Belts, Tensioners, and Pulleys

Years ago, during a pre-purchase inspection before finalizing the deal to buy the Grey Ghost, the Volvo mechanic pointed out that both belt tensioners really needed to be replaced, but that it wasn’t urgent.  I put the project off for a while–amost a year–until after successfully starting the world’s first nuclear reactor using heat pipes and Stirling engines.  After doing that, replacing a pulley seemed like it should be pretty easy.

It was almost too easy.  Just a few minutes, in fact, working at eye level and without getting dirty.  Loosened the spring tension with a 1/2″ drive ratchet, slipped the belt off, and unscrewed one bolt with a 14mm socket and the pulley was free.  I knew the other tensioner needed to be replaced, but the Cummins dealer didn’t have one on the shelf, and I was only in town (Las Vegas) for the weekend.  There was also a little sticker shock–they wanted almost $300 for that little thing.  I figured the worst that could happen was the tensioner letting the belt slip, in which case I could cut the belt and forgo AC until I got somewhere to fix it–maybe uncomfortable, but not disabling.

Somewhere along the way, I did a little searching and came across a Gates cross reference for the Cummins part number.  There was a pretty big difference in price–that tensioner would only be $120.  So it got replaced in 2014.

Both tensioners have done their jobs just fine, but recently I started noticing a bearing squeal when it was wet outside.  Knowing the tensioners (and tensioner pulleys) were basically new, that really only left the idler pulleys, one for each belt.

With some really nice weather over the weekend, I decided it was time to take care of it.  Of course, I decided this after the Cummins dealer was closed, and unlike with the tensioners, I couldn’t make out a part number on the idlers.  A little searching turned up a number that crossed to a Gates part, one that wasn’t in stock at the NAPA distribution center that would normally be the go-to place.  On a whim, I called the closes O’Reilly’s–they had the part, for about $35.  I headed down there, and while they had the part I asked for, it wasn’t the one I needed.

The clerk tried to be helpful (I was playing this RockAuto commercial in my head…), asking what it went on.  When I said a Volvo truck, he started typing and I was shocked to see the VNL listed as a model just after the V70 and ahead of the XC90.  Really?  Ok, but I have a Cummins engine–sure enough, it was listed.  Along with all of the part numbers I needed for the idlers, tensioners, and belts.  And all of them were in stock locally.

I did shop around just a little–I wanted the idler for the fan belt right away, since it was already off–and found that I could do better on Amazon by a pretty good margin.  I picked up the idler for $90, and ordered the other idler and both belts.  Combined, I was able to save $75.

This is one of those projects that takes longer to write up than to actually do.  Start to finish was well less than an hour, and most of that was just fishing the fan belt between each of the fan blades and the fan shroud.

Tools needed:

  • 14 and 15mm sockets
  • 1/2″-drive ratchet with long handle or cheater bar
  • Pry bar or breaker bar

Parts needed (pictures linked):

  • Gates 38610 AC belt tensioner, $119.78
  • Cummins 3978022, $141.14, or Gates 38504 fan belt tensioner, $85.78
  • Gates K120842 or K120842HD Fan belt, $68.16
  • Gates K060637 or K060637HD AC belt, $18.79
  • Gates 36286 fan belt idler pulley, $67.85
  • Gates 36223 AC belt idler pulley, $39.45

I mentioned that I had replaced each of the tensioners separately, but given that most of the big trucks put into RV service already have quite a few miles, I’d probably recommend doing everything at once (the extra cost is in parts, not time).

The first thing to do is get the fan belt loose.  Stick the 1/2″-drive ratchet into the square hole on the tensioner and push up (clockwise) to relieve tension.  Slip the belt off of the idler pulley above the tensioner, and release the ratchet slowly.  You may need a pry bar to support the tensioner in place while you reposition the ratchet to fully release the tension.

Ratchet on fan belt tensioner, squeaky idler on top side of belt.

Do the same thing on the AC belt tensioner, except using a 15mm socket and prying counter-clockwise.  Again, slipping the belt free at the idler pulley is easiest.

Ratchet on AC belt tensioner. Idler to be replaced is just above ratchet head.

The AC belt is easily freed with the fan belt pulled off of the water pump, while the fan belt will have to be fished over the fan blades into the space between the fan and radiator in order to get free.

The pulleys are both held in with 14mm bolts–unscrew, swap out the pulleys, and re-tighten.  Install the new AC belt first, then fish the new fan belt into the space between fan and radiator, and over each fan blade before slipping it into place.  Same operation in reverse with both tensioners, though it may require a little more effort going back together–you’ll have a stiffer, newer belt, and stronger tensioners.

Done. New belts, tensioners, and idler pulleys.

A few things were learned here:

  • Like with cars, the dealer isn’t necessarily the cheapest place for parts.
  • Unlike with cars, most auto parts places can’t look up your truck’s make and model (though neither can the dealer…they tend to want to work from your VIN or ESN).  O’Reilly’s proved an exception here.
  • Despite being bigger, the ease of access and eye-level work area made it much easier than working on a typical passenger car.
  • At about 240,000 miles, both idler pulleys were well worn, but both belts were probably fine to leave in service.  Not too different from passenger cars in that regard.

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