serpentine belt jumping off

[scoobysmak]

Well I have around 145K on the truck after buying it with 139k on it. Recently I have been driving around and my belt just jumps off. The first time it happened I had been driving it for about 45 min and took off from a traffic light. At this point I had to cut the belt out because it was twisted behind the harmonic balancer, I checked everything and saw the pulley on the tensioner was not in the best shape so I replaced it. I got a new belt from NAPA and I drove one this belt for a couple of days and this one flew off also. This belt was a "Micro-V" 25 061020. After looking at the part number this seems 10mm too long. Everything on the truck seems normal. I turned the alternator, A/C compressor, water pump, and the vacuum pump (this turns both ways with no noise and you can feel it get resistive and then release pressure as you turn it). The second time I had just started it and was accelerating from my neighborhood. This seems rather strange to me unless I got the wrong belt, but why would the first one jump off to begin with. I believe the tensioner is holding properly. Any other ideas are or things I should check would be appreciated.

[d350diesel]

A worn tensioner will hold the pulley crooked and make the belt walk off. Replace the whole tensioner assembly and that ought to solve the problem.

[sailun]

Harmonic Balancer/Crank Pulley going south ? Seems like the right mileage range. Just did mine at 153k or so, after all the bolts sheared. Just a Wild-A**-Guess.

BTW, it's REALLY difficult to stop and steer when that belt comes off again.
Try to fix before it happens on a corner, or in traffic.

[scoobysmak]

Harmonic Balancer/Crank Pulley going south ? Seems like the right mileage range. Just did mine at 153k or so, after all the bolts sheared. Just a Wild-A**-Guess.

BTW, it's REALLY difficult to stop and steer when that belt comes off again.
Try to fix before it happens on a corner, or in traffic.



Well I am kinda leaning towards this just because it happens only when I add power. Still skeptical about it but I agree trying to stop this thing and turn is a nightmare when the belt "falls" off. I might just replace it all, the harmonic balancer/pulley and the tensioner just to be safe.

Edited by cowracer to show the above in a quote

[Iain]

Just a thought, check the bearing on the idler pulley. If it is worn out, it may still be turning but may be allowing the belt to wander. I pulled my pulley off at Christmas and was shocked at how worn the bearing really was. I pressed out the old one on a small hydraulic press and installed a new $3.50 bearing and I should be good for a long time now. You may not have to replace the entire assembly if this is all that is wrong. Pull the pulley off and then spin the pulley on your finger with your finger through the shaft hole and you can feel if it is grinding and dry or if it is o.k. It will take about 10 minutes to check it.

Iain

[Cowracer]

Do not get a tensioner from Autozone. Their computer has the same tensioner for gassers and the 6.5. the pulley is too narrow and will not ride all the way across the belt

Go to napa. they have the right one.

Tim

[rjwest]

Be carefull of NAPA, not all GM Replacement tensioners are the same...
They may have a tensioner asm with a pully that does not have a lip,,
it won't work...

Note: do not know if it was a parts guy screw up or Napa cross ref, but
I had to take the NAPA back and get a GM Tensioner and belt to fix my problems..

I would reccomend GM( Goodyear ) belts only, others are a crap shoot..

[65TD]

Mine does not have a lip on it either.

[Burning Oil]

I have been going through this also and the problem I found was that (atleast with Gates brand) there are atleast 3 different pulley sizes. The one most commonly stocked in the stores around here are for the dual altenator set up (I don't know why). The pulley is smaller on that one then the single alt and puts the tensioner close to the end of its travel.
So make sure the pulley size matches up to the old one also it makes a big difference.
This problem was on my 96

[scoobysmak]

The pulley I replaced it with is exactly the same as the one that was there. The only difference being that this bearing in the middle rolls smoothly Vs. the other bearing that rolled like some sand was in it (this took two trips to Advanced Auto though, the first trip may have been my fault and I asked for an idler pulley, well that was not the answer it was under tensioner pulley even though it is the only "idler" pulley there to my knowledge but maybe other years are a bit different). I went ahead and got a new harmonic balancer/crank pulley and a new tensioner just to be safe but won't get a chance to install them until this weekend. If there is any tips on this procedure please let me know. I hope this cures the problem but either way it won't hurt to replace them both especially if a bad harmonic balancer can destroy an engine.

[Cowracer]

Be carefull of NAPA, not all GM Replacement tensioners are the same...
They may have a tensioner asm with a pully that does not have a lip,,
it won't work...



