch on the cover. You don’t have to be a trans rebuilder to understand that this would not be good. It would be much like sitting at a stoplight with a manual shift trans and letting up on the clutch pedal just enough to start to drag the disc.

Now, this is where you really need to pay careful attention, because whether you are the rebuilder or the installer, this is the point where you can make or break this trans:

Release oil goes down the input shaft, past the clutch lining to the rear side, then out the rear side exhaust circuit where it becomes COOLER supply. It then goes through the cooler and is dumped down only the gear train where the return line is connected. The cooler itself is the main restriction, and cooler is connected to converter rear side oil. If the cooler is restricted, cooler pressure, and consequently, converter rear side pressure rises. When this happens you increase the probability that the lockup clutch will DRAG! Friends, this means you can take a superbly rebuilt trans with upgraded technology, and install it with a partially blocked cooler, AND RUIN THE TRANS (kill the lockup clutch)!

If you have understood everything so far, then technically speaking you are above average, and the following should be within your perception:

PRESSURE = RESISTANCE TO FLOW. With no resistance, you have unrestricted flow. That is, flow with no pressure. When it comes to Honda cooler systems, we do not want high pressure. We want high FLOW, with low pressure. How do we achieve this?

The goal is three-fold:

1. remove any and all restrictions from the cooler lines

2. flush the original cooler as best as you can

3. add an external aftermarket cooler in PARALLEL (not in series) with the original.

The whole aftermarket transmission industry knows that when the Honda trans fails the internal filter gets plugged with lockup clutch lining, but you may not realize there’s another filter that also gets clogged. This one was in the cooler line of a 2001 Odyssey B7TA 4 speed automatic [see figures 6 and 7].

Not every vehicle has it. But you must check, because it can drastically shorten lockup clutch life! So make sure you show these pictures to whoever will be installing the trans. Point to the filter and say.. “Do you see this? Go find this filter and remove it from the cooler line! It may be buried underneath the lower radiator hose!”

The first time we found this in the line, we spent several hours at the Honda parts service counter with the VIN# from the van. The parts guy could not find it. The replacement cooler line has no filter, and the one shown here was nowhere to be found. As far as Honda parts is concerned this filter doesn’t exist. It wasn’t till a few months later we learned that this complete cooler line and filter comes packaged with a Honda reman transmission. It appears they do not trust that the OE cooler will be adequately flushed, and install this filter to catch any lockup clutch particles. So any vehicle that has had an OE replacement may have one buried somewhere in the line.

It so happens that this filter can be taken apart and cleaned, but we prefer to remove it altogether since it is a very low capacity filter and even when clean adds additional restriction to the cooler system.

If you prefer to have an external filter installed, that’s a good idea IF and ONLY IF it is a large capacity low resistance filter (about the size of an engine oil filter) that will not drive up cooler pressure when it is 30% plugged. It should have a safety blow-off well under 30 psi.

Many of the 5 speeds have a trans mounted heat exchanger on top of the trans [see figure 8]. When new, they flow a little better than the 4 speed cooler system, but we’ve cut them open to find they also clog up. Unfortunately this bad boy is very expensive to replace, so we recommend using a transmission cooler adapter. The water lines can be joined by a coupler made up of 2 barbed fittings and a straight fitting [figure 9]. These can be found in the plumbing department at Home Depot. The three brass fittings run about $12. The total cost for adapter, cooler, and coupler is still far cheaper than a replacement heat exchanger and the trans will run much cooler with the external system.

Several aftermarket versions are available. Your regular parts supplier probably stocks at least one of the available types. But we have also discovered that Honda also has adapters for many models. There are a few different ones, and the price varies, but overall they are cheaper than the aftermarket ones, so check the dealer first.

In addition, a 1/2” fitting with a hose barb can be installed in some of the cases after the heat exchanger is removed. The case can be threaded with 1/2” pipe tap and the barb screwed into case and then a cooler line directly attached. The return line can be cut and adapted to receive a hose also, so that the OE setup can be bypassed and an external cooler installed.

We have found that the OE cooler system is very restricted. By installing a new external cooler (even the smallest, like a Haydon 1401) and bypassing the OE radiator and inline filter, cooler flow is literally doubled and cooling capacity dramatically increased. This suggests that resistance was cut in half. So if you were to run the new external cooler in PARALLEL with the OE cooler [figure 10], then you would be dropping the resistance to 1/3!

That is, your flow potential would be tripled at the same pressure. But in reality, when at a stop in D at idle, the actual flow delivered to the cooler system is controlled by what is fed to the converter release (front) side, since you can’t get out more than you pump in. This is much lower than the cooler system’s flow capacity, which means therefore that you will effectively lower the resistance and pressure in the cooler system to ensure rear side pressure is always lower than cover side, and in doing so, you have preserved lockup clutch life.

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