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Posted: Thu Nov 04, 2010 11:24pm

Post Subject: petter mini 6 diesel engine

on my petter engine there is a oil pressuure switch in the charging cicuit, when the engine is running the pressure in the engine rises the contacts in the switch then close to allow the alternator to charge the battery. when the engine is switched off the pressue falls the contact open. if they dont the batteries drain through the alternator, this a system fitted by petter themselves. my problem is that the switch is no longer available. i have tried to replace it with a modern switch from a car or boat but they all close the contacts when the pressure falls not when it rises. does anyone know of a switch that contacts close when engine stops. or do i have to rewire with a modern ignition switch system


Posted: Fri Nov 05, 2010 11:56am

Post Subject: petter mini 6 diesel engine

What you need is known as a Rising Oil Pressure Switch and I was amazed that I also could not find one on line. I suggest that you talk to a few engineering companies like Thornycroft (AMC) & PB Mechanical Services. It would also be worth talking to ASAP supplies although I could not find one in their catalogue. If you now have a "standard" alternator all you need do is to ft an ignition switch (if you do not already have one) and give it a supply. Run a cable from the start position terminal down to the SINGLE thin wire terminal on your starter or solenoid. Then from the auxiliary or "ignition on" terminal run a wire, via a warning lamp of at least 1.5 watts to the warning lamp terminal on the alternator. Then runs a wire from the same terminal on the switch to supply the instruments etc. Possibly by linking it to the switched side of the rising oil pressure switch. Diagrams in the electrical notes on If you have an old Lucas 10 or 11 AC alternator it is more complicated so I would advise changing it for something like a Lucas A127 at the same time. The alternative is to get a 5 terminal relay. and feed its coil from the battery and the down to the ordinary oil pressure warning lamp switch. This will energise the relay when the engine is stationary but a 5 terminal relay has two pairs of contacts one open when energised and one closed when energise. Connect the wires from the old oil switch to then two terminals that are OPEN when the coil is energised. In this way as the oil warning lamp switch opens with oil pressure the relay will switch the alternator etc. on. Tony Brooks

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