Re: Oil pressure readings
I was able to put in a different sender to make the factory gauge work, here's what I did. I realized the original switch was either go or no go on the pressure, with the engine off, it was open, and with pressure applied it was about 30 ohms. The resistance doesn't change, from 5 PSI and up. I got a sender from Autozone for a 87 mustang I (I think that was the application), anyway you can tell it's different, just by the size of it. The one you want is about 2 1/2" in diameter and about 3" tall. Obviously it wouldn't fit in the original location, so I relocated it on to the fender well. The hole in the block is 1/8 NPT, so I plumbed it in stainless by using the following: 1/8 MNPT to 1/4" Swagelock, 1/4 stainless tubing from the block to the sender ( I added some coils to the tubing so it would flex and not break- kind of like the brake lines off the master cylinder), 1/4 Swage to 1/8 FNPT, and sender. The sender mounted to the fender well with a screw and a hose clamp. I used a piece of rubber between the sender and the hose clamp for a cushion, so as not to damage the sender. Now for the electronics part:
I wasn't sure how the dash gage would respond to a varying input resistance, so I plotted it out using some different resistors I had laying around (you could use a decade box- but I didn't have one). My gage wasn't quite linear, but it was close enough. Resistance is from sender wire to ground. Around 63 ohms the gage was low redline, 40- low normal, 20- mid scale, and and 10- high normal. Then I measured the sender that I had purchased by hooking up to a hand pump and Teed in a mechanical pressure gage. The pressure input was slowly increased as I measured the resistanse of the sender, and plotted that curve out. I wanted 5 PSI to be redline, 10 PSI low normal, and 30 PSI midrange. After a little math, I added a 20 ohm resistor in series with the sender wire, and hooked it up to the sender. Using the hand pump again, I slowly added pressure, and verified the dash gage responded, it did, so I hooked it up to the motor, and have been using this setup for 3 years without problems.