# Automotive pressure sender issue

#### leftwing27

Joined Dec 5, 2015
4
Hello all,

Looking for some help getting an oil pressure gauge working correctly in a '67 Mercury. The issue is that the original pressure sender delivered a resistance range of roughly 10ohm - 100ohm (100ohm = low oil pressure, 10ohm = high oil pressure) to the gauge, but the aftermarket senders manufactured today operate over a much larger range of resistance and pressure. So, for instance, where 40psi of oil pressure with the original sender would equate to 30ohm of resistance, the same 40psi with the new sender equates to 100ohm of resistance. These numbers are not 100% accurate, just examples to explain the issue. I know the gauge is good because grounding the sender wire pins the gauge at max oil pressure and this seems to be common issue in cars from this era when searching online about it.

So what I'm looking for, if possible, is a 12volt circuit that will take a resistance level and reduce it by half (or there about). I'm not looking for complete accuracy here at the gauge so don't worry about all the data and testing that would need to be done for that, I simply want the gauge to read an acceptable psi while the engine is running. I've hooked up a mechanical oil pressure gauge and know that the engine produces between 40 and 50psi of pressure under normal operation. With the new sender the in-dash gauge reads just under 20psi when the engine is cold and basically 0-5psi when the engine is warmed up.

I'm pretty green when it comes to circuitry and components but do have some experience assembling and repairing circuits so I should be able to follow some basic instructions. Or maybe there's something off-the-shelf that would achieve this. Any help is appreciated.

Thanks,
Paul

#### MikeML

Joined Oct 2, 2009
5,444
To get this to work, you would have to do two tests, log the data, and plot the curves...

Test 1. (In the car). Connect a 100 to 250Ω pot as a rehostat between the gauge wire and ground with the ignition on. Log the voltage across the rehostat and the current through it using a DMM at min pressure, max pressure, and whatever other marks are on the gauge...

Test2. Obtain the sender's manufacturer data for the new sender, either from a data sheet, or by actual measurement on the bench of resistance vs pressure. Need more than just the end points...

If you can get these two plots, we can do something with it. Else, I wont waste my time...

Last edited:

#### leftwing27

Joined Dec 5, 2015
4
To get this to work, you would have to do two tests, log the data, and plot the curves...

Test 1. (In the car). Connect a 100 to 250Ω pot as a rehostat between the gauge wire and ground with the ignition on. Log the voltage across the rehostat and the current through it using a DMM at min pressure, max pressure, and whatever other marks are on the gauge...

Test2. Obtain the sender's manufacturer data for the new sender, either from a data sheet, or by actual measurement on the bench of resistance vs pressure. Need more than just the end points...

If you can get these two plots, we can do something with it. Else, I wont waste my time...
Thanks, I'll try to get this data today and post it up here.

#### leftwing27

Joined Dec 5, 2015
4
Ok, here's the data. The operating range of the gauge is 0 to about 25ohms. The new sender isn't too far off but on average is delivering twice the resistance needed. I have attached a chart with the data plotted. Let me know what you think.

Thanks
Paul

#### Attachments

• 12.4 KB Views: 11

#### MikeML

Joined Oct 2, 2009
5,444
Ok, but I need the voltage across the rehostat at each point so I can compute a Thevenin Equiv. of the gauge. I'm guessing you measured V an I at each point and divided V by I to come up with the red plot?

#### leftwing27

Joined Dec 5, 2015
4
Ok, but I need the voltage across the rehostat at each point so I can compute a Thevenin Equiv. of the gauge. I'm guessing you measured V an I at each point and divided V by I to come up with the red plot?
No, the red plot is what the in-dash oil pressure gauge read at the given resistance set on the pot. The green plot is the resistance that the new sender provides at the given psi. So you can see that the in-dash gauge will basically always read 0psi (or close to it) when engine is running a normal 40psi of oil pressure. If you still need the voltage readings, let me know. I will get them during the day. Thanks again.

#### MikeML

Joined Oct 2, 2009
5,444
I would like to see the voltage across the pot at each red point...

#### tcmtech

Joined Nov 4, 2013
2,867
Given the time and components required to make any type of signal adapter you would be time and money ahead to just buy a correct replacement. The ~100 - 10 ohm oil pressure transducers were a common component used on millions of vehicles and engines in other applications so finding one should not be hard or expensive.