Dynamic functional characteristics
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ENTER CODE : ART30 AT CHECKOUT
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!