Rally dash oil pressure gauge issue.

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Dartswinger70 said:
I have no gauge movement at all when the car is started and running and the last I tried touching the sender wire to ground it moved all the way to high
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Geez!
That will burn up the gage.

Use a 10 ohm resistor or variable resistor next time for testing any of these gages.
Or do what I did. I had a bunch of 22 ohm resistors. Use one alone the gage should go the middle mark. Use two twisted together in parallel and that's 11 ohms.
Put two in series and that's 44 ohms.
 
Mattax said:
Geez!
That will burn up the gage.

Use a 10 ohm resistor or variable resistor next time for testing any of these gages.
Or do what I did. I had a bunch of 22 ohm resistors. Use one alone the gage should go the middle mark. Use two twisted together in parallel and that's 11 ohms.
Put two in series and that's 44 ohms.
Click to expand...

That may be it. The gauge burned up. If I open it is itvrepairable? There seems to be no one making reproduction ones I haven't checked e bay yet.
 
Dartswinger70 said:
That may be it. The gauge burned up.
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may be what?
The gage went to the top because resistance was practically zero.
If you left it that way long enough, then the heat wire would burn up and it want move at all. Since it moved its not dead - yet.

Here's how it works.
When we simulate a sender with 10 to 11 ohms of resistance, the gage's needle will move to the top mark.

On the graph follow the yellow line from the 10 ohms on the left across to the vertical line above the top mark.
upload_2021-5-5_21-38-57.png



To simulate the sender resistsance for the mid point on the gage, use 22 to 23 ohms.
upload_2021-5-5_21-40-19.png
 
One more. You wrote that you tested at 70 ohms, and it read low. I'll assume that was just above the first mark next to the L.
Something like this.
upload_2021-5-5_21-50-53.png


That's pretty close to what a Chrysler tester (or a copy like Mike made) says is a correctly operating gauge.
The green dots and the dotted line connecting them are the results from the test on my '67's rally gauge.
The circled C is the factory specification.
 
Keep in mind the circuit pulls a lot of current, particularly when the gauge is reading high. You should have 5 or 10 watt resistors. If you use a low wattage resistor, like a 1/4 watt, don’t leave it hooked up long or it will burn up and change value. also don’t touch it or it will burn your fingers, don’t ask how I know...
 
How sensitive is the system to resistance? I feel like the sender is good when checked with an ohm meter, and the wiring from the sender to the pc board is good. Could it be the printed circuit board? Would the guage peg high when the sender wire is grounded button respond otherwise if the pc board is bad? A short to ground is no resistance or ohms at all.
 
Dartswinger70 said:
How sensitive is the system to resistance? I feel like the sender is good when checked with an ohm meter, and the wiring from the sender to the pc board is good. Could it be the printed circuit board? Would the guage peg high when the sender wire is grounded button respond otherwise if the pc board is bad? A short to ground is no resistance or ohms at all.
Click to expand...

I think you need to test the gauge's response to at least two more resistances.
The test you did here suggests the gage and connecting circuit are working correctly, but the sending unit is not.
Dartswinger70 said:
I did an ohms test with engine running and it is 70 ohms and gauge reads low. I hooked up a snap on oil pressure gauge and I have 45 psi at idle. i am going to try a sender before going after the gauge. I seem to think or read that 70 ohms will real as "low" so I am thinking the sender is not working correctly.
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Why do I say that?
Ask yourself at 45 psi what should the sender resistance be?
 
Mattax said:
I think you need to test the gauge's response to at least two more resistances.
The test you did here suggests the gage and connecting circuit are working correctly, but the sending unit is not.

i did that before ordering a new sender, I then tested the new sender with an ohm meter. Engine off (70 OHM) then running(20-25 ohms) I do have 45 PSI at idle 55 PSI at 200 rpm checked with a snap on guage. I still have to do a resistance check, but I am in a natural mental process of elimination, the other guages work, and the dash PC board hasn't been replaced, still the 50 year old unit. The PC bords are available the guage and dash not so much people are selling junk on e bay with bent indiactoirs for $100 bucks LOL!


Why do I say that?
Ask yourself at 45 psi what should the sender resistance be?
Click to expand...

i would think that indicator would be right in the middle of the guage. Going by the tables provided.
 
I still have some checks to do but it whittles down to what is available for parts, do I want to tear my dash apart, and why if there isn't any serviceable parts available.

