Quick question about diffs (hopefully) (1 Viewer)

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seakayaker32

seakayaker32

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Oct 28, 2014
Threads
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Location
Peoria, IL
Hi guys,

So silly me forgot to unlock my hubs (yes I do have a lunchbox locker in the front) and took my fj to work last week (about 5 miles of interstate driving). I wound up breaking the pinion shaft. My fj is a 76 (Course spline pinion shaft). I have a parts fj that's a 78, which obviously has the fine spline pinion shafts. Can I just swap out pumpkin for pumpkin (including the output flange of course), or is it more complicated than that?

Do I need to swap out both pumpkins, or does it matter that my front would have fine spline and my rear has course splines?
 
Hi all,

You should not have busted the front differential pinion shaft unless the transfer case was in 4 wheel drive!

Yes, the 1978 third member (differential chunk) will bolt in as a replacement for your existing 1976 third member though there may be a pinion flange problem (requires changing the flange on either the pinion shaft or the end of the drive shaft.)

Regards,

Alan


seakayaker32 said:
Hi guys,

So silly me forgot to unlock my hubs (yes I do have a lunchbox locker in the front) and took my fj to work last week (about 5 miles of interstate driving). I wound up breaking the pinion shaft. My fj is a 76 (Course spline pinion shaft). I have a parts fj that's a 78, which obviously has the fine spline pinion shafts. Can I just swap out pumpkin for pumpkin (including the output flange of course), or is it more complicated than that?

Do I need to swap out both pumpkins, or does it matter that my front would have fine spline and my rear has course splines?
Click to expand...
 
Hmm. I know for a fact I had the tcase shifter in 2wd. Hope I don't have tcase problem, too.
 
Could you steer it before it broke? That must have made for some funky bound up handling.

I agree that to break the shaft, the pinion had to be locked to the t-case/rear drive in some way. (Although you may be talking about the cross shaft inside the differential carrier.?)

If you have access to both, then yes, swap them both. The fine spline is a better set up. The front should be good because they don't get much use, but the rear should be gone through-have the pattern checked, and the bearing pre-loads checked.
 
Your thinking is sound. You can swap them out, so long as the gear ratio is the same. The pinion flange will need that end of the donor drive shaft as well. If the gears are different then as stated here before just swap both of them out.
 
Cruiserdrew said:
Could you steer it before it broke? That must have made for some funky bound up handling.

I agree that to break the shaft, the pinion had to be locked to the t-case/rear drive in some way. (Although you may be talking about the cross shaft inside the differential carrier.?)

If you have access to both, then yes, swap them both. The fine spline is a better set up. The front should be good because they don't get much use, but the rear should be gone through-have the pattern checked, and the bearing pre-loads checked.
Click to expand...

That's just it, it was steering fine. A few seconds of vibration and before I could pull over, BAMM!!
 
Interesting. Maybe you had a pinion bearing failure at speed. Was the diff full of oil?

Post pics of the carnage.
 
have you torn it down yet? If so get us some photos please.

On the face of it - hubs locked but Tcase not engaged for 4wd nothing should break as all the hubs do is spin the wheels with the axles. But having a lunchbox locker in front (a no-no IMO) might be what caused something to bind and break.
 
image.jpeg
image.jpeg
 
Nice. Not your pinion shaft. That's the pinion. Very nice breakage/carnage.

That outer pinion bearing must have seized. I wonder why?
 
Cruiserdrew said:
Nice. Not your pinion shaft. That's the pinion. Very nice breakage/carnage.

That outer pinion bearing must have seized. I wonder why?
Click to expand...

The slip yoke was jammed, too. I thought it was because it hit the pavement at 60mph, but it could have been the cause, maybe???
 
shawnfj40 said:
The pinion flange will need that end of the donor drive shaft as well.
Click to expand...


1976 and 1978 would have the identical drive shaft bolt pattern pinion companion flange....


:meh:
 
Are you spring over?
 
Is your front differential pointed up at the t case? This can starve the pinion bearings of oil at speed. Some guys will add a filler port higher up on the front or at the top off the diff so you can fill the diff with more fluid to compensate.
 

No. I've wanted to a cut and turn because the front dl is pretty much at max articulation, but I can't afford it right now. As a temporary fix I fabricated a new slip yoke using one out of a 2000 4Runner.... It has more articulation than a 40 U-joint, but it doesn't solve the extreme angle the dl is in.
 
The the front locker and hubs engaged could have caused some drag which at higher speeds (you mentioned interstate) could cause axle wrap and/or spring compression. the diff would have been turning the driveshaft even if it was spinning freely at the tcase. It could have started binding at the flange. Were you accelerating or at constant speed? Any damage at the flange that looks like binding? Just a possible fail mode.
 
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