M38A1 Oil Pressure Sending Unit Testing

[cabinfever]

My M38A1 oil pressure gauge (0 to 60psi) shows a reading of at about 18 to 20 psi and varies little with rpm.

I want to test the sending unit using the directions provided below:
http://www.cj3a.info/sibling/milgauge/milgage.htm

The first test says:
"The ohmmeter should indicate 0 to 1 ohm reading with engine stopped, if ohmmeter does not indicate this reading or excess 1 ohm, replace sending unit."
Q: I assume that one of the ohm meter probes should be grounded. Where does the other ohm meter probe go? On top of the sending unit (the internal part that connects inside of the shell connector?"

The second test says:
"Start engine, ohmmeter should indicate a higher reading, if a higher reading is not indicated, replace sending unit."
Q: Same question, where do the ohm meter probes go?

Thanks
[/i]

[mdainsd]

One on a known good ground. The other to the disconnected terminal of the sending unit. It also pays to check the ohms between the negative terminal of the battery to the body of the sending unit.

Grounds are the "unseen" portion of the electrical system, but are just as important as the wiring in the system.

[wesk]

6. Sending Units.

a. To test function of sending units, an ohmmeter with 0- to 7,000-ohm range is recommended. The following procedure applies to any sending unit being tested.

b. Disconnect wire from gage to sending unit at sending unit.

c. Connect positive lead of ohmmeter to sending unit and negative lead of ohmmeter to vehicle ground. (G, fig. 1). Caution: Do not connect ohmmeter to ignition circuit and turn ignition switch ON as ohmmeter will be damaged.

"Red lead to center pin in terminal on top of sender."

"Black lead to sender body bare metal or block bare metal"

(2) Oil pressure gage sending unit.

(a) The ohmmeter should indicate 0 to 1 ohm reading with engine stopped, if ohmmeter does not indicate this reading or excess 1 ohm, replace sending unit.

(b) Start engine, ohmmeter should indicate a higher reading, if a higher reading is not indicated, replace sending unit.

[cabinfever]

Thanks, guys! I will test my oil pressure sending unit and let you know the results.

[4x4M38]

Have you confirmed the oil pressure is actually going up and down with rpm’s?

[cabinfever]

Have you confirmed the oil pressure is actually going up and down with rpm’s?

I'll start the cold engine and the gauge jumps to 20psi. After warm up, I'll increase the RPMs and the gauge will pretty much stay at 20 psi. It might jump up or down 1 psi when I drive it down the road.

[cabinfever]

I am more confused than ever. Here is what I find.

Gauge:
With the battery switch at OFF, the oil pressure gauge is pegged to the left.
Battery switch at ON, the gauge needle jumps to ~1psi (without engine running)
After engine starts, the oil pressure reads a pretty steady 20 psi regardless of RPM.
When I disconnect the sending unit wire, the oil pressure gauge pegs to the right.

Sending Unit:
With engine not running, I get about 5 ohms.
With engine running, about 10 ohms (+/- 1 ohm) at idle and when i rev the engine.

I've had my M38A1 for six years. To tell you the truth, I've never really paid much attention to the exact PSI my oil gauge was reading over the years. (My bad!) I may have always been running at 20 psi.

My sending unit looks old and rusty. Perhaps I should replace it.

Thanks for any additional input.

[wesk]

No reason to be confused! Your test results indicate the sender must be replaced.

[cabinfever]

No reason to be confused! Your test results indicate the sender must be replaced.

Thanks Wes for your recommendation. I'll give J. Bizal a call about a new one.

[wesk]

Remember there are two different senders. One goes with 60 PSI gauge and the other with the 120 PSI Gauge.

[cabinfever]

UPDATE

Today I swapped out the oil pressure sending unit. The old sender had the following stamped on it.
S-W 506B
120 LBS - 7728856

The oil pressure gauge in my M38A1 (1955) is 0 to 60 psi.

I guessed I had a mismatched gauge and sending unit combination.

I replaced the sender with a new 60 psi unit and my readings are good.

Idle: 35 psi
Driving: 45 to 50 psi

Thanks for all the help.

[pacho]

Good !!