I disagree. Mine does not have a lip and its been good for over 25k miles.

The gasser one has no lip and is much "shorter", i.e. the back edge of the pulley is about 3/4" to far away from the engine, causing the edge to hit about the middle of the belt

the diesel one may or may not have a lip, but the pulley is wide enough that the belt does not come near the pulley edge.

Tim

[rjwest]

Cowracer, if it works Great,

I have a late 96 with HO cooling ,OEM, my application may be a unique ..

I had many problems with the Serpertine belts, tensioners, etc.

Once I installed GM parts and Belt ( goodyear )
all problems fixed,,,

I believe the NAPA tensioner with out the lip was not correct PN call out
for my application, So MHO, Buy GM for these parts and save a lot
of money in the long run...

And, even with all gm hardware, the Advance Gates belts and the napa were good for only a few thousand miles,

[scoobysmak]

Okay advice taken, well I had already ordered the autozone "diesel want-to-be" tensioner and yeah its the wrong part. I go to put mine back in and well "snap" the pin broke. Now I need one for sure. Second problem, uh are the bolts holding the harmonic balancer reverse threaded or is there something I can do to keep from turning the motor over while I try to remove the 4 bolts on the front (humm righty tighty and lefty loosey didn't work and to answer anyones question I do not have a service manual of any kind handy, its about 700 miles from me at this moment). Besides being morally wounded from this I would like to get her back on the road ASAP. Thanks for everyones help so far.

Well to edit this I was a little upset and figured where there is a will there is a way. I took the old belt that got mutilated by the fan and wrapped this around the flywheel pulley and put a set of vise grips on it. Then with my frustration by holding the vise grips in one hand and my socket wrench in the other I managed to get the 4 bolts out ( I didn't think they would be reverse threads). To my suprise though my harmonic balancer/flywheel pulley just fell into my hand. There was no need to get a "pulling" tool out for this job. I looked inside and there is rust everywhere, I assume this is normal. I just want to make sure before I put this back together that I don't need to do some cleaning besides the lip where the new harmonic balancer will slide on. Also to make sure I am thinking straight the part behind the harmonic balancer is just a counter weight for the crankshaft (or just for rotational mass). Well hopefully I can get the new pulley on with out a problem and I can find the correct tensioner so I can get back on the road.

[65TD]

The harmonic balancer is the one in the back and it requires a puller. It also has 200 ft/lb of torque on it.

The balancer pulley just bolts up to it no puller required.

[billschall]

Second problem, uh are the bolts holding the harmonic balancer reverse threaded or is there something I can do to keep from turning the motor over while I try to remove the 4 bolts on the front (humm righty tighty and lefty loosey didn't work... I took the old belt that got mutilated by the fan and wrapped this around the flywheel pulley and put a set of vise grips on it. Then with my frustration by holding the vise grips in one hand and my socket wrench in the other I managed to get the 4 bolts out...

Next time, save your knuckles & frustration. Take a 3/8" drive 15mm socket on a ratchet and smack the end of the ratchet with a mallet. The jarring force works almost all of the time. (a little trick learned years ago working on tractors). Make sure the socket is square on the bolt head before you go a-swinging.

I just did this not more than 30 minutes ago on mine and trust me, your rig can't be as weathered & rusty as mine is underneath.

-Bill

[a5150nut]

"Next time, save your knuckles & frustration. Take a 3/8" drive 15mm socket on a ratchet and smack the end of the ratchet with a mallet. The jarring force works almost all of the time. (a little trick learned years ago working on tractors). Make sure the socket is square on the bolt head before you go a-swinging."

Also known as an Oklahoma Impact wrench, among other discriptions(;-

Now ducking before someone thows things my way!

I used to have a tool that held a socket and you hit it with a hammer on the end.................Impackt Driver IIRC

[65TD]

I took mine loose by just jerking up on the wrench suddenly will moving clockwise. To tighten it I did not really trust that so I put another wrench on one of the bolts to hold it jammed against the side of the pulley.

Of course the belt was already off.

[billschall]

Also known as an Oklahoma Impact wrench, among other discriptions(;-

Yeah, but here in Central PA, we call 'em an Amish Impact Wrench, LOL...

