Oil Pressure Gauge Repair & Calibration

I have just finished a long fight to get my original oil pressure gauge and transducer back to full and accurate health. The details may help others to do the same so here goes!

I always thought that the gauge read low on my 1976 V12 but one day it refused to indicate any pressure at all even though the oil light had gone out. O.K. - Shorting the lead going to the top of the pressure sensor to ground gave me an instant reading of 100psi so the fault was in the sensor itself.

On closer examination the spade terminal on the top of the sensor appeared to be floating about and not connected inside. As a temporary measure I fitted a mechanical oil gauge which I knew was calibrated and got pressures of 75psi cold, 60-65psi hot, 25psi hot tickover. I cut open the sender unit using a fine hacksaw blade around the circumference about 1/4" up from the crimped edge. As the top came off it was obvious what had gone wrong, the nylon insulator post which held the spade terminal and resistance wire coil had broken away at it's base due to age and excess strain on the spade terminal. At this point rather than go into a long description of the sender I refer everybody to an excellent drawing of it at B.J.Kroppe's web page (http:/www.mich.com/~kroppe/pics/xj6_oil_xducer.gif ).

Having glued the nylon post back on using a high temperature glue ("Araldite") I set about re-calibrating the sender/gauge combination by connecting +12v to the "B" terminal of the gauge, joining the "T" terminal to the sender spade terminal and then 0V to the sender base. "Oh Dear!" the gauge read full scale (100psi) when I was expecting 0psi! A close examination of the gauge revealed that it was not the usual "Hot Wire" type but contained two large fixed coils at right angles to each other, one coil between the "B" & "T" terminals and one between the "T" & case(0V). I then realised that the gauge case also had to be connected to battery 0V for the system to work properly. (note:- this is not shown in the drawing above and is the only error I could find) Having fixed this the gauge started reading normally but low as before!

The reading with no pressure applied was below zero on the gauge and about 20 psi low all through the range(checked against a calibrated air line at my work). Luckily there is an adjustment which is accessible with the sender open, a small screw & locknut. I adjusted this to give an exact reading of 0psi, using "Loctite" on the locknut as it was varnish locked originally. Having sorted out the 0psi setting I checked all through the range and the accuracy was good right up to 100psi. There is an adjustable end stop to stop the wiper coming off the end of the track at high pressures which will give a sudden zero reading, not good for the nerves! I re-assembled the top of the sensor with a washer and 8mm "O" ring on the top shoulder of the nylon post. The "O" ring was slightly compressed by the top and sealed the gap to keep out oil and also to support the post to stop it breaking loose again. It was surprisingly easy to solder together the sender case at the hack-saw cut with normal electrical solder and a 50 Watt soldering iron. For the record, these are the values I recorded during calibration:-

SENDER:- "PTR1001/10ec 700kN/m(squared)"

Pressure(psi), Resistance(Ohms): 0 290, 10 264, 20 226, 30 188, 40 154, 50 122, 60 95, 70 76, 80 55, 90 37, 100 20

GAUGE:- ACP2203/00 (100psi)

• "B" to "T" = 240ohms
• "T" to "0V" = 326ohms