Ive got a 2000 XLT 5 speed. The oil pressure gauge will come up to the low position when the key is turned on and pegs at the high level once the engine is started. I had thought it might be the pressure sending unit but what I have saw here and other places, had it went bad it wouldnt read any pressure or fluctuate around. I am now wondering if the pressure relef valve for the oil pump is clogged. Do I have the right suspicion or would that still be a sending unit problem?

 

Remove the 1 wire on the sending switch, see what needle shows, key on and key off.
Then ground that 1 wire and see what needle shows.

Ford uses a switch for oil pressure, so it is either on or off, so 0 or 1/2 on the gauge.
On most this switch is the ground for the gauge, when engine is off there is no ground, switch is open, so gauge shows 0, when engine starts and pressure is above 5 psi the switch closes and gauge is now grounded so shows 1/2 on the needle.

On others the switch is closed when engine is off, this turns on the oil light, when engine starts and oil pressure is above 5psi the switch opens and oil light goes off.

So what you are seeing on the gauge is electrical signals not actual oil pressure.

So most likely a wiring or sender issue.

 

I understand the static voltage. But why would it show and bounce off of high pressure?

 

Ok after consulting a few other people and racking my own brain, correct me if I am wrong on this; The pressure sending unit will send full ground with anything over 5psi of pressure. So in that then, the only way to get a high reading on the gauge would be for it to be getting more than the 5 volts so therefor the gauge would be bad resulting in the entire cluster needing replaced. If I am wrong please correct me.

 

Yes could be a voltage issue in the cluster or wire issue(shorting to power).

Simply unhooking the sending unit wire and then looking at the gauge with key on and then grounding that wire with key on will tell you what direction to go in.

On the 2000 Ranger is shows 3 connections for the oil pressure gauge
12v from a 7.5A fuse(looks like this is for most of the gauges), Red/yellow wire
Oil pressure Switch connection(closed if oil pressure is above 5psi), Dark Green/white wire
3rd wire is a ground labelled G201, Black/white wire

 

Ok appreciate the help. Im trying to help my dad and tell him what to do and whatnot, but He couldnt find the sending unit, so he took it to a buddy's shop, they went ahead and changed it out and it did the same thing. So they proceeded to try the grounding and it did the same. So they established that the gauge is bad. Just cut the wire and said to hell with it. Now, Where could an aftermarket pressure gauge be found that doesnt have a plastic or copper oil line ran to it? Would like to have an all electronic one if possible. And actual pressure gauge and not a dummy gauge like what they did.

 

The old PS60 senders(under $15) are electric/resistance senders so should work with any analog or digital oil pressure gauge.
These were the original oil pressure senders used pre-85.

Digital oil pressure gauges often come with a sender and can be had for under $40
You will most likely need an adapter to fit the new sender to the current "threaded hole", so save the old switch for sizing.

I found a circuit diagram for the newer oil pressure gauges, the 3 wire ones.
It looks like there are two resistance circuits in the gauge, when key is on engine off the 12v power is passing through both circuits so needle is low.
When engine is started the switch closes and becomes a better(least resistance) ground path so needle goes up.
So reads like the one resistance circuit has less resistance now, so needle starts higher with engine off, then goes very high with engine on, so it is working but needle is wrong.