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I have a 1964 ford galaxie ,390,4spd car, car has a rear differential ford part number c1aw4025d, ratio is around 390. I am unable to find ifo on this differential, 28 spline axles, it is not a posi rear, limited slip maybe?what fluid to put in it? Thanks for any help
 

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shelby367,

How do you know it is not a posi rear? From the tag or just from the behavior? Have you had it apart? I ask because a posi rear with bad clutches will behave like a non-posi. Of course if the clutches are bad you will need more than fluid to fix it. If it's truly an open (non-posi) axle then standard gear oil will do. Buy it in the squeeze bottle so you can squeeze it in there.

- John
 

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shelby367,

How do you know it is not a posi rear? From the tag or just from the behavior? Have you had it apart? I ask because a posi rear with bad clutches will behave like a non-posi. Of course if the clutches are bad you will need more than fluid to fix it. If it's truly an open (non-posi) axle then standard gear oil will do. Buy it in the squeeze bottle so you can squeeze it in there.

- John
shelby367,

How do you know it is not a posi rear? From the tag or just from the behavior? Have you had it apart? I ask because a posi rear with bad clutches will behave like a non-posi. Of course if the clutches are bad you will need more than fluid to fix it. If it's truly an open (non-posi) axle then standard gear oil will do. Buy it in the squeeze bottle so you can squeeze it in there.

- John
Thanks for your answer, I do not have a tag for the differential, If I could decode the ford numbers it would tell me what the unit is supposed to be, open, posi,or limited slip. thanks for your help
 

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…rear differential ford part number c1aw4025d, ratio is around 390. I am unable to find ifo on this differential, 28 spline axles, it is not a posi rear, limited slip maybe?
Unfortunately, C1AW is the housing number, and bears no information of what parts were installed into that case. Either the door tag (warranty plate) or the differential tag would have the original info. As your tag is missing, it is possibly not original, and could have anything inside.

BTW, "posi" is short for Positraction, a GM trademark term for their limited-slip differentials. Ford used Equa-Loc (simple clutch-plate system) and later the Traction-Lock or Trac-Lok (a modified clutch system that increased clutch pressure with increasing torque). Unfortunately, but obviously, it's easier and more common to use "posi". :cautious: Ford also rarely used the Detroit Locker, which is a true "toothed" locking differential, and you'd know if you had one by the mild popping noises and light jerking they make in turns. Some hate it. I like it. 😁 While there are versions of all of those that will bolt right into your case along with the 2 and 4-pinion open-differentials; you'll have to pull your 3rd member if you want to know for sure what you have in there.

David
 

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Unfortunately, C1AW is the housing number, and bears no information of what parts were installed into that case. Either the door tag (warranty plate) or the differential tag would have the original info. As your tag is missing, it is possibly not original, and could have anything inside.

BTW, "posi" is short for Positraction, a GM trademark term for their limited-slip differentials. Ford used Equa-Loc (simple clutch-plate system) and later the Traction-Lock or Trac-Lok (a modified clutch system that increased clutch pressure with increasing torque). Unfortunately, but obviously, it's easier and more common to use "posi". :cautious: Ford also rarely used the Detroit Locker, which is a true "toothed" locking differential, and you'd know if you had one by the mild popping noises and light jerking they make in turns. Some hate it. I like it. 😁 While there are versions of all of those that will bolt right into your case along with the 2 and 4-pinion open-differentials; you'll have to pull your 3rd member if you want to know for sure what you have in there.

David
 

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Thanks for your reply , I removed the third member, it turns out to be a equa lock differential 28 spline axle, 390 ratio. I cleaned it up and put it back in the car. The unit was low on oil because the carrier was leaking oil from the area where the housing gasket was supposed to be. I will replace the carrier if I have any problems, it is cheaper to replace it than try to fix unit. thanks again for your help
 

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Equal-Locs are rebuildable, and the 5-tab clutch 'n steel kits are available and easy to do. About $100. 8", 9" and 9-3/8" are the same clutches. Crack yours open in order to verify if only the clutches are required. Full instructions and specifications for servicing are in your Ford Service Manual set.

David
 
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