A short while back, upon starting my truck (1994 F-150, 5.8l) in a grocery store parking lot, I noticed the oil pressure guage was reading less than 0. Engine had appropriate amount of oil in it. I don't know how long it had been doing this, but this was the first I had noticed.

I took a chance and drove it 4-5 miles home (uphill most of the way) without a problem. I didn't hear any tapping (dry lifters) or anything else that sounded wrong (except for my exhaust leak which is another story).

So I guess I have one of the following problems:

1) No oil pressure
2) Bad guage
3) Bad sensor

I just started it again yesterday, and ran it for about 10 minutes. Would that seem to indicate there actually is oil pressure?

I stuck an ohmmeter on the sensor, and it reads into the M ohms when not running. When running, it drops into the K range. Anybody know what the optimal readings are? It would be a lot easier to go this route than yanking the sensor and either replacing or inserting a manual guage.

Once again, with the ignition off, the guage reads 0. With the ignition on, engine running or not, the guage pegs on the other side of 0.

Any ideas?

 

Do you have any other gauges acting funny? The voltage reg on the back of the instrument cluster could be going bad. Have you looked at the wiring harness for corrosion in the connections on the firewall? Short of that I would replace the sending unit.

 

Go ahead and replace the oil pressure switch. It is an "idiot gauge" setup, meaning it uses a pressure switch, not a sender like a real gauge setup would. A real sender would vary resistance based on the pressure. This setup instead has a simple on/off pressure switch. When the oil pressure is above the switch minimum, the switch closes to ground. The "gauge" has one end fed by 12 volts, the other end goes through a resistor to the switch. So with the switch closed, it reads on the gauge. Totally phony. Could have just as well used the idiot light. Anyone doubting this, look at the gauge at idle, then rev it to 3000 rpm and hold it steady. A real gauge would really go up, as the oil pump shaft is spinning a lot faster. But the idiot gauge just stays there. Why did they do this? Cheap, and fools most into thinking its a real gauge like the others?

 

Anyone have any quick tips on replacing the sender unit? Once again, this is a 5.8L Windsor.

In the "old days" I remember just grabbing a pair of channel locks, and "cranking" the old ones out. The sender on this engine is buried behind brackets and such.

Is there a "short-cut" tool available? Or, any tricks for making one out of common materials?

Thanks,

 

There is a special socket available that fits these sending units.
It should allow you to replace the unit without removing any brackets or accessories.
DannyP

 

Once again, the problem I'm having is that as soon as I turn on the ignition, the oil pressure "guage" pegs below zero. After starting the engine, it stays there. I've driven it around a while and haven't heard any noise from the lifters so......

I finally went to my friendly Napa parts store and got a new sender. The guy behind the counter asked for the usual information, and then asked if I had a light or a guage. I told him "guage" and he brought out this huge sensor (compared to what I have on the engine). I told him my current sender was much smaller and had a threaded center post. He asked again if I had a light or a "guage". I said guage. He asked if someone had modified the set-up, and I told him I had no idea (since I bought it used). He said he had only one other listing, and it was for a light. He went and got it, and it looked like the one currently on my engine. I bought it (even though he acted like he didn't want to sell it to me).

I just installed the new sender today. When I turned the ignition on, the needle went almost to the center position. After starting the engine, it pegged below zero. I shut-off the engine and tried it again. Same thing. I put the old sender back in, and I now have the original problem back.

Any ideas??

Thanks,

 

You bought the wrong sending unit. Go back to Napa and buy the "gauge sending unit" that the guy was trying to sell you. I'll bet you'll have no more problems after you install it! Good Luck!!!

 

Regarding the sending unit:

I had two choices for the new sending unit.

One looked identical to the one that is currently on the engine. Small. Screws into the block. Has a threaded center pole fot the electrical connection.

The other one was 3-4 times larger (both length and girth). It also had a spade connector.

I picked the only one that would fit the current configuration.

Any more ideas?

 

Well, I decided to dismantle the instrument panel just for kicks. I needed to change a lamp for the right turn indicator anyway.

Just as has been mentioned previously, this is definitely an "idiot" guage.

Per all the good info that linked to:

http://www.ford-diesel.com/contents/getitems.php3?Real%20Oil%20Pressure%20Gauge%202

there is a 20 ohm resistor in line with the "guage". In other words, when the sender(switch) gets good pressure, it grounds-out. The ground going through the 20 ohm resistor sets the needle to what looks like a good reading. No ground, and the needle pegs to the left. I grounded the connector just to prove this out.

So, I'm going to try the new sender one more time, just to make sure I wasn't high on something yesterday.

So, I either have a bad new sender, or maybe I just have "bad" pressure. Like I said, I haven't heard any noises like clacking lifters, so unless I find a mechanical guage to hook-up, I won't know what kind of pressure I'm getting. Maybe just enough to keep things quiet, but not enough for a big load.

By the way, is there an oil pressure tester I might be able to rent somewhere, or is buying a mechanical guage the only way?

We'll see......

 

For about 25-30 bucks you can pick up a mechanical gauge and be much better off than trying to get the stock gauge/light to work.
Just my opinion.........
DannyP

 

just go out and buy a small cheap pressure gage with the same thread as the sending unit and screw it in. Look at the guage and see what it reads. If it shows pressure you have an electrical problem. The thing to do would be to get an electrical schematic and find all the grounds that are connected to that circuit and make sure they are good.

 

Went to a parts shop (B&B) that had correct listings. Their "guage" sending unit looked almost exactly like the idiot light sender. The only real difference was it had a plastic cover that slipped over the electrical connector, like the one that was originally on the engine.

The differences between the senders are: one is normally open and the other is normally closed. One opens to shut the idiot light off, the other closes to turn the guage on.

The NAPA parts system is incorrect in that they want to give you a pressure "sensor", which is for a real guage and won't work in this application without modifications (per previous messages).

Thanks for all the input.

 

I noticed something strange today on my 86 F250 w 351W. The gauge was reading quite low at idle...engine warm. If I remember right it was reading right on the beginning of the normal line of L. I held 2-3000 rpm and the gauge came up to almost center normal. I did not hear anything out of normal...hell what is normal I have owned the thing less than a week. I am not too concerned, I didnt hear anything and press came off of the bottom. I think I need an oil change, I have no idea when the last one was...the level is good though

 

Anyone still reading this, I have begun to look further in this oil issue. I changed the oil and have the same problem. The pressure gauge shows above normal, around the A+L. Then slowly fell to middle and in 5 min it was back to the usual N+O. Well at some lights the pressure drops all the way to zero. A couple kicks of gas and the pressure rises just as slowly as it fell. I drove up the driveway and had zero pressure, opened the hood listening for tapping valves. I dont hear anything unusual, just normal engine running. Kick the gas and the pressure rises very slowly to between N+O and stays there for about 5 min until I shut it off. The volt gauge doesnt work, the gas gauge works sometimes, the only thing that works seems to be the temp.

I have been looking into getting a mechanical gauge setup but dont know what fittings I need or if I have to order the gauge for my application 351W?


Will
Alsip, IL
86 F250 4x4 351W
-Plow Truck, Western 8'6"
97 Explorer XLT 4x4 4.0 SOHC
-Lifted 2.5"
99 Jeep Sahara 4x4 4.0 I-6
-Stock for now

 

Go into search in this conference, and key on "oil pressure".

Before I posted this string, I remember seeing some information on voltage regulators or something of the sort. Something to do with flakey guages, and their power source being bad.