Most of us are all too familiar with the effects pumps can have on a transmission when they fail. No move, delayed engagements, clutches and/or bands incinerated and more. It is interesting to note the type of pump a transmission is using, and the advantages and dis-advantages associated with that style of pump. Take for example a gear and crescent style pump. This is otherwise known as an internal gear, positive displacement pump. It has been the primary fluid mover for years in automatic transmissions having three basic stages, it fills (sucks), transfers and delivers.

They have a limited number of moving parts and offer higher pressures and outputs than vane or lobe style pumps. And, they have better suction capabilities than an external gear style design and are well-matched to high viscosity fluids. One downside is that a Positive Displacement Pump will produce the same flow at a given speed (RPM) no matter the discharge pressure. A Positive Displacement Pumps is a “constant flow device” which must never function against a completely closed circuit continues to produce flow until the pressure in the discharge circuit has increased to the place where something bursts, or the pump is severely damaged, or both.

A relief or safety valve on the discharge side of the Positive Displacement Pump is absolutely necessary. Most gear pumps have circuits equipped with relief and/or safety valves for this reason. The draw back to any type of sudden pressure dump is the loss of horsepower (Fluid Power Horsepower (hp) = pressure (psi) x pump flow (gpm) / 1,714). If horsepower is lost too many times or for too long, transmission malfunction occurs.

A Variable Displacement Vane Pump on the other hand can maintain high pressures when needed at near 0 displacement (When the cam ring is centered on the axis of rotation and the chambers are equal size at the inlet and outlet. In this position, no fluid is pumped; displacement is zero).  A higher rated relief valve is in the system for safety only, but this style pump can maintain pressure without providing a continuous flow of oil. This style pump also has three basic stages, zero, minimum and maximum displacement output. Having this type of control over the pump not only provides to the transmission what it needs under various driving conditions, it also increases fuel economy. But it too is susceptible to failure.

Jon Fisher from Bell Transmissions dealt with a 2010 Chevrolet HHR that uses a 2.4L engine to power a 4T45E transmission that would exhibit a couple of issues. It came into the shop with complaints of falling out of 4th gear when easing off of the throttle at highway speed. The customer also stated that sometimes it would be in neutral at a stop after driving at highway speeds but would be okay again if the key was cycled.  The ATF was full, clean and did not smell burnt.  This compact sport utility vehicle was scanned and showed no fault codes.

When the unit was initially disassembled, no problems were found until the oil pump was split open. The pump slide springs was broken in several pieces (Figure 1).  The pump slide spring pushes the slide into the maximum output position. Decease oil pressure is delivered to the outer perimeter of the slide and is used to lower the output of the pump. With the spring broken, this pump remained in a minimum displacement state causing the falling out of gear complaints. There was not enough volume of oil being delivered under light throttle conditions.