Serpentine belt tensioner is jumping and acting weird

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Trevor B

Trevor B

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The belt tensioner is acting strangely – can anyone help diagnose this problem?

Engine: Crate Magnum 380/360 with a serpentine set-up from a mid-90s Jeep. Everything was fine before I changed the belt set-up: I deleted the power steering pump and added a dummy AC idler on top to get the belt geometry right, and fired it up for the first time yesterday.

It took some experimenting to find the right length belt but I finally found one that keeps the tensioner in about in the middle.

The belt tension “feels” right in that I cannot quite turn it over 180 degrees.

Symptoms:
It is unclear exactly what is happening but almost acts like there is a hitch in the belt or something that is momentarily (and randomly) grabbing the tensioner pulley as it goes by and is jerking it inward (toward “looser”) and then letting it snap back. It is quite audible and also you can see the belt loose tension around the crank pulley momentarily.

First thing I did was run the engine with no belt for a few moments and it sounds like it should.

So I went out and bought a new tensioner and put it on. Same problem.

Next I put added washers under the dummy AC idler to add a little bit more tension. Same problem.

Any thoughts?

Thanks in advance!
 
There is typically an indicator where the tensioner is in its sweet spot.

A video of the tensioner jumping would be helpfull.

Some movement is normal
 
The fact that you finagled your belt length tells me that's where the problem is, and the symptoms indicate a belt that is too long. As the slack builds in the belt, the tensioner moves to take it up, and then when the tension momentarily increases, the tensioner jumps to compensate. If the belt is the right length, it'll pretty much run with a fixed tensioner, and the spring action is only there to compensate for wear, not 'adjust' the tension.

The AC-delete pulley that I used (before I bought the proper AC-delete alternator bracket), uses the same length belt as stock. It doesn't matter if you have Jeep, Ram, Van, whatever....if the PS bracket is from that system, the AC-delete pulley puts the extra roller in the same spot so you DON"T have to go hunting belts.
If you use the proper Mopar non-AC bracket (which is a ***** to find, even in Alaskan salvage yards), the belt length is not the same even though the overall routing isn't much different. And don't even think about some of the half-assed 'make it work' re-routing solutions you see online. There's the right way, and then there's that way.

81183-81408.gif


The stock belt doesn't go on easy. To me, they usually seem 'too tight' when I put them on.

My guess: the OE-length belt will be shorter than what you have on there now, and will solve your problem.

FYI the pulley I used initially looks like this, and had three mounting bolts.
91a7sJbLyFL._AC_SS350_.jpg


Edit: And then I re-read OP's post and saw that he deleted the PS pump. Same advice; get a tighter belt.
 
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jos51700 said:
The stock belt doesn't go on easy. To me, they usually seem 'too tight' when I put them on.

My guess: the OE-length belt will be shorter than what you have on there now, and will solve your problem.

Edit: And then I re-read OP's post and saw that he deleted the PS pump. Same advice; get a tighter belt.
Click to expand...

This right here is gold. I'm still new to the whole serpentine thing.
I will find a shorter belt and try it out! Thank you!
 
Does your alternator have a solid hub or is it a 'decoupler'? The decoupler is sort of a one way clutched pulley found in newer serpentine systems. It really mellows out a crazy belt if your using the correct sized belt. Check this vid out of OAD in action
 
probably not your exact problem, but this happened to my son. He bought a rebuilt alternator for his 318(5.2L) G. Cherokee, and he just turned in his old one for core. It bolted right on no problem, but it was shredding the serpentine belt edge. After dinking with it quite a bit, he realized the pulley on the rebuilt alternator, had one more (or one less) groove. Of course, when he went back to O'Reilly's his alternator core was gone, so then he had to go to a wrecking yard to find the correct pulley to install on the new alternator. Moral: count the grooves on all the pulleys to make sure they match.
 
I agree with above. Belt length is critical. Tension looses tension the further out it goes. They are meant to be tight and slack out when worn. They bounce a lot when the belt is stretched/ worn. So if bouncing right off the bat the belt is too long. Could be only 50mm too long. Enough to cause issues.
 
If you got a buddy at the auto parts store, he'll let you try out several in the parking lot.

That said, write the part number under the hood somewhere if that takes care of it!
 
Slip off the belt and move the tensioner. If it moves smooth and has even tension, run it. If it is sticky replace it. They will move around a little when the engine is running.
 
This is an ongoing problem with my '93 F250 (7.3 IDI). The stock length belt does not result in the tensioner staying in the middle of its range after a few minutes of breaking-in a new belt. One that's half an inch shorter (I think) lasted a while.
Now it's chirping again. As pointed out above, if it's too loose the tensioner can't keep it tight all the time. On my old truck there is also plenty of oil and other fluids around, some of which undoubtedly get on the belt. Once there's a glazed spot I think it will always chirp.
 
My daughter's 96 Jeep Grand Cherokee chirped all the time.
I replaced the belt with a Goodyear "Gaterback" and it has not chirped yet.

My understanding is the chirp is air getting trapped in the groove and squeezing out making the chirp.

Screenshot_20200920-120851.png
 
Okay:
- Tensioner is good
- Original (too big) belt was 81 5/8" (Gates K070810)
- Tried next two sizes (down) of belt: 80 3/4" (K070801) and 80" (K070795)
- 80 3/4" went on 'a little too easy' so went with 80"
- 80" definitely snug fit when brand new and no rattle... for a while. There was very little range left on the tensioner with this belt. Meaning it was almost all the way in and would therefore have nearly its entire travel to stretch the belt out.
- However, after running engine for a while and doing some tuning, eventually the tensioner starter bouncing again. First, a little, then kind of violently, then started to kill the belt. The inner-most rib came apart and shot a piece out of the engine bay.
- I should note that although there may be discrepancies between rib count (will check when I get to the shop later), the old belt was exactly the same rib count (7) and never had a problem.

Today's experiment will be to run the 80 3/4" belt and add the washers as spacers underneath the AC delete pulley if/as needed.

I bought this car, a forgotten and neglected 1973 Dart Sport 340 in March to replace my poor wrecked Duster (Death of Huey the Hooptie) and bracket race with it. I tore it down to the shell and fully rebuilt it over the last few months with as much of the Duster as I could.

I'm THIS CLOSE to being able to hit the track tomorrow afternoon (we only have amateur bracket races on some Wednesdays at Sonoma Raceway). I've got an alignment appointment tomorrow and just need to get this belt thing under control! Why is it always the weird little things?!?
 
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No need for my comment, but I have been saying the same thing since May. Still not drivable, but close. Put one fire out and find that a spark from the first fire started two more.
 
Problem found!
The AC delete pulley has three mounting holes, through which 5/16" bolts go through and into the front cover, as shown in jos51700's post #3 above. The holes are significantly bigger than 5/16" and there is thus a lot of "adjustment" possible.

What was happening is the pulley was too far back and the belt was trying to jump over the front of it, or getting hung up on the edge. I've pulled it as far forward as possible and cranked down on the bolts and it is running smoothly.

Not too excited about the prospect of constantly having to check the AC delete pulley and will be looking for possible solutions for taking up space in the holes. But it works! And it works well enough that I was able to run the car around the block for its maiden voyage. Tomorrow is an alignment in the morning and then spend a few hours beating up the car on the highway to see if I can break anything... then off to the races!
 
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