Oil pressure keeps going to 0

[vtboy51]

This has happened about 4-5 times over the past year, I'll start the bronco and get 0 oil pressure on my stock gauge. For obvious reasons the first time I freaked out, thinking I had a major problems so I bought a mechanical gauge, installed it and had 50psi. After confirming this, I reinstalled the original oil pressure sending unit, restarted the bronco and the gauge now read correctly.... I'll also add, before swapping in the mechanical gauge I grounded the wire that connects to the sending unit and my gauge moved, which told me the gauge was good.

Like I said this has happened a few times and each time removing the sending unit and reinstalling it fixes it, but I'm not sure why. Does the sending unit need a good ground, which might be intermittent??

 

[phred]

Sound to me that the sending unit is defective.

 

[vtboy51]

Sound to me that the sending unit is defective.

Sorry, I forgot to mention I have replaced the sending unit after this happened a couple of times. This is why I was thinking it might be a grounding issue, but I don't understand how the system works. Does it rely on a ground path through the engine block?

 

[Broncobowsher]

Does the fuel and temp gauge also drop to E/cold? could be an IVR.
Did you get any valve clatter when it read 0?
Next would be a (long term/permanent) install of a mechanical gauge.

Still have not determined if it is a gauge issue or an engine issue.

 

[vtboy51]

Does the fuel and temp gauge also drop to E/cold? could be an IVR.
Did you get any valve clatter when it read 0?
Next would be a (long term/permanent) install of a mechanical gauge.

Still have not determined if it is a gauge issue or an engine issue.

All the other gauges work fine when this happens and I get no engine noise. I almost immediately swap in the mechanical gauge to confirm I have pressure when this happens and I do.

 

[jamesroney]

Sorry, I forgot to mention I have replaced the sending unit after this happened a couple of times. This is why I was thinking it might be a grounding issue, but I don't understand how the system works. Does it rely on a ground path through the engine block?

You actually do understand how the system works, because you stated it in your last sentence.

The gage uses a traditional d'Arsonval movement and relies on current passing thru the gage to produce a torque on the coil attached to the needle. The current path starts at the positive terminal of the battery, goes thru the ignition switch, thru the IVR, then to the + terminal on the gage, to the - terminal on the gage, to the engine harness wiring directly to the post on your oil pressure sender. The oil pressure sender is a variable resistor with one end tied to the 10-32 terminal, and the other end grounded via the 1/4 NPT nipple.

The vast majority of failures as you have described are due to intermittent contact between the (now 50 year old) white/red wire, and the molded 90 degree boot that slips on to the sender terminal. This can be easily verified by using a high reliability alligator jumper wire to make the interface between the sender and the wire. That wire itself also gets old and brittle.

There's nothing wrong with your engine. You've done everything right. Keep going, and don't worry. It will run for a long time without an oil pressure gage.

 

[tirewater]

Sorry, I forgot to mention I have replaced the sending unit after this happened a couple of times. This is why I was thinking it might be a grounding issue, but I don't understand how the system works. Does it rely on a ground path through the engine block?

Yes, and James description is as good as you'll get. Your scenario sounds like a loose connection. In my experience, a poor ground between block, extension and sender - possibly due to overzealous use of teflon tape or sealant - manifests as an inaccurate gauge rather than on & off behavior. Check the entirety of the brown wire. From the gauge attachment to the entrance in the firewall.

 

[bmc69]

Yes, and James description is as good as you'll get. Your scenario sounds like a loose connection. In my experience, a poor ground between block, extension and sender - possibly due to overzealous use of teflon tape or sealant - manifests as an inaccurate gauge rather than on & off behavior. Check the entirety of the brown wire. From the gauge attachment to the entrance in the firewall.

I've seen the use of teflon tape on the sender threads cause the issue OP is seeing. I never use teflon tape myself.

 

[vtboy51]

Thanks for all the feedback. I definitely have been using Teflon tape on the threads so I'll remove it and see if that helps in addition to inspecting the wiring.

 

[36Fan]

I would start at the simplest place, the wiring harness for a bad wire.

 

[Broncobowsher]

The teflon PASTE works fine for electrical connections. The tape can be to be an insulator.
Paste fills voids and acts as a lubricant as the threads engage. But squishes out of the way for metal to metal contact. The tape doesn't always squish out and can remain an insulating layer.

 

[Steve83]

...Teflon tape on the threads so I'll remove it...

Just run a temporary jumper wire from the sender body to a known ground to check briefly if the gauge comes back to life.

 

[DirtDonk]

You said when you grounded the fitting, the gauge moved. That’s not enough.
When you grounded it, then went in and turned the key on, did it go through full sweep? Or did it just move a little?
When it’s fully grounded the needle needs to go all the way past the top. If it doesn’t, then you still need to narrow down where the problem is.
Is it the connector, the wire, or the gauge.

I didn’t catch it, but what year is your bronco? If it’s a 70 or older, check the firewall connectors. The two big rectangular connectors that pass most of the wires through the firewall.
Could be as simple as disconnecting, then reconnecting them, to break a little oxidation on the contact points.
Best to do this with the battery disconnected!

 

[vtboy51]

You said when you grounded the fitting, the gauge moved. That’s not enough.
When you grounded it, then went in and turned the key on, did it go through full sweep? Or did it just move a little?
When it’s fully grounded the needle needs to go all the way past the top. If it doesn’t, then you still need to narrow down where the problem is.
Is it the connector, the wire, or the gauge.

I didn’t catch it, but what year is your bronco? If it’s a 70 or older, check the firewall connectors. The two big rectangular connectors that pass most of the wires through the firewall.
Could be as simple as disconnecting, then reconnecting them, to break a little oxidation on the contact points.
Best to do this with the battery disconnected!

Finally got some time to check in on this. I grounded the wire, turned the key on, and the gauge did a full sweep to full. I'm thinking that I may of "over tefloned" the fitting creating a bad ground. I've cleaned things up and will keep an eye on things.

 

[DirtDonk]

Sounds good. Sometimes it’s just a matter of the original fitting stretching and having a loose connection to the sender. Sometimes the Teflon tape gets in the way.
You could try holding the fitting tight while someone watches the gauge. Or you could also take a tiny wire brush and clean the inside out. Just in case it’s getting corroded.
That happens as well. Kind of normal in fact…