1991 1500 Blazer 4wd problem

The problem has been happeneing for a while but mostly when it is cold, but much more now. When I first start the truck and drive it it is fine for the first 1000 ft or so. Then I start to hear a noise that sounds like a splined shaft bouncing on a splined hole, or gear teeth bouncing on another gear but not engaging. Then when I slow down to stop, when I get below 5 mph the noise stops and the truck cluncks repeatedly until I stop. Then when I start up again it clunks repeatedly until I get above 5mph when the gear bouncing noise starts. The frequency of the noise goes up as I speed up and decreases as I slow down. I put the truck up on my lift and found a hose coming out of the rear of the transfer case. It routes above the transmission and up between the firewall and the bell housing it stops at a clamp. It has what looks and feels like a plug or valve in the end of it. There is another hose that comes out of the front of the transfer case and routes along the bottom of the driver's side of the motor. It then goes through a coupler tube and connects to another hose and goes to a purge tank on the drivers side fender. There is one hose that comes out of the purge tank and goes to a fitting on top of the front differential and one more that goes to the bottom of the throttle body at the manifold. There is one more hose that comes out of the front differential that routes along the bottom of the radiator and comes up on the passenger side of the motor and stops at a clamp with a plug or valve in the end of it. The plugs or valves on the end of the hoses seem loose, not tight like they are sealed. The hose is the original hose but I didn't see any cracks or holes. Could they be just porous? Any ideas would be GREATLY appreciated. Thanks in advance! :confused:

Honestly, it sounds like blown bendix/U-joints in the driveshaft. Get the truck up on Jack stands and rock, wiggle and twist your drive shaft and see if it moves more than a 1/4" with a clunk. Push up and down near the pumpkin, and side to side as well, wherever you see a joint. Get back to us with what you find after trying that.

Honestly, it sounds like blown bendix/U-joints in the driveshaft. Get the truck up on Jack stands and rock, wiggle and twist your drive shaft and see if it moves more than a 1/4" with a clunk. Push up and down near the pumpkin, and side to side as well, wherever you see a joint. Get back to us with what you find after trying that.
Thanks Wolfox. This is what I discovered while back up on the lift regarding the driveshaft. The u-joint seems solid. There is only 1. It's at the transfer case. The other end goes into a housing that sticks out about 18" from the front pumpkin. If I turn the front driveshaft by hand the passenger wheel gets driven, but the drivers wheel doesn't. I hear the noise coming from inside the hub on the driver's side. It's almost as if the front axle is sitting flush with the hub and only needs to be pushed in to engage. If I don't turn the driveshaft and spin the driver's side wheel I don't hear the noise and the wheel spins freely. Also this only happens in 2WD. 4WD is fine, except it is rough on the tires on dry pavement... I have never been in the front hub assembly so I don't know what is going on in there, but it seems like I might have a broken hub or hub bearing. I was thinking it might be the 4WD engagement mechanism though.

We need someone with a little more experience than I do to help you here. I am not certain, but there might be a "drive yolk" that slides back and forth on a splined shaft to engage your front wheels on one side. If memory serves on other, older 4x4 designs, this yoke may need to be lubricated. However, I am uncertain if there is such a think on the full-size Blazer. (I am assuming an older, bigger brother vehicle here from what you have described - you will have to supply us with year and 4x4 system type for us to really draw a bead on your problem. Pushbutton, floor shifter on demand, manual transfer case+front, manual locking hubs, etc) Had a Rodeo that needed to have a front shaft yoke lubed up at every oil change 'cause when it went dry, it would make this horrendous clunking noise in 2wd modes. See if you can find a grease zerk underneath that old beast and give it a shot of lube. Anyone here with any more thoughts?

A blown bearing will tend to make a hum and buzz in no matter which vehicle you drive. Loose or worn bearings may give you a clunk, but only when cornering, or wiggling the elevated wheel. (Jacked up off of the pavement) Check your hub pre-load with a torque wrench and replace rusted or lost cotter pins in the often found castelated nut - this keeps it from backing out again. I hope an expereinced fellow comes along soon with a more precise diagnosis I have been able to give...

EDIT: Witty thoughts come to mind when one walks away - Do you have a solid axle up front with these huge knuckles that tie into your hub assembly? Those too require regular greasing. If left dry for too long, they can wear horribly, also creating the problem you describe. If only on one side, try pumping it up with grease. It may disappear if it was just dried out, saving you the tear-down and rebuild time. Then on general principle, hit the other side too. Fill them only when the truck has been sitting in 35 degree or warmer weather for at least an hour, and fill only to the point where you see some of the old grease and gunk start to come out. Wipe clean and test drive...

