8-4 Overdrive Electric Solenoid Malfunction

“A” Type Overdrive


BritCar ServiceElectric solenoid operation of the Laycock DeNormanville “A” type hydraulic servo overdrive require two windings within the solenoid to overcome the 475 psi pressure acting on the operating valve. A
pull-in winding draws approximately 25 amps supplying the force required to overcome the system pressure. However, this winding is engaged only momentarily as the heat generated within the winding will cause it to rapidly self-destruct. As the solenoid plunger is completely drawn in, an internal switch is opened and the pull-in winding is disengaged leaving a hold-in winding of a mere 2 to 3 amps to hold the operating valve in the open position.

The solenoid ground circuit for the “A” type solenoid is effected through the mounting flange and mounting screw contact and must be adequate to conduct the 25 amp draw. As the overdrive casing is aluminum and the solenoid and mounting screws are of steel, dielectric corrosion can inhibit current (amperage) flow to ground. This fault can be seen as either no operating valve movement, (no solenoid pull-in), or operating valve movement when there is no pump pressure, such as when stationary, (weak solenoid pull-in). This second type of fault mimics primary or pull-in solenoid winding failure.

Replacing these solenoids is an expensive exercise, accurate diagnosis of the problem is therefore, recommended. As it is difficult for the average owner to verify this condition without a thorough knowledge of the Laycock DeNormanville “A” type operating principal, removal and cleaning of the solenoid ground contact area(s) is recommended before discarding the solenoid as defective. If bench testing of the solenoid shows a pull-in winding draw in the order of 25 amps, (momentary energizing of the solenoid with the plunger removed), and a hold-in winding draw of 2 or 3 amps, (energizing of the solenoid with plunger installed), then the solenoid is functional and the problem of weak solenoid operation will have been a poor ground path due to corrosion.