Oil Pressure Sender Surgery

Frankensender

I was taking a drive and my oil pressure dropped to zero and then came back. While in my cold sweat, I got home quickly and parked the car. After reading several threads on this subject, I figured that it was most likely a gauge/sender problem….but I needed to prove it. I realize that since there are many threads on this but in order to stay comprehensive, there is a lot of repeated info (and maybe some new stuff).

The gauge was pulled (I’m thinking about wing nuts on my dash since this instrument panel removal is getting old). The gauge is actually quite simple. There are two separate senders feeding this gauge. The first is a low pressure warning switch (see pics). This switch simply turns the little red light on. The second is the analog sender which sends a variable resistance proportional to the pressure. Since I could not find any specs on this resistance, I tried different values. I got the following results.

Gauge reading
in Lbs---Resistance (ohms)
0-------300
- ------178
85------110
- -------68
170------5 (pretty much a short to gnd)

The warning light was then checked with +12V along with the dash night light. I didn’t want to pull the dash again for bad bulbs. I am glad that I didn’t have to open this little guy because it looks to be quite tightly fastened together. If you turn it upside down, it wants to read differently. I suspect it is not balanced internally. So don’t drive upside down.

The gauge was put back in the dash. The sender wire on the engine was disconnected at the connector. A 110 ohm resistor was connected from the connector (to the gauge) to ground. The ignition was turned to where the fuel pump starts running and the needle jumped immediately to 85 lbs. If you short this connector to ground, the needle will go to 170 lbs (max). The needle should always read something or there might be an open circuit.

After proving that the gauge was reading correctly, I went after the oil pressure sender. The unit was mounted to the left of the oil filter casting (as you face the front). The sensor on the right of the casting was the low oil pressure switch and came off easily (I’ll get to that later). I could see the sender and even touch it but I could not get a wrench on it. The casting was removed after removing the large oil cooler hose first and then three nuts. The casting was then put in a vice (gently… its aluminum) and the sender came off easily. The unit read “open” with my DVM (high impedance). As I messed around with it, it read about 600 ohms (reading about 300 ohms too much) every now and then so I knew that I had a bad connection internally.

The lead into the device was cracked (see photo) but wiggling it did not get anything. It only read something when I pressed on it really hard. Since the sender was basically trash at this point, I decided to cut the end off. You can see the corrosion inside. I suspect that the moisture seeped in through the wire connection where it enters the sender back because the front seal looks to be good. Closer inspection shows a rubber gasket with a poor seal where the signal lead exits the outer wall of the sender.

Here is how this thing works. The oil pressure is sensed through a small hole in the front of the unit. I cannot see into this side of the sender but I am assuming that there is a diaphragm between the little hole in the front and the resistor in the back. The diaphragm pushes a small pin in the same direction as the hole (I could see the back of the pin when the red board was removed). There is a tiny mechanism that converts this linear motion to the rotary motion of the resistor wiper arm. The red thing in the picture is a 328 ohm wire wound resistor. As the oil pressure increases, this little arm rises and slides across the exposed wires of the resistor changing the resistance between the base of the slider and the small contact on the upper right of the board. This thing is really a poor design. I counted four places where pressure makes electrical contact. Any one of which can cause an “open”. An “open” will cause the gauge to drop to zero. The 328 ohms was measured on the resistor by itself after being removed from the housing.

Looking at the design it is no wonder that…

1) Oil pressure drops to zero. This is caused by an intermittent contact that can happen in four spots and can be caused by vibration, corrosion or contact (metal) fatigue. It can also happen anywhere in the wiring on it’s way up to the dash.

2) Oil pressure “dances” up and down. Same as #1

3) Oil pressure falsely reads high. Not likely given that the resistor arm is spring loaded to drive to high resistance (low pressure). The unit can however, have a short due to corrosion and this can cause your meter to read very high or full scale. A short anywhere in the wiring can also cause this.

4) Oil pressure falsely reads low. Same as #1. Your old wiring can also have some residual resistance and it all adds and shows up reading lower pressure on your gauge. There are three connectors external to the sender to bring the signal to your gauge and they should all be cleaned.

Of course, you can actually have bad pressure but I would check your sender first, then your gauge.


Cleanup

The corrosion was cleaned out of the unit with tapping and shaking. There are two rivets holding the board and they had to be ground off with a Dremel motor tool to pop the board off of its mounts. The board comes out easily from between the double wiper arm. Naval Jelly did the rest to clean it up. The mechanism was removed with one screw and there were just a few parts. The mechanism was cleaned and reassembled. On this sender, the actual failure was that the contact on the red resistor board was under pressure by the contact from the case housing containing the single wire to the sender. The board had actually rotated on its mounts, over time, so that it could not “push back” and therefore could not make contact. The board was remounted with epoxy for support and to hold it in position. It is not under any real load; it just needs to stand still as the wiper moves.

Calibration

The sender was connected to a small PVC contraption with a compressor fitting on one side and the metric thread of the sender on the other (see picture). The compressor was set to 85 PSI (should be 110 ohms). The sender first read 160 ohms at this pressure (not too far off). The small mechanism had a tension wire for actuation. The wire had a “U” shaped loop that was squeezed with a pair of needle nose. I only squeezed it a bit and the sender read 107 ohms – done. The signal wire was soldered to the large contact on the upper right of the wirewound resistor and brought through the outer cap.

Finally, a copper pipe cap fitting ($3.50) was epoxied to the sender with JB Weld. The unit was baked at 220 degrees F for two hours (for any out gassing) and then the small hole where the wire comes out was sealed with epoxy. I scoured the hardware store looking for a smaller diameter cap but I couldn’t find one. I suspect there might be a better fit out there.

Low Pressure Warning Switch

This little sender should read zero ohms when there is no pressure. This will cause the gauge to light up the warning light (totally independent of the other sender). Much to my dismay, it read “open”.

I had cut it open too. It consists of a small pin that is actuated by the oil pressure much like the resistive sensor. The pin pushes a spring loaded contact that should open the circuit. This one was just dirty and making poor contact. After cleaning it internally, the switch was reassembled and also JBWeld’d shut. It was tested under pressure and it now works fine. I don’t know what the pressure is but I suspect that it doesn’t really matter and that it is really set up to alarm the driver with a sudden real loss of oil pressure.

When my ignition is on but not in the start position, the gauge goes from the far left peg to about “0” Lbs. This is because power is now applied to the gauge and the sender is reading about 300 ohms. The Low Warning Light is on (because there is no pressure). When the car is started at idle, the gauge now jumps to about 85 lbs and the light goes out. After revving the engine, it goes up to what looks like about 90lbs.

It is already much better that what I had and I cheap-skated out of the $225 for a new sender. If anyone has found a cross for this part at a reasonable price, please let me know.

-Rjay
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Guys,

Thanks for the nice comments. The sender has been working well. Oil pressure pops up a little past 85 right on ignition and is very stable. It climbs just a bit on higher RPM but is very smooth. Even though that might be considered a bit high, I am pretty sure things are ok - and I am not going to mess with it. Mike Florio is in the neighborhood and we are trying to hook up. It is perfect timing because I'm into the dist's and the points right now. Besides, I am looking forward to meeting a fellow Fchatter and share stories.

Jonathan, I have been following your thread on the Electromotive. Very enticing right about now

Rjay