We need someone with a little more experience than I do to help you here. I am not certain, but there might be a "drive yolk" that slides back and forth on a splined shaft to engage your front wheels on one side. If memory serves on other, older 4x4 designs, this yoke may need to be lubricated. However, I am uncertain if there is such a think on the full-size Blazer. (I am assuming an older, bigger brother vehicle here from what you have described - you will have to supply us with year and 4x4 system type for us to really draw a bead on your problem. Pushbutton, floor shifter on demand, manual transfer case+front, manual locking hubs, etc) Had a Rodeo that needed to have a front shaft yoke lubed up at every oil change 'cause when it went dry, it would make this horrendous clunking noise in 2wd modes. See if you can find a grease zerk underneath that old beast and give it a shot of lube. Anyone here with any more thoughts?

A blown bearing will tend to make a hum and buzz in no matter which vehicle you drive. Loose or worn bearings may give you a clunk, but only when cornering, or wiggling the elevated wheel. (Jacked up off of the pavement) Check your hub pre-load with a torque wrench and replace rusted or lost cotter pins in the often found castelated nut - this keeps it from backing out again. I hope an expereinced fellow comes along soon with a more precise diagnosis I have been able to give...

EDIT: Witty thoughts come to mind when one walks away - Do you have a solid axle up front with these huge knuckles that tie into your hub assembly? Those too require regular greasing. If left dry for too long, they can wear horribly, also creating the problem you describe. If only on one side, try pumping it up with grease. It may disappear if it was just dried out, saving you the tear-down and rebuild time. Then on general principle, hit the other side too. Fill them only when the truck has been sitting in 35 degree or warmer weather for at least an hour, and fill only to the point where you see some of the old grease and gunk start to come out. Wipe clean and test drive...

On vehicle type/trans type. It is a 1991 1500 full-size Blazer with a 5.7L and 4WD. The trans has a column shifter. The 4wd has a floor shifter that I believe is manual. The front hubs are automatic, i.e. I don't have to get out and manually lock them. I have to admit I don't know what my hub pre-load is but I have several torque wrenches... Your description of the front axle sounds correct and now that you mention a dry hub that could be it. I'll get it up on the lift on Saturday and look for the grease fittings and pump them up if I find them. Yes, this is an old beast but it is has been solid as Sears and still runs great with only 114k on it. Winter is coming I have no fear it will get me through another winter just fine, providing I can get this problem sorted out. Thanks again for the input. :thumbsup:

Well, I was thinking it was a dry knuckle, honestly. The hub rides on a wheel bearing in which the axle shaft protrudes from so it turns with engine torque, and then a set of bearings keep it aloft and spinning. Getting a look at it will involve nothing short of disassembly. And if you went that far - might as well poke in a new set of bearings and press them tight. Anyway -

Wiggle on your front tires, side to side, top to bottom, grasp it with a hug and lift it up and down. If it clunks, dollars to donuts you have a bad bearing, or at least, it is starting to go. If it's solid, lube the knuckles behind it on the inside, behind the brake rotor. Turning the wheel to either side sharply before doing this will give you some room to maneuver hands and your grease gun.

Barring that, follow the axle shafts themselves and see if there is indeed a locking yoke and lube that. If this is the case, and if it's a Dana axle, there will be a locking yoke actuator motor on one of the intermediary shafts. (Speaking from the viewpoint of a Dodge owner, it is located under the driver's side of the truck, half-way in between the knuckle and the front differential housing) Remove the wire for the indicator switch, then remove the vacuum line(s). If there is more than one, label one of them and mark which side it was on the vacuum actuator. The disconnect housing should then have 4 bolts that can be removed. Carefully lift the actuator out of the housing, there will be a fork that moves a locking collar back and forth on a splined shaft that's split in the middle. See if there is any oil left in there. If it's gunky, it can be siphoned or vacuumed out. Make sure that it is lubricated properly, and inspect the vacuum motor's shaft seal (o-ring) to make sure that it is intact and clean. You may have to rebuild the cork seal that keeps this happy and safe, but a bead of Permatex silicone will work well here. However, while it is open - lubricate the splines, fork, and collar with your finger and gear oil - I say this, because you can feel if there is any grit or broken splines. (The slippery oil will marginally protect your finger from snagging on something sharp) Once re-assembled and sealed - unscrew the actuator position switch and fill the hole with about 5 ounces of GL5 80w-95 gear lubricant. Screw the switch back in snugly, and then re-attach it's wire harness and the vacuum motor lines.

Ummmmm, the only other thing that comes to mind is that if you do not find a yoke on one side of the front diffy, the locking mechanism may be contained inside. If this is the case, check for fluid level and condition. You may have no benefit of a fill/drain plug there so prepare to get messy and drop the cover on it. If it's dry, that may be your problem. Worse yet, it's filled - but not with oil....I have seen clear water run out of differentials before; or this thick, tar-like substance that was once-upon-a-time hypoid gear lube. *laugh*

I am afraid I just do not know much on the full-sized Blazers. Jeeps, Subaru, Isuzu (Rodeo), Dodge Pickups - 80's style A-frame GM and my current Blazer is all I know about from the experience of ownership and maintaining.

