Rover Sd1 3500 Oil Pressure Isolater Switch.
by Bill Beddows (with information from Doug Lloyd)

The switch is located on top of the oil filter housing, it has 3 connections and the original fitment connects the wiring with a molded plastic socket. The primary function of the switch is isolate the fuel pump if the oil pressure drops to zero and the ignition is still on, for example, after an accident which stalls the engine.

To assist in starting the car when cold the fuel pump is connected while the starter is cranking.

Wiring connections
White    12v supply (ignition on )
White/purple  to fuel pump
White/brown  from starter solenoid

If the wires are a different colour then disconnect them from the switch and with the ignition on and engine off check with a voltmeter to find the 12 v supply, shorting this wire to the other two in turn will cause the fuel pump to run and the starter solenoid to kick in.

Switch connections

The 3 prongs are in a U shape.

Emergency operation

If the switch fails the fuel pump will not operate, in this case the 12v supply can be directly connected to the fuel pump wire. No damage will be caused by doing this.

N.B. There are at least 4 possible reasons for non operation of the fuel pump :
1. The fuse has blown
2. The fuel pump is faulty
3. The oil pressure islolater switch has failed.
4. The fuel pump earth connection is faulty

By-pass switch

If the car is fitted with a Weber 500 or a Holley carb and a very open air filter such as a K & N when it is not used for some days the fuel in the carb evaporates and the pump has to run for several minutes to refill it. In these circumstances it can save wear and tear on the starter and battery to fit a dashboard mounted push button operated by-pass switch.

I used a 10amp push button switch mounted in a short piece of suitably sized steel strip which I screwed to the inside of the dashboard so that the switch is just below the lower edge of the dash. This avoids drilling quite a large hole in the dashboard and renders the mod reversible.

To attach the wires to the switch just cut off the connectors for the 12 volt supply and the pump wires and crimp and / or solder new connectors on with both the original wire and one of the switch wires in each one. The solenoid wire is now redundant and can be left unconnected and safely taped back onto the other wires.