Oil Pressure Sensor for 6-volt Stewart-Warner Gauges

I’m an engineer by profession. As a Senior Chief Electronics Technician in the Navy, I ran calibration labs. My retirement hobby is Maples Road Instrument Services, specializing in analog test equipment from the 1970’s and 1980’s. That’s why the gauges in my ’48 Willys have driven me up the wall!

Try as I will, sensors don’t match the gauges. Gauges read bass-ackwards, go to full scale when they should be at a quarter or half. The instrument cluster guy tells me that I’m not alone, the old senders and sensors just aren’t available – it takes an adapter in many cases.

With that in mind, I was pleased as we ended the restoration of Ham’s ’48 truck to see that the oil pressure gauge with the original sensor on the engine was close enough to be credible. A mechanical gauge told us it had about 10 PSI of oil pressure at idle and about 32 PSI at cruise speed. Then came the aw-pooh. One day we started the truck and the oil pressure gauge pinned at way far right – more than 50 PSI. A quick look showed that something, someone, had smacked the sensor on top of the case and smashed it. We figured out Mr. Notme did it.

The replacement sensor wasn’t right. It said 30 PSI at idle and 50+ at cruise, so I added a resistor in series to center it up at cruise, but it was still kind of funky at low speed. It turns out that if you can possibly repair an old sensor, you’ll probably have a better chance of it working properly. As the YF carburetor guru said “Just because its new and shiny doesn’t make it the right part”.

After I'd driven a while, I wondered if I could fix the old sensor. After all, it’s broken; maybe I’ll have better luck with it than I did with the ignition switch. (It was so worn the contacts were just shot…).

I’m not going into much detail about Stewart-Warner gauges – there are plenty of sources – Google will find them for you. A SW gauge was basically a bi-metal strip driving a needle pointer; the hotter the strip, the more it bent and more needle deflection. But, the other end of the system is even more intricate. Look at the pictures and I think you’ll be able to follow what’s being described.

The oil pressure sensor operation starts out with a copper diaphragm against which engine oil under pressure pushes. As pressure increases, the diaphragm bulges and moves an arm. This arm has a contact point on it which touches another point mounted on a bi-metal strip wrapped in a nichrome heater wire. While these points are together, current flows through the panel gauge, causing its pointer to move upscale. Even with the engine off, the gauge pointer reads below zero, so some amount of heating is needed to bring the pointer up to zero. There is an adjustment wheel on the sensor base that appears to set this initial zero point to let the gauge preheat to bring the pointer up to zero. When the wire has heated the bi-metal strip enough, the strip bends, which opens the contact points. Current flow ceases, the strip cools, contact points close and the process repeats.

Now, assume we’ve started the engine. The copper diaphragm bulges, pushing the arm against the bi-metal strip with more force. This bends the bi-metal strip, and it has to heat more to open the contact points. Since the points stay closed longer, the bi-metal in the gauge heats for a longer period of time and the pointer moves upscale. As the engine moves from idle to cruise, the diaphragm bulges even more, causing the pointer to move even further upscale.

Here are some other details of the oil sensor on the ’48. It is stamped “50” on the threaded boss that screws into the block. I would assume that designates it as a gauge for a 50-PSI panel gauge. In the photos you can see a tubular object mounted in clips; that’s a resistor, about 7-ohms, that limits current through the gauge and sensor to less than an amp or so. The heater on the bi-metal arm has very low resistance cold, a couple of tenths of an ohm, but as it heats it will increase (one of the non-linearities I spoke of).

Now, just what can you do if you have one of these that you want to try to fix? First, make a quick check of its internal resistance. If your oil pressure gauge went to zero (scared you, didn’t it) but you figured out you still had oil pressure, your sensor probably went open. Take an ohmmeter and read resistance from the threaded base to the brass contact on the lid. It should be about 7-ohms or so; not 50 or 60 – a resistance I saw in a universal replacement gauge. If it reads zero, like mine did, something has probably bent the case into the mechanism.