I would think there is away to repair the gauge if it is bad or burnt.
 
Dartswinger70 said:
25 ohms.
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About. Preferably closer to 20 ohms.

Here's the tester values someone here provided.
c-3826-jpg-jpg-jpg.jpg


Pink lines show why I say probably closer to 20 ohms.
Given 45 psi, go up to the green line. Then go across to the the left to see how much resistance is expected.
upload_2021-5-6_9-59-25.png
 
Dartswinger70 said:
I still have some checks
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Yes.
All of the things can be found if needed. Some will be a bigger PIA than others.
Finish the checks, then you'll know.
So far this is what we know for sure.
At approximately 45 psi, the sender was at 70 ohms.
upload_2021-5-6_10-6-46.png


And a sender at 70 ohms would cause the gauge to read near the Low mark. Which it did.
upload_2021-5-6_10-10-41.png


So the gage is responding properly to the input of the two tests you've conducted, 70 ohms and 0 ohms.

The pressure sender is way off based on the one test you've conducted. It was at 70 ohms when it should have been close to 20 ohms.

Now maybe you can help me out.
When you measured the resistance, was it very steady or did the needle bounce a little?
Video of that here Oil Pressure Gage and Sending Unit Revisited
 
Mattax said:
Yes.
All of the things can be found if needed. Some will be a bigger PIA than others.
Finish the checks, then you'll know.
So far this is what we know for sure.
At approximately 45 psi, the sender was at 70 ohms.
View attachment 1715733654

And a sender at 70 ohms would cause the gauge to read at near the Low mark. Which it did.
View attachment 1715733655

So the gage is responding properly to the input of the two tests you've conducted, 70 ohms and 0 ohms.

The pressure sender is way off based on the one test you've conducted. It was at 70 ohms when it should have been close to 20 ohms.

Now maybe you can help me out.
When you measured the resistance, was it very steady or did the needle bounce a little?
Video of that here Oil Pressure Gage and Sending Unit Revisited
Click to expand...
I will check again ad document it this weekend. I had a sender that was for a "Idiot light" originally. That was "open" while engine running hence the 70 ohms.

I have since purchased a "gauge" sender. I will check and post those readings and go from there. I also have to locate my resistors. I use a digital meter but I do have analog if that will show a "bounce" of the needle.
 
I'm sorry I thought you were using an analog meter. If you look at the video I was using a digital meter and it was jumping around. I was hoping someone had done the same test and could say if that was normal.

I'm going to test the sender I bought from Bill Rolick. Will use air pressure to see if its resistance matches the Chrysler specs and what my car's gage does.
 
No worries, I will get more into mine also. I will post what I find. It would be good for Rally dash owners to be able to fix what they can on these issues.
 
have you verified top end oil pressure at/beneath the sender? A gauge shows what you have and what you dont have.
 
Yes I have oil pressure verified with Snap on mechanics gauge. 45 at idle 55 at 2000 rpm. I did the inline resistor test with a 10 ohm resistor. The oil gauge went to what would be just under half on the gauge, or what would be a quarter tank on a fuel gauge. I also have pulsing voltage on my test light. At this point I am thinking it's the sending unit or a bad ground on the sending unit. It is new and I think it had thread sealer on it. I am going to pull it and take a second look.
 
Measure from the sender case to engine block to see if sender ground is good.

With a 10 ohm resistor the gauge should have gone to full scale.
 
Dartswinger70 said:
High resistance is low pressure and low resistance is high pressure?
Click to expand...

Yep, the 10 ohms at the sender should be full scale on the gauge. The lower the resistance, the more current flows. The more current flowing the more the gauge heats up. More heat means more needle deflection.
 
I should be able to hook a ohm meter up and check the sender with one lead on ground, the other on the wire terminal, and it should read high engine off, at approx 70 ohms, and with engine running and 45 to 50 psi it will be lower say 20 ohms. I want to do that as a test next.
 
This iswhere I am at. I did hook up a 10 ohm resistor, and the guage read somewhere near 1/4 tank or a little below half. I started the car with a digital ohmmeter and it read 10 ohms running. the guage stii didnt move. I am thinking possible dash printed circuit board. I had to solder pins on it when I had it apart, maybe not a good connection. Thinking I should pick up new ones if i pull the cluster may as well put in new boards. Worth a try as far as i can tell my guage should read a little below half and its not . I am thinking it should at least move...:)
 
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