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Automatic Front Axle

YoungMoneyVII

YoungMoneyVII

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I have a 1991 Full size blazer parts truck I took the motor out of and I noticed the front axle doesnt have locking hubs. Does this mean its a automatic front axle and I dont have to switch them when I go into 4WD or its permanently in 4WD? I also noticed it has 2 shocks on each side in the front compared to my 90 full size blazer where it only has 1 shock on each side. If the axle on my 91 is better what would I need to remove from it to convert this onto my 90? The 91 was a plow truck for a construction yard and the body is rotted away
 
YoungMoneyVII said:
Does this mean its a automatic front axle and I dont have to switch them when I go into 4WD or its permanently in 4WD?
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What it means is you will be in 2WD when you want to be in 4WD. :)

Yes, they are "automatic". Something like 10' of forward movement after you shift into 4WD it will engage (theoretically, if they work perfectly and the magnetic pull of the moon is at the proper angle to the hubs) and you'll be in 4WD. Turning the front driveshaft (in a roundabout way) locks them in. So it's not like you are turning all the front components all the time due to tire rotation.

Often, they will simply not lock in, and you will be stuck in 2WD. Some people have good luck with them. The problem is that normally people put the truck in 4WD when they NEED 4WD, and that's when you find out it's failed. Happened to me, they just seem pretty finicky in regards to working properly.

Don't forget to back up for a few feet (I always went way overboard just in case) after putting it back in 2WD, to unlock them.
 
so theyre known to commonly fail? do you know the mechanical part of it that would need to be adjusted or replaced if this occurs
 
Agree with everyone on the hubs. Don't swap axles unless the 91 axle is more desirable for some reason. i.e. better gear ratio, better condition, etc.

Also, the dual shocks were an option - even on your 90, the buyer just didn't get them. In my opinion, there's no real advantage. 2 standard shocks will give you a stiff ride. 2 softer tuned shocks would be ok, but it's still no better than a single shock setup with good shocks like bilsteins or whatever.
 
YoungMoneyVII said:
so theyre known to commonly fail? do you know the mechanical part of it that would need to be adjusted or replaced if this occurs
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No adjustment or replacement except of the whole unit. Aftermarket or earlier Spicer hubs (which were stock) are common swaps. I honestly can't recall if there is one particular thing that causes them to "fail". I don't think it's hard parts typically, they just don't work right 100% of the time.

Dual shocks are stupid today, shock technology is so much better that the dual shocks are simply unnecessary. From my experience they are also problematic (not to mention more expensive to replace) as the bolt that is used to retain the lower end of the shock tends to bend on the outer position. Not a huge issue, but just another thing to cause problems and unnecessary with modern technology.
 
thanks guys. how can I tell which axle would be better? Both front and rear. Im going to be junking the 91 and I dont want to let it go with a better diff. I have the locking hubs on my truck the 91 is the parts truck.

and if I planned on attaching a plow to my truck having 2 shocks in the front wouldnt really make a difference on the wear itll put on the truck?
 
Personally if you have them, and not being a plow guy, I assume that is pretty hard on the truck, I'd keep them as spares no matter what if you have the space.

They are the same axles. If you want to verify the ratio (and see if the innards are in good shape) pop the cover off of each diff, and while checking condition, note the two numbers stamped onto the edge of the ring gear. Should look like 41:10. Divide the first number with the last to get ratio.

Realistically the rears are pretty junky, but if the half ton rear axles have served you well, no reason not to have a spare. When you need it, finding one will be either hard or expensive, guaranteed. :)
 
so the 3.42 ratio diff is the front axle and the 3.08 ratio diff is the rear axle? sorry if what im saying isnt making sense I dont know much about differentials havent serviced many at work. I just really dont want to junk anything worthy so im asking alot of questions
 
YoungMoneyVII said:
so the 3.42 ratio diff is the front axle and the 3.08 ratio diff is the rear axle? sorry if what im saying isnt making sense I dont know much about differentials havent serviced many at work. I just really dont want to junk anything worthy so im asking alot of questions
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You have to have the same ratio axle front and back or you'll have problems.
 
Yes. MUST have same ratio front and rear for 4WD to work right. If you have 2 axles 3.42 and 2 3.08, you can still save them as spares, you'll just have to swap both front and rear as a set if you ever need to. Or re-gear your other axle to match, but this isn't worth the trouble.
 
The axle "parts" for the most part are all the same, there were no different axle assembles offered those years, the ONLY difference will be the ring/pinion ratio. You can't just take the ring/pinion out of one and dump it in the other and have it work (without serious luck) but things like axleshafts, housing, spindles, brakes, etc., are all the same.

Obviously it's best if the spare set is the same ratio, but even if not, spares are never a bad thing to have IMO.
 
whats the benefit/ difference of one ratio to the other. If its significant enough I will remove the carriers
 
YoungMoneyVII said:
whats the benefit/ difference of one ratio to the other. If its significant enough I will remove the carriers
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Higher numerical gears (3.42 in your example) will give you better low end. Lower numerical gears (3.08 in you case) will be better for highway driving. Things can get confusing when talking with people though. As some call different numbers by different terms. Higher, lower, steeper, etc... When someone says they put "lower gears" it usually means they went to a higher ratio (3.08 --> 4.10)

Think of it this way, a higher number is like a longer lever. More rotational advantage.

If you are thinking about plowing, I'd go with the 3.42's. They will make life easier on the parts ahead of the gears as well (trans, transfer case, motor, u-joints).

Realistically the difference in 3.08's and 3.42's isn't that much. Only a couple hundred RPM on the highway. You still can't mix and match the two though.
 
how can I tell what I have? id prefer not to take the cover off because I dont want to put new fluid. My 90 truck doesnt have a vin because it was stolen before I owned it
 
1. Auto hubs; first mod to the Dante's Peak Suburban. As said not to expensive like $80 at Napa. They work or they don't! WARN premium are life time warranty sold at Napa under Napa part number but in a WARN box.

2. The 3:08 axles will give you better mileage (87Blazer 350 got 17mpg) if you do highway driving. But off road they are a little gutless. The DP burb has 3:73s and is great off road, 93 Blazer has 3:42 and a 350 gets 14 mpg.

Do the trucks have the RPO tag in glove boxes?
IMG_3134.jpg
 
Jack up one of the rear tires and make a mark (or pick a spot) on both the driveshaft and a rear tire. With the transmission in neutral, spin the tire by hand through one rotation, and count the number of times the drive shaft turns. The number of driveshaft rotations is your rear end ratio.

For example, if the driveshaft turns about 3 times, the ratio is probably 3.08. If its almost 3 3/4, the ratio is probably 3.73. Approx 4 turns, 4.10

Open gears, only have to jack up one tire, locked differential, have to jack up both tires.


I'm also one of those people who have yet to have any auto hub issues, so far. I'd say, run it till it stops working, then go get some warn manual lockers.
 
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