9" Limited Slip Question
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Cargo Master
Joined: May 2011
Posts: 2,387
Likes: 138
From: Northern California
9" Limited Slip Question
I have had my 75 F100 4x4 for 2-1/2 years now... I have gotten to know it well but it also continues to surprise me.
This weekend, I was parking my boat; backed up my concrete driveway (a bit of a slope - 10' long - dry) to the side if the house. During the process, the relative light weight of the truck showed...
Basically, if I wasn't careful, the rear end would start spinning. I was able to park the boat in 2x4; just had to nurse the clutch. There is nothing wrong with the truck but I made an observation.
Both rear tires were spinning. So its my assumption this thing has a really tight limited slip? It wasn't spinning for very long at all; as soon as I felt it let go I stopped. Damn 390 has so much torque it was breaking traction at idle.
Many 4x4's I have owned previously did not have the limited slip... So this is new to me. Did such a feature come from the factory? If so, is it normal for it to be working so well 40 years & at least 200k (way beyond mechanical limits) later? Not complaining at all but I thought the slips wore out after a while.
This weekend, I was parking my boat; backed up my concrete driveway (a bit of a slope - 10' long - dry) to the side if the house. During the process, the relative light weight of the truck showed...
Basically, if I wasn't careful, the rear end would start spinning. I was able to park the boat in 2x4; just had to nurse the clutch. There is nothing wrong with the truck but I made an observation.
Both rear tires were spinning. So its my assumption this thing has a really tight limited slip? It wasn't spinning for very long at all; as soon as I felt it let go I stopped. Damn 390 has so much torque it was breaking traction at idle.
Many 4x4's I have owned previously did not have the limited slip... So this is new to me. Did such a feature come from the factory? If so, is it normal for it to be working so well 40 years & at least 200k (way beyond mechanical limits) later? Not complaining at all but I thought the slips wore out after a while.
Cargo Master
Joined: Jul 2005
Posts: 2,309
Likes: 364
From: Kansas
if both rear tires had equal traction, they could still both spin, even in the absence of a trac-loc...the only way to know for sure is to jack up the rear end and try turning one wheel and see what the other one does. you could look at the axle code on the door, or the tag bolted to the diff, but after this many years one can't be certain they are original and correct...
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
Likes: 784
From: Simi Valley, CA
9" with Limited Slip optional beginning in 1968. 1959/67 F100's with Limited Slip came with Dana axles.
If the AXLE code stamped on the Warranty Plate begins with a letter: Limited Slip. If with a number: no Limited Slip.
9" axle ID tag will also reflect Limited Slip if an L is located within the ratio (example: 3L25). The tag is located under one of the bolts at approx. 5 o'clock that retains the carrier to the housing.
But, by now...the tag could be missing.
btw: Limited Slip uses 4 lined and 4 steel clutch plates, when the lining wears down, Limited Slip no longer works.
Limited Slip also requires 4 ounces of friction modifier that is added to the 90 wt. gear lube.
If the AXLE code stamped on the Warranty Plate begins with a letter: Limited Slip. If with a number: no Limited Slip.
9" axle ID tag will also reflect Limited Slip if an L is located within the ratio (example: 3L25). The tag is located under one of the bolts at approx. 5 o'clock that retains the carrier to the housing.
But, by now...the tag could be missing.
btw: Limited Slip uses 4 lined and 4 steel clutch plates, when the lining wears down, Limited Slip no longer works.
Limited Slip also requires 4 ounces of friction modifier that is added to the 90 wt. gear lube.
Junior User
Joined: Jun 2014
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Yes - put it in neutral. (block a front tire before jacking it up.)
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
Likes: 784
From: Simi Valley, CA
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Cargo Master
Joined: May 2011
Posts: 2,387
Likes: 138
From: Northern California
It did... Power assist anyways... Was leaking everywhere so I have been running it as manual steering until I collect all the parts to finish the 78/79 power steering conversion.
Kinda surprised about the ratio... Its so freaking low already I assumed it had 4.10s. I will check this when I put it up to confirm the slip is still working.
Kinda surprised about the ratio... Its so freaking low already I assumed it had 4.10s. I will check this when I put it up to confirm the slip is still working.
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