Well, I was thinking it was a dry knuckle, honestly. The hub rides on a wheel bearing in which the axle shaft protrudes from so it turns with engine torque, and then a set of bearings keep it aloft and spinning. Getting a look at it will involve nothing short of disassembly. And if you went that far - might as well poke in a new set of bearings and press them tight. Anyway -

Wiggle on your front tires, side to side, top to bottom, grasp it with a hug and lift it up and down. If it clunks, dollars to donuts you have a bad bearing, or at least, it is starting to go. If it's solid, lube the knuckles behind it on the inside, behind the brake rotor. Turning the wheel to either side sharply before doing this will give you some room to maneuver hands and your grease gun.

Barring that, follow the axle shafts themselves and see if there is indeed a locking yoke and lube that. If this is the case, and if it's a Dana axle, there will be a locking yoke actuator motor on one of the intermediary shafts. (Speaking from the viewpoint of a Dodge owner, it is located under the driver's side of the truck, half-way in between the knuckle and the front differential housing) Remove the wire for the indicator switch, then remove the vacuum line(s). If there is more than one, label one of them and mark which side it was on the vacuum actuator. The disconnect housing should then have 4 bolts that can be removed. Carefully lift the actuator out of the housing, there will be a fork that moves a locking collar back and forth on a splined shaft that's split in the middle. See if there is any oil left in there. If it's gunky, it can be siphoned or vacuumed out. Make sure that it is lubricated properly, and inspect the vacuum motor's shaft seal (o-ring) to make sure that it is intact and clean. You may have to rebuild the cork seal that keeps this happy and safe, but a bead of Permatex silicone will work well here. However, while it is open - lubricate the splines, fork, and collar with your finger and gear oil - I say this, because you can feel if there is any grit or broken splines. (The slippery oil will marginally protect your finger from snagging on something sharp) Once re-assembled and sealed - unscrew the actuator position switch and fill the hole with about 5 ounces of GL5 80w-95 gear lubricant. Screw the switch back in snugly, and then re-attach it's wire harness and the vacuum motor lines.

Ummmmm, the only other thing that comes to mind is that if you do not find a yoke on one side of the front diffy, the locking mechanism may be contained inside. If this is the case, check for fluid level and condition. You may have no benefit of a fill/drain plug there so prepare to get messy and drop the cover on it. If it's dry, that may be your problem. Worse yet, it's filled - but not with oil....I have seen clear water run out of differentials before; or this thick, tar-like substance that was once-upon-a-time hypoid gear lube. *laugh*

I am afraid I just do not know much on the full-sized Blazers. Jeeps, Subaru, Isuzu (Rodeo), Dodge Pickups - 80's style A-frame GM and my current Blazer is all I know about from the experience of ownership and maintaining.
Well, I got it on the lift and the front wheels were not loose. I picked them up and manhandled them around and no cluncking. I took the hub cover off and all looked ok except, here is what I found. Looking at the hub there is a splined shaft that comes into it from the front diff. There is a splined cylinder that mates with the splined shaft. It is held in place with a spring that apparently is supposed to keep this cylinder out as far as it can go. This spring is loose. It doesn't keep the cylinder away from the shaft. There is about 1/4" of play in it and with this assembly at rest the cylinder is rubbing up against the shaft. It wasn't dry but the grease, or what used to be grease was waterlogged or appeared to be and there wasn't much of it. I packed the whole assembly with grease and put it back together. It was ok for about 1/4 mile and it started to come back. I guess the grease kept them apart. If I could get my hands on an exploded drawing of the front hubs I think I can fix this. My guess is the front hub or part of it is shot. I'm thinking of subscribing to AllData for the $15 fee just so I can get a drawing. BTW, there is no way to grease the knuckle behind the drums. I pushed some grease in there but I don't think it did any good.

Might very well be weak springs in this case. I slipped out the OEM lockers on my front axles in an old Jeep Cherokee by just unbolting the works, slipping on a set of WARN upgrades. If the ends of your axle splines are not chewed up, you may very well benefit from the same upgrade. Pretty cheap too - did mine at the time for $50. Blown/weak return springs will make the trouble you are having now when the spring gets tired and not pushing that collar fully out into the retracted position. When you get the replacement lockers, there may be an exploded view packed in the box. Mine didn't - they just assumed what you were doing. Luckily I had a Chilton's on the truck. Save the $5 and pick up yours at any auto parts shop before blowing the bucks on online subscriptions. I had the rotten luck of getting one for help on a Subaru that saw 3 production splits in the same manufacturing year. None of the on-line data on critical brake and driveline parts/specs were correct. Nothing short of taking it apart and checking a Hayne's book got me what I needed to know on it. ;)