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Low Oil Pressure or Bad Sending Unit?

 
  #1  
Old 03-14-2019, 04:02 PM
tgun
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Low Oil Pressure or Bad Sending Unit?

Since I bought my '86 F-150 302 EFI new in 1986, the oil pressure gauge needle has stayed about half way in its range when the engine is warm. Now, the needle is on the lower end of the range, pointing at the N in normal when the engine is warm. The needle does not move if the engine rpm is raised when the truck is idling and the engine rpm is increased. The oil pressure sending unit was replaced last fall. I find a test procedure online for disconnecting the electric wire to the sending unit and grounding the wire while the key is on. This procedure is supposed to cause the gauge needle to swing fully to the right. If the needle swings all the way to the right when the wire is gounded, then the sending unit is bad. If the needle does not swing all the way to the right when the wire is grounded, then the gauge is bad. But I also find that during the 80's, Ford used both a switch for oil idiot lights and a sending unit for oil pressure gauges. Since my truck has a gauge and a sending unit that looks like this:

am I correct in surmising that (1) I have a sending unit and not a switch, and (2) that the testing procedure of grounding the wire will tell whether I have a bad gauge or bad sending unit? Thanks for any insight.
 
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Old 03-14-2019, 05:53 PM
kr98664
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Does your truck have a temperature gauge or an idiot light?

If you have the temperature gauge, you can use that to troubleshoot the oil pressure gauge circuit. The temperature, oil pressure, and fuel gauges are the same electrically. The only difference is label on the face. So I'm assuming (Danger! Danger!) your temperature gauge is indicating correctly. If so, this means that circuit has a known-good sender and gauge.

Use a pair of long test leads to swap the connections for the oil pressure and temperature senders. Your suspect oil pressure sender is now connected to the known-good temperature gauge. Your known-good temperature sender is connected to the suspect oil pressure gauge.

Start the engine and watch the gauges. The temp gauge should quickly drive to the middle, as it is actually showing oil pressure. The oil pressure gauge should slowly increase, as it is actually showing coolant temperature. By observing the temporarily cross-wired gauges, you can quickly determine what's wrong with the oil pressure circuit.

Wiring diagrams are here:

Fuel Tank Selector & Gauges - Gary's Garagemahal (the Bullnose bible)

 
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Old 03-14-2019, 09:43 PM
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Originally Posted by tgun View Post
Since I bought my '86 F-150 302 EFI new in 1986, the oil pressure gauge needle has stayed about half way in its range when the engine is warm. Now, the needle is on the lower end of the range, pointing at the N in normal when the engine is warm. The needle does not move if the engine rpm is raised when the truck is idling and the engine rpm is increased. The oil pressure sending unit was replaced last fall. I find a test procedure online for disconnecting the electric wire to the sending unit and grounding the wire while the key is on. This procedure is supposed to cause the gauge needle to swing fully to the right. If the needle swings all the way to the right when the wire is gounded, then the sending unit is bad. If the needle does not swing all the way to the right when the wire is grounded, then the gauge is bad. But I also find that during the 80's, Ford used both a switch for oil idiot lights and a sending unit for oil pressure gauges. Since my truck has a gauge and a sending unit that looks like this:

am I correct in surmising that (1) I have a sending unit and not a switch, and (2) that the testing procedure of grounding the wire will tell whether I have a bad gauge or bad sending unit? Thanks for any insight.
No; grounding the wire only tests the dash gauge & wiring to it. You still need to determine if the problem is the sending unit or something else.

Yes, you have a sending unit and not a switch.
 
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Old 03-15-2019, 07:26 AM
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My truck is a 1986 with gauges. It had a leaking oil pressure sending unit, so I replaced it with a NAPA unit. The gauge read low and NAPA replaced the sending unit and it, too, read low. I bought one from O'Reilly [BWD] and it has been in the middle of the normal range ever since.

Perhaps your "new" sending unit is at fault?
 
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Old 03-15-2019, 03:04 PM
tgun
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Thanks for the comments. I did ground the wire to the sender and the gauge needle swing fully to the right so it seems the sender is bad. Bought a Bosch 8216 manual gauge and may just install it or may get a Motorcraft sending unit and keep the OEM gauge. Have to thread a new connector into the oil port in the side of the block either way so it seems either is workable. Thanks again.
 
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Old 03-17-2019, 03:06 PM
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Tried to install the Bosch 8216 yesterday and the plastic line that came with the gauge was apparently old and kinked in several places during the install. Had seen an 8216 install on youtube and the installer had used 1/8" air line which was much better than the OEM plastic line. Was unable to find any 1/8" air line locally but was rescued when th Motorcraft sending unit that I ordered late Friday afternoon showed up on Saturday about noon and saved me having to reinstall the old part until I could order some 1/8* air line and install the Bosch 8216. Installed the new Motorcraft sending unit and noted the bad one that was installed last October was also a Motorcraft. Replaced the 5W30 oil that the oil change place (the mechanic we use when necessary owns a Rapid Rotation and they do oil changes as well), with 20W50. Started the truck and had oil pressure with the gauge needle slight left of midline. Drove the truck a couple of miles to see if the pressure would hold and it did. Went out this morning, checked the oil,started the truck, and had no oil pressure but the engine sounded normal so I drove it about four miles to a park where we walk, shut it down, and left it for about an hour. Started it up to return home and we had oil pressure with the needle above the "m" in Normal, best it has been. Drove it home and oil pressure remained constant. Later this morning, we drove the truck about 50 miles to a nearby town and the gauge needle stayed above the "m" and actually moved above the "a" in Normal at 65 mph. The oil pressure remained at this level on the return trip and the engine seemed to run fine. I am unsure if the improved oil pressure is due to (1) the new gauge, (2) the thicker oil, or (3) the fact that I noted the wire connection to the sending unit was slightly loose so I crimped the connector a little before installing it on the new sending unit. Regardless, the truck is much improved and I appreciate the helpful suggestions from the forum members. Thank you.
 

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