30 GEARS January / February 2006

In the last edition of Basic Hydraulics we covered the dif-ferent types of pumps used in

automatic transmissions and how they function. We also discussed the fact that pumps primarily create volume, which when contained, produces pressure.

Pump pressure must be regulated to control shift feel and to keep pres-sures from exceeding a level that will do damage to the transmission. When it comes to pressure regulation, there are only two basic types of pumps: non-variable and variable. In this edi-tion of Basic Hydraulics we’ll use a few examples to illustrate both types of regulator valves. We’ll cover the theory of how these valves work with-out including the use of math - that will come in future articles.

Regulator Valves for Non-Variable Pumps

With non-variable pumps, the vol-ume that the pump produces goes up and down with engine RPM. Therefore the regulator valve must be designed to exhaust a lot of oil. The regulator valve movement can be broken down into three basic positions; stage 1, 2 and 3.

Stage 1 Position (figure 1)

In this position, the valve is in the deepest part of the bore and is not exhausting any oil from the pump. This can also be referred to as the max pres-sure position because the valve is not regulating. As the line pressure rises, the balance pressure will eventually be high enough to push the regulator valve into the stage two position.

Stage 2 Position (figure 2)Once the regulator valve is

pushed into the stage two position,

pump volume is sent to the torque converter. At lower RPMs, this will usually be enough pump volume loss

don't get upset...don't get upset...CONTROL YOURCONTROL YOUR PRESSUREPRESSURE

by Larry Frash

LIGHT_0

LIGHT_5LIGHT_6

Line Pressure

Converter/Lube

ExhaustBalance Oil

From Pump

ExhaustTo Converter/Lube

LF

Figure 1Stage 1 Position

LIGHT_0

Line Pressure

Converter/Lube

ExhaustBalance Oil

From Pump

ExhaustTo Converter/Lube

LF

Figure 2Stage 2 Position

BASIC HYDRAULICS

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32 GEARS January / February 2006

to control line pressure from exceed-ing the desired amount. As pump vol-ume increases above this controllable level and line pressure rises above the desired amount, balance pressure will also rise. When this happens, balance pressure will push the valve further, compressing the spring into the stage three position.

Stage 3 Position (figure 3)

In this position the valve sends pump volume to the torque converter, and exhausts pump volume out a large exhaust passage. This will allow enough of a leak in pump volume to control the desired amount of line pressure.

The regulator valve will usually float back and forth between stage two and stage three during normal opera-tion. The heavier the regulator spring, the harder it is for the balance pressure to move the regulator valve to a regu-lating position. Accordingly, a heavier spring will raise line pressure.

Because automatic transmissions need to have lower line pressure at light throttle operation, and higher line pressure at heavier throttle, the regula-tor valve needs to be able to raise line pressure on demand. To do this, a boost valve is located on the spring side of the regulator valve (figure 4). At heavier throttle openings, pressure supplied to the boost valve is increased. This essen-tially adds force to the spring side of the regulator valve, making it harder for balance pressure to move the regulator valve into the stage two or three posi-

tion. This raises the line pressure when needed at heavier throttle openings.

Boost pressure will fluctuate with throttle opening, and by theory, is the

Basic Hydraulics

LIGHT_0

Line Pressure

Converter/Lube

ExhaustBalance Oil

From Pump

ExhaustTo Converter/ Lube

LF

X

EPC Pressure

Boost Pressure

LIGHT_0

Line Pressure

Converter/Lube

Exhaust

Balance Oil

ExhaustTo Converter/Lube

From Pump

LF

Figure 3Stage 3 Position

Figure 4

To decrease side of pump slide

From pump To converter and lube

Mainline Regulator Valve - Stage 1 Position Figure 5

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34 GEARS January / February 2006

same whether it’s controlled by a vac-uum modulator, throttle valve or an electronic pressure control solenoid.

Regulator Valves for Variable Pumps

Regulator valves for variable pumps are basically the same in the stage one and two positions as the regu-lator valve for non-variable pumps. In the stage one position, the valve is not regulating (figure 5). In the stage two position, the valve sends pump volume to the converter charge circuit (figure 6). The major difference is in the stage three position (figure 7). In this posi-tion pump volume is not only sent to the torque converter, it’s also delivered to the decrease side of the pump slide. This forces the slide to a lower volume position. Remember, in the last article

we discussed that in this position, much of the pump volume was carried back to the intake side of the pump.

The regulator valve in a variable pump is usually very small in diameter, and cannot exhaust enough fluid to control pump volume to maintain the desired line pressure at higher pump RPMs. If the slide sticks in the maxi-mum volume position, the pump will put out far more oil than the regulator valve can control. This will create very high, uncontrollable line pressure.

Reverse BoostDue to the extra loads put on

clutch packs and bands in reverse, transmissions need higher overall line pressure. To obtain this, reverse apply oil is supplied to the reverse boost valve (fi gure 8). This adds more force to the

spring side of the regulator valve, which in turn creates higher line pressure in reverse.

Line to Lube Modifications

Most transmissions use the con-verter charge oil to lube the transmis-sion after the oil has passed through the cooler. Most mainline pressure regu-lators only supply the converter and lube circuit when the regulator valve is regulating. To improve lubrication, it’s often necessary to make a modifi-cation to bypass the valve, and have a small amount of line pressure directly feed the converter charge circuit. To do this, you must have a good hydraulic diagram.

Sometimes you can grind a small, flat spot across a land of the regulator

Basic Hydraulics

To decrease side of pump slide

Mainline Regulator Valve - Stage 2 Position Figure 6

From pump To converter and lube

To decrease side of pump slide

From pump To converter and lube

Mainline Regulator Valve - Stage 3 Position Figure 7

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GEARS January / February 2006 35

valve, as in a 4L60E. Other times it is necessary to drill a hole through a wall that separates line pressure from con-verter charge.

When doing this modification there are three important things to keep in mind. First, keep your modifications small. If you drill too large a hole, or grind too much off the land, you’re taking a chance of overcharging the

converter and/or losing too much pump volume. Second, drilling a line to lube will cause a small amount of converter drain back. So again, if you’re doing these modifications, keep them small. A good hole size is about 0.062”. The last thing to keep in mind is to make sure you do this modification correctly. Drilling the hole or grinding the valve in the wrong place can cause serious

problems in mainline regulation.Well, that’s all the room we have

in this edition of GEARS. Stay tuned for additional topics related to Basic Hydraulics.

Figure 7

Reverseboost

Reverseboost valve

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