Hi everyone,my 1988 f-150 4x4 with 5.0 stock motor oil pressure gauge is pegged full left side/low side with engine on.It does have the bosch s334 sender,but was working fine until i replaced wiper motor.I hooked up a mechanical gauge good psi 15-50.Should there be power to the plug at sender with key on?There is only 1 wire to it,also if there is a short would the engine light come on?I want to keep the stock gauge working i teed mechanical gauge off the original sender.Thanks

 

If you disconnect the single wire at the sender/switch and turn the ignition
On the needle would peg the LOW scale and the engine light will be on.

If you short the single wire to ground and turn the ignition On, the gauge
should peg the HI scale.

If you had a oil pressure switch and replaced it with a sender to make your
gauge function as a real oil pressure gauge, did you remove/short out the 20
ohm resistor on the back of the oil pressure gauge circuit board area?

Also your problem could be a bad sender ground if you used teflon tape
on the pipe threads of the sender or switch.

 

The above post is very accurate.

 

Thanks for the info!

 

Ok guys im back, i grounded the plug at the sending unit with key on the gauge stayed pegged all the way past low, i did notice that when the key is off the needle creeps back into the low side.Could it be a bad wire?

 

If you had a good ground on the wire, it does appear you have an open circuit.

The factory oil pressure gauge was not a real oil pressure gauge.
It used an oil pressure switch. Not a sender. When 7psi of oil pressure
was sensed, the switch would close and 20 ohms of resistance would
cause the gauge to register about midway on the scale.

On my 88 there is a 20 ohm resistor on the back of the gauge.
When you short the signal wire to ground, and turn the ignition on, the
needle is half way on the scale.

If you unplug the wire and turn the ignition on, the gauge pegs low.

If you want to convert the gauge to a real oil pressure gauge, you
need to remove the 20 ohm resistor and solder a wire across where
the resistor was. When you short the signal wire to ground and turn the
ignition on, the needle will peg high.

When you turn the ignition off, the needle could point anywhere and
is not an indication of what the oil pressure is.

You also need to replace the oil pressure switch with an oil pressure sender.
The replacement oil pressure sender has to have the same resistance range as what the factor gauge used.

Here is a picture of the switch as used in the factory setup.



Here's a picture of what a sender looks like.

 

Thanks again i`ll look for a short in that wire.I finally got 2 days off to fix alot of small stuff.I put blue leds in the dash,LOOKS BAD @#$ turned the puke green into a teal color almost looks like FORD blue.