How do you know if you have positraction?

[71CHEVYSHORTBED402]

71 Chevy 1/2 ton short with 12-bolt rear-end. I have no reason to believe it's been rebuilt. I've seen positraction on a SPID before, but it's not on mine. I know nothing about rear ends (no pun intended), but this one has high gears. Doesn't much even burn the tires until it chirps when the transmission hits second gear.

My SPID is below, not that it necessarily matters:

[Fitz]

Originally Posted by 71CHEVYSHORTBED402

71 Chevy 1/2 ton short with 12-bolt rear-end. I have no reason to believe it's been rebuilt. I've seen positraction on a SPID before, but it's not on mine. I know nothing about rear ends (no pun intended), but this one has high gears. Doesn't much even burn the tires until it chirps when the transmission hits second gear.

My SPID is below, not that it necessarily matters:

Put the truck on a patch of dirt, put in gear and mash the gas to the floor (or pop the clutch) at start. If you find both wheels dug holes, it's a posi. If only one spun, it's an open carrier. If you don't care about tires, Universal joints, your trans or clutch you could perform this test on pavement.

[cdowns]

you could take the rear cover off and drain the 45year old fluid out and start with freash clean good stuff that way you can look at the gears too

[68350]

I agree, if you believe it still has the original fluid, open it up and change it out. Look at gears and post pics if you are unsure if its a posi. You will find some numbers on the gears, for example 41:10 which means 4.10 gears. Divide first number by second. Get yourself some good gear fluid and a new gasket and you'll be set to check your rearend out

[Stocker]

Always a good thing to have fresh fluid. Just to check for posi, jack up the rear enough to raise both tires off the ground.... parking brake off and trans in neutral. Rotate either rear tire by hand. If the opposite side tire rotates in the opposite direction it's an open diff. If it rotates in the same direction it's a posi.

[truckster]

Originally Posted by cdowns

you could take the rear cover off and drain the 45year old fluid out and start with freash clean good stuff that way you can look at the gears too

The absolute best way and a nice favor to do for that rear gear that's been working so hard for all these years. Just be aware that if it is a posi, you'll either need to add friction modifier to the fresh gear oil or use a synthetic rated for posi (I put Royal Purple in mine - it doesn't require a modifier).

[71CHEVYSHORTBED402]

Ha!..............The fluid has been changed before, but I've always paid to have it done. I'll be doing this in the future. Not certain what to look for, but if I haven't figured it out by then, then I'll take a picture.

Apparently one way is to lift the rear end, emergency brake off, and put it in neutral. Spin one tire clockwise. If the other tire spins counterclockwise, then it's not positraction.

[Stocker]

Originally Posted by 71CHEVYSHORTBED402

Apparently one way is to lift the rear end, emergency brake off, and put it in neutral. Spin one tire clockwise. If the other tire spins counterclockwise, then it's not positraction.

Yeah, I've heard that somewhere before...

[special-K]

Keeping fluid changed is the best prevention for rear end failure. But, we can only assume yours is one that wasn't ever changed. Some trucks out there may have had the fluid changed every 10k like clockwork. For a simple way to answer a simple question, do as Fitz said and put the tires on grass or dirt and see if one or two wheels spin.

[Hook'em]

https://www.youtube.com/watch?v=6qgREMBTE28

[trkfrmr]

Jack both rear wheels off of the ground, turn wheel by hand,if both wheels turn same direction,its posi.You can also get a close idea of gear ratio at this time by marking driveshaft and wheel and counting revolutions of driveshaft to each turn of the wheel.

[dave`12]

Since this has been answered, hopefully nobody will think I'm hijacking the tread. When I jack mine up and turn one wheel, the other does not turn at all. What does that mean?

[BLT gmc]

It means your posi is wore out, out of tune, or you don't have a posi. This is why sometimes jacking up or peeling out doesn't work(when the posi clutches are wore out or out of adjustment). Bruce
Posted via Mobile Device

[71CHEVYSHORTBED402]

Originally Posted by dave`12

Since this has been answered, hopefully nobody will think I'm hijacking the tread. When I jack mine up and turn one wheel, the other does not turn at all. What does that mean?

You sure it was in neutral?

[dave`12]

Originally Posted by 71CHEVYSHORTBED402

You sure it was in neutral?

yep, double checked

[71CHEVYSHORTBED402]

Originally Posted by trkfrmr

Jack both rear wheels off of the ground, turn wheel by hand,if both wheels turn same direction,its posi.You can also get a close idea of gear ratio at this time by marking driveshaft and wheel and counting revolutions of driveshaft to each turn of the wheel.

