I have a 1990 F350 7.3 and need to locate the power source because the gage doesn't register and the engine light is on all the time.

 

Look at the rear of the engine, passenger side. Easier to see if you take the air cleaner off. The sender should be back there, possibly screwed into a short riser pipe.

 

Thanks, I did find that but it's not getting any power so the gage doesn't register and the engine light stays on. The wire that is attached to the sender has no power. Do you know where that power source is?

 

I assume that you changed the sender.

The power wire from your oil pressure sender should be White/Red Stripe.

It goes through the main 8-pin Glow Plug Harness Connector (that connector on wiring diagrams will be labelled (C129) at the position I've marked with an 'X' - I'll mark the two pins that connect the two main power wires to the GP Controller Relay with a 'P' for reference. Those are the largest wires going through the connector.

O O O P ----toward front of truck--->
X O O P

From that pin (X) on the Starter Relay side of the 8-pin connector (C129), the wire remains the same color, White/Red Stripe, It goes into the loom that goes toward the Starter Relay but does not make a connection at the relay. It continues on into the main harness that comes from under and up beside the passenger side battery where that harness is attached to the inner fender....

...continuing in that harness to the main connector on the fire wall, which is connector 'C100' in wiring diagrams. There are no other connectors, fuses, fuse links, splices or components in that circuit under the hood. In looking for the cause I would advise checking the harness back to the main connector on the firewall, for evidence of rodents (ie: mice) chewing on the wiring.

From that main connector (C100) on the Fire Wall, the wire goes to the connector on the back of the gauge with another short White/Red Stripe wire coming out of that same gauge connector going to a connector at the 'Engine Warning Indicator Module' (thus your engine warning light being on).

It looks like the voltage supplied to this Oil Pressure Gauge wire circuit is coming from it's connection to the 'Engine Warning Indicator Module' which is energized when the key is turned on and that is working so there is an open circuit (broken wire, bad connection, etc)
in that White/Red Stripe wire.

NOW, Not shown in the wiring diagrams of my first edition of the 87-91 factory shop manuals, is a RESISTOR in this circuit which in your truck is probably on the back of the instrument cluster.

I want to post this right now before I mention that resistor because there will be a delivery truck arriving here very shortly and I have to work with these guys to show them how to get in and do what they have to do so I don't want to get stuck not being able to post this until that job is done.

The resistor has to do with the change in these trucks from using a full variable resistance oil pressure sender to a partial one that functions more like a temperature switch.

More on that later

 

Thanks for so much great info. The C129 connector shorted out and part of it got burned. I wanted to replace the whole connector but couldn't find one so I'm keeping part of it and splicing the wires that got damaged. I would really like to find the C129 connector with diagrams to match the wires properly so if you have any more great info I would appreciate it. Again, thanks.

 

technically, the oil pressure sender does not get "power". it is a ground circuit.