Oil Pressure Sender

[david.g.chapman]

Just a post to tell you about my oil pressure sender unit that needed adjusting.

It is similar, but not exactly the correct one for my +2 with the 60 psi smiths gauge, but I am assuming the innards are the same.

As mentioned in an old post, it is possible to open the steel casing without hacksawing it in half. I made a few small hacksaw cuts (about 6) in the outer flange at the base of the unit, evenly spaced around. I could then use an old screwdriver and hammer, to ease back each flange section. The top can then be carefully prized off the base.

Inside my sender was a 3-sided rectangular strip of metal with a contact at one end and a winding around one side. This winding is connected between the spade terminal at the top and earth via a curved piece of springy metal pressing against a brass plate.

The other contact is fixed to a metal strip that passes over a screw adjuster and a metal rod coming up from a diaphragm in the base.

I wish I had taken a picture!

I believe the sender works as follows:
For no oil pressure and ignition on, the screw adjuster is set to give 0 psi on the feeder oil gauge. In this state the contacts are making and breaking (much like the way the voltage regulator works) constantly to give current pulses through the gauge and hold it steady at 0. Note that this is not the ignition off position which is on the gauge stop below 0. The feeder current heats up the upper strip using the winding which breaks the contact which stops the current. The strip cools down and contact is remade, and so on.....

As oil pressure increases the diaphragm raises the lower strip and contact, increasing the heating effect required to break the contact again. As a result the current pulses get wider and the gauge starts to move away from the 0 psi mark. The damping in the gauge smooths out these pulses, so you get a steady reading.

Understanding this meant I could adjust the zero point with the screw adjuster, and also the way the gauge reads by twisting the 3 sided upper strip between my fingers - being careful not to break the winding! The settings are interactive so need to be repeated a few times.

So there was no need to add any other external resistors in my case.

I then pressed the case together again and resealed with 200 degrees C RTV.

I hope this is of some use.

Cheers

Dave Chapman.

[john.p.clegg]

Dave

I think you forgot to mention the resistor inside the sender unit?

John

[david.g.chapman]

Thanks, John.

I did forget!

There is a 0.5 watt carbon resistor across the contacts as well. Changing this if the value had drifted is a good idea, or you could change this to another value to alter the response if you like. It may not have a linear effect though.

Cheers again,

Dave.