ScottMH

how can i test the oil pressure sending unit for the gauge? my gauge stays in the center no matter what so i want to know what ohms it should be sending out for what pressures to see if its the gauge or the sender.

71coupe454

You can ground the sender plug to see if the gauge goes full sweep or not

ScottMH

good idea and i will do this. but how about testing the sender itself? noone knows the psi to ohms ratio?

71coupe454

I have never went that step, allways check the gauge first then replace the sender. But if theres a answer, I'm sure it will pop up in here soon.

0Willcox Corvette

What year is the car?

The electric oil pressure sender output should read.

0 ohms = 0
45 ohms = 40 lbs.
90 ohms = 80 lbs.

Ohm out the sender and see what it's putting out.

On the electric gauge..

If the needle is at 3 o'clock, it is missing the input ohms signal, has power and ground so check for a break in the sending unit wire.

If the needle is at 0, the sending unit wire is grounded out. The gauge has power and ground.

If the needle is at 10-15 lbs constant, the resistor bad on the back of the gauge. The gauge has power and ground.

Willcox

71coupe454

There ya go Scott, theres the good answer.. Are these gauges different than other cars or am I confused with a temp or fuel gauge? I'm sure one of them is suppose to peg out when ya ground it. I guess its a good thing my Techs are in the bays fixing cars and I stay on the computer/phone most the time..

0Willcox Corvette

Pat-

You are correct, the temp gauge will do this! To remember which gauge does what use this as a good rule.

A gauge without input will always do what you think is a good thing.. Example.. fuel without signal will go full, oil pressure will go to 80 plus and a temp gauge will go cool.

The opposite will happen when you ground out the ohms input wire.

Willcox

ScottMH

Thank you very much! Its on my 81 Corvette by the way.

ScottMH

ok guys here is what i got. I changed the oil so it has fresh 10w40 in it and started it up. the gauge went from 0 to about the 20 mark once the engine was started. if i rev the motor it twitches a little bit higher like in the 22-24 range if i had to guess. so from there i grounded the signel from the sender and it dropped the gauge to 0. is there a way to get it to peg? apply battery voltage? or will this mess it up? also. how would i set up my multimeter to read ohms at the sender?

0Willcox Corvette

To test the sender, put your multi-meter in the 200 ohms setting.

However, from what you describe, you have blown the resistor on the back of the dash unit. This resistor should read around 90 ohms.

You can see where the resistor is located in the picture below.



Willcox

jotto

Ernie,
What is the power rating of that resistor? I know you have said it should be a 90 ohm but would that be 1/4 watt, 1/2 watt 1 watt etc?

Think mine may be bad and want to try one just to make sure. Is it available as a spare or will any from an electronics store be OK?

ScottMH

i put in a manual gauge just to make sure. im makeing 40psi at idle and 50psi at 2000rpms. so it must be the gauge. thanks for all the help!

G dad

I think it would be 1/4 watt not sure though i am real rusty at this stuffl

0Willcox Corvette

a 1/4 watt will work just fine.

7T1vette

I think you will need something bigger than 1/4 watt. At the max draw on that resistor, it will see something like 3-4 watts (if my calculations are correct). I'd go with a 5 watt resistor, just to be safe.

r16678

Curious if anyone has looked at one of those new old stock oil pressure senders on ebay, wondered if it is ok to pick one up or will the internals of the sensor be to old.

mikem350

If you look at the orig resistor, its a high wattage wire wound part.