Oil pressure sender unit

The following information is provided as a 'for interest' item. Having found the fault I decided to continue on and find out if I could make the unit work again. In the event I succeeded but how reliable and long it would last for I cannot say. If you decide to try this repair yourself then good luck but it is entirely at your own risk re accuracy and reliability of what is an important monitoring device.

 

The sender unit has two terminals, one of which (ALERTE) is not used on an M201. It is to operate a low oil pressure warning light that was not included. When the oil pressure gauge (manometer) stopped working on my jeep it was the sender unit rather than the gauge that was faulty.

You can check this with the ignition on by earthing the sender wire which should make the needle on gauge on the dashboard move. If it does so then the sender is faulty.

The obvious solution is to change the sender for a new one but these are increasingly difficult to find either new or second hand. I managed to obtain one and the problem was solved but rather than throwing the old one away I kept it with a view to finding out what's inside and what goes wrong with them.

The cover with the terminals on is stamped onto the main body and the only way of separating the two parts without destroying the unit is to carefully file a chamfer all the way round the edge at 45 degrees watching carefully to stop as soon as enough metal has been removed. (I used a Dremel for this.)

Once a ring has appeared all the way round the two parts can be separated but they are a tight fit and the process must be done carefully or the inner workings will be damaged. Put a couple of marks on the casing using a permanent marker to aid lining up on reassembly before separating the two parts. I forgot to do this! 

Pressure acting on a diaphragm underneath the base plate acts through a lever system to move a sliding contact across a rheostat made of ultra fine resistance wire. It is quite a marvel of miniature engineering but is easily damaged so take care!

Look carefully at the photos above and you can see the problem. The fine resistance wire has broken at a point on the rheostat.

I decided to see if it could be fixed by carefully smoothing down the wire into place then bridging across the broken area with conductive silver paint having decided that attempting to use a soldering iron would be futile. This special paint is available from electronics suppliers and also from some auto shops where it is sold for repairing rear windscreen heater elements but be warned it is expensive!

 

The paint contains silver metal and is actually silver in colour but appears pink in the photo as a result of reflected light. It needs time to dry and harden before it becomes fully conductive and obviously a few turns of the resistance wire have been taken out of use but not too many. To my surprise it appeared to have worked when the rheostat coil was checked from one end to the other using a multi-meter.  Don't expect to find continuity between the 'MANO' terminal and the body. I couldn't work this out at first until I realised that the mechanical design includes a ramp that the wiper arm travels up when there is no pressure lifting it away from the resistance wire to make sure the gauge reads zero when there is no pressure.

Before putting the unit together again clean the surfaces of the contacts with switch cleaner and make sure they are lined up before pushing the two parts together, that's where the marks I forgot to put on would have come in handy. Be careful to make sure that the orientation is correct otherwise the terminals will be wrongly marked. I then sealed the rim with two part epoxy adhesive (Araldite) to prevent moisture getting in and to hold it all together. I have tested it using a foot pump and it appears to work but as I said at the start I really only did all of this to see if it was possible. I have no idea how long it will hold up for in proper service and certainly the gauge reading will not be completely accurate but I will give it a try when I get the jeep out next season and let you know.

Jan 2010

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