Before you start to open it up, make a quick check of the diaphragm. If it’s split or has a hole, the sensor is beyond anything I can figure out to do to fix it. Suck on the sensor fitting and block off the hole with your tongue. It should hold a vacuum. Be cautious, if the diaphragm has split, there will be oil in there and you could get a slug in your mouth. It probably won’t be the first time…

If the diaphragm is OK, you can take a chance. Just remember, it’s already broken so you don’t have much to lose! With a cutting wheel in a Dremel tool, make two cuts about 90-degrees apart around the crimped edge of the case. You can see when you get through the steel case and start to cut into the brass sensor base. Once you get your cuts made, pry the 90-degree sector out as little as you can to get some working space. Work around the 270-degree sector as needed to tilt the base plate assembly out of the case. There is a gasket in the case, try not to tear it up.

With the sensor assembly out of the case, you can make an inspection of the innards. I’m not an expert, I’ve only taken one apart, but there seem to be a limited number of failure points unless the unit has gotten full of water and is has corroded.

My best guesses to check are:

BROKEN WIRE

Look at that little wire from the bi-metal strip to the contact arm. In my unit the point at which it was soldered (maybe welded?) had a large glob of corrosion. (A lot of solder joints in the ‘40’s were made with corrosive solder flux). The wire was still connected, but after I brushed the corrosion away, the whole connection looked suspicious. I didn’t want to put it back that way, so I resoldered the joint. I had to file away the base metal on the arm to get down to copper/bronze/brass that would take solder and the heater wire was very difficult to solder. I had to scrape a bit of the insulation away before I could get to a section that would take solder. As you can see, it’s not a beautiful solder job, but it is secure. Take your time, think twice, and scrape once.

DIRTY CONTACT POINTS

Back in my early days in electronics, things were full of relays. Relays have contact points. Points get pitted and dirty and as a junior guy in the shop, I got the job of cleaning the relay contacts in the XYZ-199 whatever. I learned that the less cleaning you do, the more chance you have of being successful. In my sensor I took a small piece of 800-grit paper and pushed it between the contacts as gently as I could. (Don’t bend anything)! I burnished each contact, and then cleaned them with a piece of paper soaked in isopropyl alcohol. I didn’t see any dirt - that was good.

OPEN RESISTOR

My sensor had a 7-ohm (+/-) resistor in a set of contact clips. The resistor could be open, or just as likely the clips have failed to make contact. Check for about 7-ohms at the base of the clips. If not, I’d suggest getting a 7.5-ohm, ½ watt resistor and soldering it into the clips. The resistor is either going to be OK or open in just about every case, they very seldom change value. (The color code didn’t seem to match the RETMA standard color code).

SENSOR READS CLOSE TO ZERO OHMS

My sensor read zero because the case had been bashed into the mechanism. I persuaded the case back into shape and that cured the shorted condition. I suppose something could cause a short – perhaps a failure of some insulators or spacers where the unit was riveted together. If this turns out to be your problem, I leave it to your ingenuity to go from there.

Now, how to check this out – short of putting it on the truck?

For a first cut, measure resistance from the brass contact strip to the base. You should have about 7-ohms, or the value of your internal resistor. If that’s OK, now its time for a functional check. Get a six volt lantern battery and a 6-volt bulb. Connect them in series, and put the sensor in the circuit, like a blinker, one wire to the curved brass contact strip – one wire to the base. The bulb should light, then after a bit, go out. ON, OFF, ON, OFF. It may only flicker a bit, but you should see the ON – OFF. That tells you the sensor is working! Yeah….. Success so far.

Now, put the sensor back into the case. I cleaned my case with isopropyl alcohol to make sure it didn’t have a bunch of crud in it and let it dry well before I reassembled it. Don’t forget the gasket. Before you do anything else, recheck the sensor operation with the light to make sure the brass contact strip is doing its thing. You may have to jiggle things a bit since the sensor is loose in the case, but this step will verify that you didn’t screw things up putting it back together. (That may never happen to you…)

To put the case back onto the base, I just took a small ball-peen hammer and tapped the pried up places back down. I wasn’t able to get the case to fit as tightly as the original crimp, so I worked some RTV sealant into the little gap between the case and the base to keep water out.

Well, did it work?

Yep. I took out the replacement sensor, removed the adjusting resistor I had added and the oil pressure gauge read very close to what it had originally read – about 10 PSI at idle, close to 30 PSI at cruise. Not a bad result for an hour or so of work.