Oil pressure sending unit

[hogi6123]

In my 1981 MC-9 with 8v92ta, the oil pressure gauge in the dash stays at about 4 psi, engine running or not.  I disconnected the wire at the sending unit at the engine, and the oil pressure gauge reads below 0 psi.  I then touched that wire to the frame of the bus, and the gauge reads max pressure.  The dashboard and gauges look all original to me.  Is my gauge acting correctly?

There is a manual gauge mounted next to the sending unit that reads what I expect, so there shouldn't be any blockage.

I think I need a new sending unit, does that sound correct?  Can I get one locally or do I need to call Luke?

Thanks.

[hogi6123]

My gauge says:
461D
24V.NEG.GND.
7G-13-11
OIL PRESSURE GAUGE
ASSEMBLED IN MEXICO
9705
It has 3 pins on the back:
G (ground)
S (sensor I think?)
I (24v I think?)
It was wired this way.

My sending unit says:
G

The gauge responds opposite of what I expected:
32 ohms 100 psi
45 ohms  90 psi
55 ohms  76 psi
68 ohms  75 psi
230 ohms  5 psi

The sending unit is fixed 230 ohms.

I searched online but was not able to find any more details yet.

[richard5933]

Some of these gauges work by making contact with the ground connection, some by breaking the connection. That would explain the two directions they go.

If you're sending unit is stuck at one output, then that would lead me to believe it's the culprit. Might be a good idea to remove it, clean the threads thoroughly, and reinstall it. I've been taught that using Teflon tape on these is a bad idea since it will make the ground connection less stable.

[hogi6123]

I pulled the sender out.  No Teflon.  I measured the ohms:

0 psi  235 ohms
20 psi  275 ohms
30 psi  694 ohms
35 psi  1000 ohms
40 psi  17,400 ohms
50 psi  16,000 ohms

Looks like the sender is working also... Definitely does not match my gauge though.  And has a max pressure of around 35 psi.

[richard5933]

Then you might be repairing a previous owner's failed attempt to get the gauge working.

Have you ever seen it work properly? If not, then I'd ask Luke which end is correct and then get the needs parts for the other end. Shouldn't really matter which setup you use, as long as the sender and gauge are the same. If the gauge is in good condition it's probably easier to replace the sender.

[hogi6123]

I'll ask Luke.

I don't remember whether the gauge worked.  It probably didn't since the previous owner told me he had just put in an engine from another bus.  That seems like such a long time ago...

[hogi6123]

I received a new sending unit from Luke.  A quick bench test shows its resistance is 240 ohms at 0 psi and decreases as the pressure increases, so I expect it to work fine with my gauge when I do install it.