Conversion Trico switch PS-33A (3 positions) to Trico PS-4A (2 positions)
Trico manufactured different switches for automotive applications. Although they look very similar, there is a distinct difference in their functionality.
The Trico PS-33A switch was used as a direction indicator switch with 3 positions: left, off and right. This switch was used by a number of British car manufacturers and can sometimes still be found in NOS condition.
Jaguar used the Trico PS-4A switch as an overdrive switch from 1955 till the early Sixties. It is a two-position switch (on/off) but in fact largely identical to the aforementioned PS-33A as the internals of the switch are common.
The difference is that the warning light inside the switch has internal connections in case of the PS-4A, whereas the PS-33A has additional connectors on the bottom plate (which carries the warning light) but the correct internal connections for the warning light are missing. The way Trico made a two position switch from a three position is simply done by blocking the third position by using a rubber buffer that prevents the handle to move.
The PS-33A has only the letter B (for battery) positioned as an indication for the centre one of the three connectors on the phenolic back plate. It is easier to mark the other two contacts with E (for earth) and L (for “load” which in case of Jaguar is the solenoid of the overdrive).
The conversions requires two modifications of the PS-33A switch:
1. After the bottom cover has been carefully removed it is possible to slide down the back wall of the switch. Beware of the ball that will jump out because of the spring behind it (see photo below right). A (semi-hard) rubber buffer of 12.7 x 10 x 7.0 mm is now placed (and preferably glued) in the aluminium housing at the side where the mark E has been put. This is done to block the third position. The 7.0 mm dimension is important and should be correct in order to allow the handle to rest in the centre position but not reach the third position.
2. The warning light inside the PS-33A switch has two contacts. As the PS-4A has internal electrical connections, for the conversion external electrical connections have to be made to contacts E and L. The third contact B has an internal connection in case of the PS-33A but will not be used. There are two possibilities for making the electrical connections:
a. Simply connect the existing LH contact on the bottom plate to the E terminal of the back plate and the RH contact to the L terminal.
b. In case one would the PS-33A switch to look more like the PS-4A on the outside, then first remove the bottom plate and slide away the back plate (see photo above right). It is possible to (carefully!) remove the existing brass terminals on the bottom plate by pulling them out of the rivited brass connection. Then take two thin wires and solder them to the lamp contacts at each end of the bulb and connect one to the E and the other to the L connector on the inside of the switch.
The photo below shows the final result of the conversion (left) and an original PS-4A (right) for comparison.