Thanks

[71CHEVYSHORTBED402]

Originally Posted by BLT gmc

It means your posi is wore out, out of tune, or you don't have a posi. This is why sometimes jacking up or peeling out doesn't work(when the posi clutches are wore out or out of adjustment). Bruce
Posted via Mobile Device

How about options D through E?: "You don't have posi, the rear end is out of tune or wore out"

[Ironangel]

A Posi has springs like this, A floater 12-bolt does not have the springs like this, You can raise both rear wheels off the ground and turn either one in either direction. If the opposite wheel turns in the same direction, then you very well have a posi. If the opposing wheel turns in the opposite direction, then you have an open rear diff,(non-posi)...The best way to tell of course is pop the cover. You will need a Fel-Pro RDS-13391 gasket...

[71CHEVYSHORTBED402]

Originally Posted by Ironangel

A Posi has springs like this, A floater 12-bolt does not have the springs like this, You can raise both rear wheels off the ground and turn either one in either direction. If the opposite wheel turns in the same direction, then you very well have a posi. If the opposing wheel turns in the opposite direction, then you have an open rear diff,(non-posi)...The best way to tell of course is pop the cover. You will need a Fel-Pro RDS-13391 gasket...

Thank you. I suspect I don't have posi. Will know soon enough, and yes I'm pulling the cover anyway. Got a "new" one to put on.

[j_cst_10]

I doubt we need anyone else chiming in, but I will anyway. I have a couple vehicles with limited slip/posi units and the jacking the car up and spinning the tires with the trans in neutral doesn't really work. In theory yes, but anything with a little age won't respond correctly.

Both vehicles will lay a furious yard or two with both tires, but won't spin in the same direction when jacked up and put in neutral. Pulling the cover is the sure bet or doing a burnout!

[71CHEVYSHORTBED402]

Makes sense 10.

One reason I asked is I got a smoking deal on a red posi tag. Very curious, does anyone know what the correct posi tag is for the 71? I've seen red tags that mount with the filler plug (I suspect not right due to plug on my truck) and a metal tag mounted with a cover bolt.

[EARNHARDT#3]

I see Fitz is suspended again LMAO

[71CHEVYSHORTBED402]

Does anyone know what the correct posi tag is for the 71? I've seen red tags that mount with the filler plug (I suspect not right due to plug on my truck) and a metal tag mounted with a cover bolt.

[Ironangel]

Originally Posted by j_cst_10

I doubt we need anyone else chiming in, but I will anyway. I have a couple vehicles with limited slip/posi units and the jacking the car up and spinning the tires with the trans in neutral doesn't really work. In theory yes, but anything with a little age won't respond correctly.

Both vehicles will lay a furious yard or two with both tires, but won't spin in the same direction when jacked up and put in neutral. Pulling the cover is the sure bet or doing a burnout!

Pulling the cover is indeed the "eyes on" best method to making sure you have a Posi or not...On the tire rotation method, if both wheels fail to turn in the same direction and a Posi unit is in the carrier, that unit could use at least some cleaning and a check of the friction plates. I too have had them so loose that the clutch pack preload is not to spec or not even enough to hold the required 40 foot pounds to disengage the clutch pack. In other words, if one tire fails to rotate in the same direction, your posi's slipping. They do require periodic maintenance and rebuilding. On the tag issue...Do you have a pic of that tag? I am not aware of any tags that designate correct units? There are Posi tags reminding owners and mechanics that a Limited Slip unit is installed and LS additive is required. The Eaton Positration unit for a 71 C-10 is EAT19556 and will accept 3:73 and higher numbered gears. Yukon makes an almost identical unit for about 50 clams less. Need a part number as well as a manufacturer of that red hot tag deal? "Pics please" Classic Parts as well as others have the replacement tags. http://www.classicparts.com/1971-81-.../#.Vk_29F5a59k

[71CHEVYSHORTBED402]

Here are the tags I was referring to. I don't know which is correct for a 1971, but I suspect it's the first one:

One goes on the cover http://www.ebay.com/itm/64-72-Chevel...hVQWM8&vxp=mtr

Two goes with the plug: https://s3.amazonaws.com/gearbox-yea...es/j/ja902.png

Three goes with the plug also: http://chevelle.com/media/extendware...on-tag-ce1.jpg

Yes two and three differ. You may miss it at first.