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Canadian Poncho Homepage -> Canadian Poncho -> 65-70 Full Size Body Tech Forum -> 65-66 US gauge cluster oil sender recalibration
Post Info TOPIC: 65-66 US gauge cluster oil sender recalibration
4SPEED427


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65-66 US gauge cluster oil sender recalibration
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Some of you may be tired of me talking about these nice US clusters (with oil and temp gauge) that we never got in our Canadian cars. 

They are hard to find, expensive to buy and on top of that they use a unique oil pressure sending unit that was never used in any other GM vehicle. Those senders are tough to find and I've seen them sell for $500 US when they do come up for sale. The thing that makes them unique vs other GM oil pressure gauge sending units is the resistance the unit provides for the gauge ground. Most GM sending units start at 0 ohms resistance for 0 psi and then go up to 90 ohms. They have two different 90 ohm senders, one is 90 ohms at 60 psi, the other is 90 ohms at 80 psi, depending on the gauge in the particular vehicle. 

The one for this cluster is from 0 ohms to about 31 ohms and the gauge goes to 60 psi, so if you use one of the other senders it pretty much pegs the Pontiac oil gauge as soon as you start the engine. 

Years back a guy on a US Pontiac forum took apart a 90 ohm 60 psi sending unit and rewound it so that it is 31 ohms at 60 psi. He did this by removing about 2/3 of the resistor wire on the rheostat in the sending unit and rewinding the remaining 1/3 of the wire along the same distance on the resistor board. Today I decided I needed to try this. I had a used 90 ohm 60 psi sender so I ground down the crimp and opened it up. 

This is what an the sender looks like before starting. I ground around the crimp that holds the can to the base, made it pretty thin, then went around with a screwdriver and pried it open.

os1a.jpg

This is what you see when you remove the outer can of the sender. 

os1.jpgos2.jpg

My crappy pic shows it poorly but the plastic board held on with 2 rivets has a wire wrapped around it, each strand being spaced apart from the previous strand, going from one rivet to the other. The copper contact slides along the board in an arc and you can see it slightly in the top of the two pictures above.  The 2 rivets need to be carefully drilled enough to tap them out and then the board is free from the unit.

os3.jpg

os4.jpg

Once that is done the wire is removed. Put an ohm meter probe on one end and then put the other probe on places along the length of wire until 31 ohms shows on the meter. Determine where the moving contact "peaks" on the board and rewrap the wire so the 31 ohm spot on the wire lines up with that spot on the board. When the contact gets to the high end of its travel it will be sitting on the 31 ohm spot. Put the board/wire assembly back in place, and push the rivets through. I held it in place with the 2 blue plastic clips for testing purposes. My primitive test setup using the air tank and tire gauge is shown.

os5.jpg

os6.jpg

Once I determined I had the unit recalibrated for 31 ohms @ 60 psi I used JB Weld to hold it in place since I sure don't have any little rivets like that!

os7.jpg

Tomorrow when the JB Weld has cured I'll put the can back on, use JB on it as well and double check to make sure it's working. No guarantees this will work but the guy in the US has done a number of them with success. This is my second attempt, first attempt about 12 years ago failed when I broke the little plastic board...

Updates to follow once I get more done. Sorry for the crappy pics, my old cheap phone is awful.



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
cdnpont


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Excellent Carl! I've never seen or even thought to think of what was in those cans. Low and behold it's actually a electro mechanical device. Basically just like a tiny fuel sender.

I'm guessing as long as you space the winding such that the contactor will make the wire continuously on the sweep up, it'll work fine? Would another way be to use a less resistive wire full wound for the full length? 

 

Took me a minute to figure the path, but I assume it's this?

Screenshot 2024-12-20 102045.jpg

 



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4SPEED427


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cdnpont wrote:

Excellent Carl! I've never seen or even thought to think of what was in those cans. Low and behold it's actually a electro vac mechanical device. Basically just like a tiny fuel sender.

I'm guessing as long as you space the winding such that the contactor will make the wire continuously on the sweep up, it'll work fine? Would another way be to use a less resistive wire full wound for the full length? 

 

Took me a minute to figure the path, but I assume it's this?

Screenshot 2024-12-20 102045.jpg

 

 You have it well analyzed Mark, precisely. 

I was thinking as I lay in bed last night, I need to measure the resistance of the left over wire and see if someone makes a wire with about 1/3 of the resistance. If that is possible then I could do exactly as you suggest, a full rewind with the same result, 31 ohms at 60 psi.  (Because I removed about 2/3 of the wire.) Yes, I've wound it so it never has an open anywhere along it's travels from low to high but it would be nice if the windings could be tighter.

I'm hoping to find an early GM 60 psi sender in this stubbier style (below) to cut open and see if they might be a different design, maybe easier to modify? Who knows until I find one. I used to have one but can't find it, I must have used/sold it.

662.jpg

 



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
Canadian Poncho


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Could a common sender be used with a resistor in the harness to reduce the reading from the sender to match the gauge?


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cdnpont


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Would adding a resistor in line not make the gauge fail to read zero?

Is there any way where you could limit (or modify) the travel of the contact point? Have it max out at the 31 ohm point on the winding.



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4SPEED427


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cdnpont wrote:

Is there any way where you could limit (or modify) the travel of the contact point? Have it max out at the 31 ohm point on the winding.

 I'm going to do up a drawing later that will illustrate it but I don't think that works. 



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
CdnGMfan


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Yes, you too can perform surgery in the comfort of your own home. All you need is an unobtainable sending unit and nimble little fingers. And someone to wipe your brow and pass the needle nose pliers... wink



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4SPEED427


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cdnpont wrote:

Is there any way where you could limit (or modify) the travel of the contact point? Have it max out at the 31 ohm point on the winding.

ops.jpeg

Ok, so a very crude drawing of an oil sender function. This looks nothing like the sending unit actually does but the principal is right. Oil pressure causes an arm to move which swings a copper contact along a board that has resistance wire wound around it. Just before the contact reaches the far end (rivet #2) the arm hits a stop (when 60 psi or greater pressure is achieved) and that is the point where I need approx 31 ohms vs. the original 90 ohms.

My thinking is if I limit the travel, it narrows the amount of contact travel leading to less accurate readings (the farther the contact travels, the more accurate the reading) but more importantly, if I limit the travel of the arm that means it won't move after a certain psi is achieved which would be less than 60 psi. i.e. if I limit the contact travel to half the distance between rivet #1 and rivet #2 then once we get past 30 psi the gauge would always read the same.

They do make this same sender in 80 psi but my thinking is that would be even worse because it takes 80 psi to move the contact from rivet #1 to rivet #2 whereas this one originally did that with 60 psi.

This afternoon I measured the piece I removed from the sending unit and didn't use. It is about 16" long. I never thought to measure the piece I used but I'm guessing it's about 8" so there would have been 24" of wire in there originally and that 24" piece gave 90 ohms resistance. To me the only solution is to find wire that will give me around 30 ohms resistance from a 24" length and then I can rewind the sender with the whole 24" of wire.

Is that all making sense?

 



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
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