[scoobysmak]

Humm another update. The balancer/pulley got replaced when I got the old one off, and the next day I got a GM tensioner. This came with the idler pulley installed (with no lip, not that it matters). I have been about 250 miles this week and figured I was safe from the belt monster. I bought 2 belts when I put everything back together. I got the cheap autozone version and a goodyear belt to put on. I didn't want to chew up a good belt incase it wasn't fixed. I had another belt that I used before but some of the edges were a bit chewed up from the problem. Needless to say I deceided to take a trip on the interstate and needed to hit the pedal pretty hard to join traffic. Well about a half mile later the cheap autozone belt jumped off. This one wrapped around the fan and looked like jigsaw puzzle after I stopped. I had my spare belt that was a bit chewed and put that one on figuring this problem wasn't over, well 2 miles later this belt died a hard life around the fan also (I drove with a lite gas pedal but it didn't help). This time I put my goodyear belt on and started it. With this belt I noticed that it wanted to jump off while at idle, the others looked normal. As your facing the engine at the front of the vehical, from the top left the belt runs from the A/C compressor to the tensioner pulley, then heads around the alternator, then the power steering pump, and the problem begins here. When it goes from the power steering pump to the water pump it starts to hop on the front edge of the water pump (the fan blade side). After this it starts to walk off the crank pulley, the vacuum pump, and then when it hits the A/C compressor again it folds over and does not correct its self (not that it should have done this to begin with). After driving around 5000 miles what would cause this problem, my power steering pump bolts are tight and it does not move. Does the power steering pump pulley have an adjustment on it that I am not familar with or am I missing something. Thanks for everyones help so far.

[scoobysmak]

Another update, well due to the fact I just moved to this area and most of my tools are over 700 miles away I took it to a shop close to work that everyone here recommened. I drove it there without the belt (only about .25 miles). I put the belt on and showed him what was going on, he said it looked like my vacuum pump was going bad. Everything seemed normal and lined up but when it was idleing he noticed the tensioner was getting a jerk from time to time. After that I took the belt back off and he rotated everything and said my vacuum pump seemed really tight. He did say he has not worked on very many 6.5's but didn't think it should be that tight. Guess it may be time to just get an electronic controller anyway but just shooting in the dark now.

[KenDavis]

I had this same problem this past month, lost several belts. I replaced the tensioner and still had problems. The last time however the upper bracket holding the alternator snapped in half. We discovered that the lower bolt on the alternator was just a bit loose, just enought for the alternator to wiggle unnoticed, but enough for the belt to come off during heavy accelleration. So, be on the safe side and check the bolt and check for cracks in the upper bracket. Seems to have solved my problems.

[4inchsnappy]

I'll add my 2 cents- I had a belt jumping problen on my 95 suburban, i thought it was my vaccum pump, It was quite noisy, i removed the vac pump because I had a mechanical waste gate on the truck, went to the shorter belt. it still jumped. with a friends helping eye we noticed and something was out of line. turns out someone had removed the engine hanger bracket that mounts behind the power steering pump bracket, we replacet that 1/4 piece of steel and i have not had the belt jump since. p.s i'm running a new tensioner with a lipless pulley.

[TJ Moose]

I had the cracked alternator housing scenario too. Hard to see as the crack was on the bottom side of the housing. But it didn't look straight and moved funny when I untensioned the belt. Upon starting to take Alt bolts out - what was left of hgousing snapped off and fell to the ground. Its so much happier with the new one.

[simon]

Last week I was pulling up a hill and felt i lost the powersteering , then alternator light came on ,drove to a safe place to stop and found the belt had jumped of. I could not figure out wy, till I noticed a little antifreeze leak at the corner of the rad on the inside. There was a pebble stuck in the core .
How the heck could that pebble gotten in there to flip the belt of and wanged into the rad like that. Luckely It was a copper core so it was easy to fix.

[Shuck]

Bringing this thread back to the top. I've got a similar problem with a 95 K2500 pickup. The belt hasn't come off, but I can see it running on the outer lip of the vacuum pump. I got a pulley puller and moved the pulley out almost a full 1/2" on the vacuum pump and the problem is still the same! Almost worse.

Is there some way the vacuum pump shaft or mount could have gotten bent by the previous owner (who was a mediocre shadetree mechanic)? If the mount was bent to the outside (towards passenger side) would that cause the belt to want to slip off the edge of the pulley? How on earth would someone bend that mount? Is it more likely that someone forgot to put a part back in while replacing the water pump? (say, a mediocre shadetree mechanic)]

Ryan