My stock oil pressure gauge has not worked in a long time
Maybe 14 years ago I ran a wire directly from the sender to the post on the gauge in the dash.

That worked ok for a couple years until the wire burnt up, got in my way, and I ripped it out.

Since then the quality of my engine harness deteriorated. I periodically
hacked in wire to keep it running.

Since completing my HEI installation, I pulled the engine harness apart, tracing back every wire to the ESA/lean burn module connectors, associated senders/sensors etc, distributor and coil, and cut them off 4" from the harness connectors (there are two at the firewall).

By the time I was finished, one connector had only 2 wires left connected to it: the wire to the torque convertor lockup solenoid, and one 20-something gauge wire I have yet to verify the purpose of. I just cut off the connector, and connected these wires with crimped on spade terminals to the corresponding terminals on the connector at the firewall.

In so doing, I found the oil pressure sender connector wire was broken inside the insulation. I crimped on an eye, and assumed attaching that eye to the post with a nut would get me my oil pressure gauge back.

No such luck.

I found that I could get a new Echlin sender for $20, so I did, and swapped it in. (I am no advocate of part-changing when I don't know what the problem is yet, but it was only $20 and the thing is from 1987. It doesn't make sense that it still worked--besides when I started thinking of a setup to bench test the sensor, I was most of my way to the $20 mark already.)

I installed the sender. It behaves no different. Oh well...

I halfway pulled out my gauge cluster (leaving the speedo cable and main connector attached) and pulled the connector that serves only the temp and oil pressure gauges and associated lamps.

I removed the OP gauge and traced the printed circuit post holes to the pins for the connector.

I identified + to the gauge and - out to the sender.

I checked continuity to the sender. It was fine.

I checked for voltage at the + pin. It read battery voltage.

I started the engine. It dropped while starter was on, jumped up when alternator kicked in, then just went nuts.

It was showing all kinds of random voltages.

Entertaining the idea that I can abandon that wire, and run a new + wire, once I figure out where to run it from, I moved on to just testing the gauge and sender independent from the wire.
I ran a hot wire to the + side of the gauge and touched the - side directly to the post on the sender.

The needle went to 1/5-1/4 of the scale.

I had my wife give it gas (first to about 1500 rpm, then about 2500--by ear. no tach attached) the needle didn't move.

Looks like I have to buy a oil pressure tester and consider that I may actually have a real oil pressure problem.

Does anybody know where the + wire on the gauge should feed from? Is there a legitimate reason for that voltage to jump around?

I hooked up a test gauge:

46 PSI @ idle

48 PSI @ 2000 RPM

Engine was already fairly warm when I hooked it up. (I wasn't really assessing the condition of the engine, just checking the gross accuracy/operation of the electric gauge.)

I guess the minimal increase in pressure explains why the needle doesn't move noticeably with engine speed, but fails to explain why it only registers 1/4 up the scale.It used to run @ 3:00. Now 4:45-4:30 (i.e. in the 1990s)

It could not have been running at 90 PSI, right

I don't know the specifics of your vehicle, but most mopars fed the (fuel, temp, oil press) gauge (+) pin from a 5 Volt regulator. This was a small electro-mechanical device on the cluster circuit board that worked like a turn signal flasher unit. Current flowed through a bi-metal contact that was wrapped with a heater wire. As 12V went through the unit, it heated up the wire & caused the contact to open briefly. So voltage was 12V or 0V- & averaged around 5V. Feeding a constant 12V to any of those gauges would burn them out.

The (-) side of the gauge goes to ground through the sender unit- it's just a variable resistor. The fancy way to test things is to look up the resistance specs on the sender & use different value resistors to check for gauge movement- 1/4, 1/2, 3/4, full scale. An easy way to test is to ground the gauge (-) through a test light & see how the gauge reacts.

_________________
63 Valiant Wagon
225 - 4 bbl

^^^ As above, the voltage limiter pulses the 12 volts on and off about once per second. Newer digital voltmeters will just shows what they sample at the moment and the readings will bounce around.

If you are getting such a low reading on the gauge with 12v fed direct, then likely the gauge is damaged.

With that level of oil pressure, measure the resistance through the sender. It will be high (80-100 ohms or more) when the engine is off. At 45-50 psi, I'll venture to estimate that it will read 'round 20 ohms.

At least you know your pump and pressure relief valve work well!

Thanks for the replies.

The vehicle is an 87 B-150 225 auto intentionally stripped of options (originally AC, cruise, tilt, lean burn--now AC & cruise gone, straight column, HEI).

If the gauge is bad, I guess I won't replace it with another OEM unit.

While I had the dash apart, I realized I could easily modify the amp meter and OP gauge pods to accept aftermarket volt and mechanical OP gauges, as they each have their own 2"+ round lens. There is a back-lit screen sandwiched between the body of the instrument panel and the lens.

I could easily cut a piece of sheetmetal to the size and shape, and cut mounting holes for standard 2" gauges.