Oil Pressure Gauge Science Project

Well, I started out what I thought was going to be a fairly simple project to figure out Oil Pressure Gauges and Sensors for M38/M38A1’s after a discussion on the Forum several weeks ago that left us kind of scratching our heads. Well, silly me! Like most Jeep stuff, the deeper you dig, the further in the hole you get.

Pelago’s oil gauges puzzled and frustrated us. I looked in the KWAS catalog and there is an Instrument Panel Oil Pressure Gauge Kit that seems to fit everything from an MB to an M38A1 and a lot of the CJ’s in between. I thought this would be a good kit to look at and see just what is what with oil gauges. I asked Mike if I could borrow one of the kits for a Science Project and he obliged and sent me the kit. I’ve looked it over and done a bunch of testing and I’ve found several interesting things. I will try to describe my findings in as logical sequence as I can.

As the KWAS catalog says, this is a NOS (New, Old Stock) replacement kit from some stash of government stock. This is a military kit. It comes in an AC commercial looking box, but everything inside is packed in Marvel Wrap MIL-B-1210 packing preservative paper dated 8/68. This was packed a year before I first became a father! It would appear that someone, somewhere, decided to make a one-size-fits-everybody Oil Pressure Gauge kit. Now, when you try to accommodate 6, 12 and 24-volt electrical systems and oil pressures from 0 to 120 PSI – there are going to be compromises. Let’s talk about the components of the kit. Here is a link to the catalog so you can see the parts we are going to talk about:

https://www.kaiserwillys.com/vehicle...a-3b-m38-m38a1

The Gauge

Yep, it’s a Jeep gauge. It mounts in a nominal 2 1/4-inch hole in the panel with the usual bracket. The dial reads 0 to 120 PSI, but that 120 PSI range is spread over only about 120 degrees of the gauge face; not much resolution. As you rotate the gauge around, the needle swings back and forth, it is not mechanically damped with no power connected to the gauge.

The face of the meter has two numbers; an Ordinance part number 7412564 and an AC part number AC1507825. There are two terminals on the back of the meter, both insulated screw post terminals – one marked IGN (Ignition) and one marked SEND (Sender). Note that these are screw posts, so if you are working with an M38/M38A1, the original connectors will need an adapter. The gauge is marked “AC Made in USA”. It’s not a Chinese copy of some sort.

The meter is a balanced coil meter. The coil from IGN to SEND measures 31.5 ohms, the coil from SEND to the meter case measures 26.3 ohms. The meter case must be grounded for the meter to operate properly. I would suggest adding a connection from the screws that hold the meter bracket to the panel to a good electrical ground, don’t count on the mounting bracket to provide a ground. If there is not a ground, the meter stays pegged at 0. I also discovered that if the connection to the sender is open, the meter pegs at full scale.

Since the kit is intended to operate with just about every electrical system, the gauge is designed to be a 6-volt gauge with dropping resistors added for 12 and 24-volt systems. If you just hook this gauge up to a 12 or 24-volt system without the dropping resistor – the gauge draws a bunch of current and probably will burn up. I didn’t try to verify this. This wasn’t intended to be a destructive evaluation.

Dropping Resistors

There is a Military Instrument Replacement cheat sheet included in the box that cross references AC part numbers to Ordinance numbers. The numbers don’t tell me much beyond “When replacing dash or sending unit both dash and sending units must be replaced to get a correct indication”. There are two resistors included in the package; one for 12-volt systems and one for 24 volt systems.

This cheat sheet information includes:

“For 6-volt system, replace dash and sending units only”. For a 6-volt system like an MB or GPW, add a wire to hook the IGN terminal of the gauge to the ignition of the vehicle. Connect SEND on the gauge to the sending unit and you are in business. You will need a Douglas or Packard connector adapter for the wire you add for this gauge since the MB/GPW was originally a mechanical gauge.

“For 12 volt systems add single resistor adapter to dash unit terminals”. One dropping resistor assembly has a single wire-wound resistor. I measured this as a 29.5-ohm resistor. It appears to dissipate about 1.2 watts, so if you were going to use a modern resistor for a dropping resistor I would recommend at least a 3-watt unit. I am not aware of what 12-volt vehicle system this kit might be used with. The 12-volt system does not have any direction to change the sending unit, but if I was installing this kit I would install the included sender.

“For 24 volt system add double resistor adapter to dash unit terminals”. The other dropping resistor has two wire-wound resistors in parallel. The 24-volt dropping resistor measures 99.2 ohms. It dissipates about 3.6 watts, so as a replacement I would use at least a 5-watt resistor.

I have attached a picture of one of the dropping resistors mounted to the terminals of the gauge. The tab connected to the SEND terminal is used only for mechanical support – it is not an electrical connection. The terminal screw on the right end of the dropping resistor is the new IGN connection to the vehicle.

Sender – 120 PSI

The Sender is an odd looking unit, shaped more like a lemon than the usual cylindrical can type sensor. Because of this unusual mechanical shape and size, it may not fit everywhere a standard sender fits. It has a standard fitting for the oil pressure port. The sample orifice is a very small hole – only a few thousandths. Electrical ground is made through the oil port fitting, so be careful not to insulate the fitting from the engine block (ground) with Teflon tape. If you do, the gauge will pin at full scale because of the loss of ground. The electrical connection to the vehicle harness is a Douglas connector, so an adapter will be needed for some installations. Another choice you could add to the vehicle harness is a female Packard connector. The pins of the Packard and Douglas connector are the same size, but the connection will not be waterproof and will not have the Douglas locking feature.

Our discussion several weeks ago centered on sender resistance vs. oil pressure and I was able to get an idea of how things worked by pressurizing this sender and looking at its’ resistance. I made some measurements to see just what the gauge was. Electrically the sender is a standard 0 to 30 ohm sender. I don’t consider these measurements to be lab grade precision. They were taken from a Harbor Freight portable air tank using a digital gauge on the tank’s charging air Schrader valve - but they do give a good indication of what is going on with the sender.

Digital Tire Gauge Sensor Resistance

90 PSI 24.9 ohms
60 PSI 17.9 ohms
40 PSI 13.0 ohms
30 PSI 10.5 ohms
10 PSI 3.0 ohms
5 PSI 0.7 ohms
0 PSI 0.1 ohms


How Well Does It Work?

So, I hooked everything up and checked air tank pressure vs. gauge reading.


Air 24 volt gauge 12 volt gauge 6 volt gauge
Pressure (PSI) (PSI) (PSI)
93 100 105 118
55 65 65 65
40 60 58 55
15 28 28 28

In summary, the gauge and sensor read high with all three electrical options. Below 15 PSI in the tank, the needle was just below 30 PSI on the gauge. At low pressure, less that 15 PSI, the gauge read significantly higher than the actual pressure. This could lead you to think you had more oil pressure than the engine actually is producing – not a good condition.

Late Note: When this gets posted, the formatting of the tables gets fouled up. I think you can figure it out, but if not - drop me a Private Message and I'll try to translate.

Wow LarrBeard, some serious work in that post!