My oil pressure gage is dead. I replaced the sender, and no luck. I did a continuity check to the point at which the wire enters the firewall. That's OK too. Is there an easy way to troubleshoot the gage without removing the instrument cluster? What is the easiest (and cheapest) way to get a schematic of the dash? Thanks in advance!

1991 F-150 2WD No A/C Manual 4.9L EFI

 

The cheapest way to get a good schematic is to check for a Chilton's or Hayne's manual for your year. I also understand that you can check your gauges by swapping the oil and temp leads and seeing if the oil gauge will move to a relative position that your temp gauge should show.

Slik

 

If your oil pressure sending unit has only one wire connected to it, try grounding it to the engine block. The gauge should read full scale deflection.

 

The gauge is actually an amp gauge. The sending unit it a resistor that varies acording to oil pressure. As the oil pressure increases, the resistance in the sending unit decreases, allowing more current to flow. More current flow = higher gauge indication.

A quick and dirty trick to determine if it's the gauge or sensor that's bad, is to remove the wire to the sending unit and, with the key in the "run" position, momentarilly ground the wire. The gauge should peg to the right. If it did, then the sender is suspect. If it didn't move, check for voltage at that wire. If there's voltage, it's time to pull the panel, as the gauge is probably dead or the connections are dirty. (gauge at a junk yard should be $10).

good luck!

 

It may be worth your time to check the connector between the engine compartment and instrument panel before you pull the panel out. I had a similar problem with the temp guage and found the contacts in the connector were covered with green copper oxide. I, unfortunatey, did not check this until after I pulled the panel and found the circuit from the sensor was open.

By the way, on my 1995 F150 Lightning, the oil pressure guage is a joke. It is wired to a pressure switch (on/off)sending unit just like an idiot light. It drives the guage through a 20 ohm resistor -- switch off = no pressure guage reading, switch on = a mid range normal pressure guage reading through the 20 ohm resistor -- doesn't matter what the pressure really is as long as it is above the switch point. Have not found the actual switching pressure in my Helm service manuals, but I suspect it is really low. On my 1995, grounding the wire at the sensor gives this same nice normal reading on the guage. I checked this out while I was in there because I had noticed the oil pressure on my truck never changed -- as soon as it started it popped up to the normal reading an held there without any variation. Hot, cold,low or high RPMs, always dead steady.

Hopefully your 1991 actually has an oil pressure guage driven by a sending unit as described in the above response -- either way I would check the connector before pulling the panel.



>My oil pressure gage is dead. I
>replaced the sender, and no luck.
>I did a continuity check to
>the point at which the wire
>enters the firewall. That's OK too.
>Is there an easy way to
>troubleshoot the gage without removing the
>instrument cluster? What is the easiest
>(and cheapest) way to get a
>schematic of the dash? Thanks in
>advance!
>1991 F-150 2WD No A/C Manual 4.9L
>EFI

 

I'd heard the same thing... that the sending unit is actually a switch. Seems to me that you could use an earlier model sending unit to get the gauge to be a "gauge", as opposed to an idiot lite!

 

Good point about the earlier unit. Jumpering over the 20 ohm resistor on the instrument panel board would not be difficult. What year would you recommend -- I am new to Ford trucks, only had it a few weeks.

The oil pressure and temp guages do appear to be identical even though I could not find a part number on them. The Helm manual says the temp guage should read right on the Hot mark (top of range) with 9.7 Ohms and on the Cold mark (bottom of range) with 74 Ohms. Same resistance range would be needed for the older oil sending unit.

 

I'd use something in the late 80's. Try a bone yard or aftermarket. I doubt the engineers put too much into changing this into an idiot lite / gauge. Probably the same gauge / different sender. If it was me, I'd pull one at a junk yard a see what happens.
DM

 

I checked my Helm's manuals that I have for a 78 Mercury Cougar. The manual states that 22 ohms should make the gauge goto about half scale. 73 ohms should make the gauge goto the L mark. If you were to use the sending unit from this vehicle you would need to jump the 20 ohm resistor out. By the way if you try this make sure you get the sending unit for the 78 with gauges. Gauges were optional that year.

 

The other folks are correct your truck has a 23 ohm resistor soldered on the back of the instrument panel and the when the engine makes 4.5 lbs of pressure it closes the switch in the oil sender unit thus completing ground and then the gauge shows about mid scale or so. I just changed this on an 89 I soldered a jumper accross the resistor. (don't get things to hot you can destroy the printed circuit) then replaced the small sender with a standard auto part number ps60 sender. This is alot larger canister looking part. Fired up engine and got almost a 3/4 gauge cold and just below half hot and the thing would move up and down with engine rpm. If this is a small block ford v8 you will have trouble fitting the canister in where the old one came from. On the older Ford small blocks they made an extension just for this purpose. If you ground the plug you should see your normal oil pressue setting if I does nothing then there is a gauge/wiring problem. If it works then you have a sender/engine problem. Make sure the engine is making the neede 4.5 lbs of oil pressure.