Volvo D24T Cruise Control Throttle Spool Repair

The Volvo D24 and D24T Cruise Control (CC) throttle spools are notorious for their high failure rate.  The natural shake and rattle of the D24 and D24T engines cause the CC throttle spools to vibrate and thus to wear rapidly.  If neglected for too long, the bushings wear out, then the spindles start to wear, and before too long, the whole assembly becomes unusable, and must either be replaced or taken to a machine shop for repair. Today’s post shows how to repair the D24 and D24T CC throttle spools and how to modify them to greatly reduce the rate of wear.

This post was revised on July 11, 2017 to show lessons learned while rebuilding the first dozen CC throttle spools. To skip to the new material, click here.

Unfortunately, these CC throttle spools are no longer available, so it is imperative to slow the rate of wear as much as possible. The D24 and D24T CC throttle spools are identical, except for the sheet metal mounts.

The following photo shows four D24T CC throttle spools, one new, the other three repaired as described in this post:

The one at the far left is brand-new, but unfortunately the factory made a mistake and installed internal springs and keepers for a B230 CC throttle spool.  Since the B230 throttle spool turns in the direction opposite to that of the D24, the internal springs and keepers are mirror images, so the whole assembly was unusable when I purchased it.  The last three D24T CC throttle spools that I purchased (all in 2011) suffered from the same defect.  I repaired all three of them, but this one was never installed on a vehicle.

The other three CCs in the above picture represent, from left to right, the first, second, and third repair design iterations, as described below.

After removing the “E” clip and the top spool, you will probably be faced with removing the 8mm x 10mm bushing that holds the spring keeper on:

The bushing is pressed on, so it probably won’t come off easily.  To get it off, simply tap around it lightly with a hammer.  That will stretch the metal and you should then be able to get it off using pliers.  Of course, this destroys the bushing, so you’ll have to find a replacement, which is pretty easy, as you will see later.

Note added 7/11/2017:  Below, I will show a better method, using Vise-Grips.

Once you get the bushing off, you will be able to remove the spring keeper, two of which are shown in the next photo:

The one on the left came from the CC throttle in the previous photo and is so badly worn (due to the engine shaking) that it spins on the spindle, making the CC inoperable.  The one on the right is brand-new.  That’s right, brand-new…. I had the foresight to buy several of those back when they were still available.  Sorry, no, they’re not for sale.  If you need some, I suggest you find a junk CC and cannibalize it; there are two per CC, and the lower one is much less prone to wearing out.

The next photo shows the same CC throttle spool on its way back together:

That new spring keeper that was shown above has been installed, along with a spring, a used upper spring keeper, and a brand-new bushing to replace the one I had to destroy to get off.  The bushing was made from a piece of 8mm x 10mm steel tubing, which is readily available from multiple sources, e.g.:

Notice that the above CC throttle spool spindle has been drilled and tapped for a #10-32 screw, as has the similar CC throttle spool in the next photo:

The above photo shows my first design for this repair, consisting of (from left to right, along the screwdriver):

  1. Spring washer, 8mm
  2. Flat washer, 8mm
  3. Spring washer, 8mm
  4. Flat washer, 8mm
  5. Bronze bushing, 8mm ID
  6. #10-32 screw
  7. Brass bushing on screw, 0.310″ OD
  8. #10 Flat washer
  9. Two #10-32 nuts

The next photo shows the completed repair, along with a package of the spring washers:

The #10 nuts can be adjusted to set the tension on the pair of spring washers.  That spring tension damps the vibration so well that it is virtually eliminated.  The Stainless Steel spring washers are MSC part number 03714854.

There’s nothing really wrong with this design, but I thought it was a bit ugly and somewhat cumbersome, so I came up with an alternative that I will describe shortly.  But, before we get to that, take a look at the next photo:

Notice that the inner steel bushing sticks up above the surface of the top spool.  The spring washer would rest atop that inner steel bushing, and if not installed the “proper” way around, would never exert any pressure on that spool.  And even if installed the “proper” way around, the compressive force that it can transmit is limited, and once that limit is reached, the force will get no higher, no matter how much the nuts are tightened.  I had several options for fixing this problem, but here’s the one I chose:

I ground away the top of the steel bushing with my Dremel tool.  The resulting “scar” of bright (unrusted) steel can be seen (sort of) in the next photo:

Notice that the #10-32 screw has been inserted into the spindle and tightened down into the partial threads at the bottom of the hole, then cut off with a hacksaw.  That thread interference should ensure that the screw never shakes loose.  Red Loctite is used to hold the screw into the piece of 5/16″ brass rod shown in the photo.  The brass rod has been threaded with an 8mm x 1.25mm die.

Here’s the same CC with the threaded brass rod installed:

And here’s that same CC with the various spring washers, flat washers, spacers, and nuts installed:The nuts have been left loose so you can see the spring washers.

Next is the finished product with the tension set on the spring washers and installed on a running D24T engine:

With the engine running, there was no perceptible vibration of the CC.  This just has to greatly reduce the rate of wear of these CC throttle spools.

Now, let’s move on to another design that I actually like a bit better than either of the above.  But first, take a look at these CC parts and pieces:

At bottom right is a spring and some new and used keepers. The CC at bottom center has a spindle that is worn beyond repair, and the bushing in the top spool is very worn and loose. At top center is another badly worn CC spindle that is well beyond repair.  To the right of that are the top and bottom spool pieces that came with that CC.  Note the badly worn surfaces where the two tabs that are supposed to mate have worn edges that got so bad that the two parts actually slipped past each other.  I will show, below, how I repaired that top spool for use on the CC at the lower left.

On the left is a CC that I acquired on a “core” engine, shown just as I received it.  The top spool is missing and the middle one is broken.  But, the spindle is unworn, so I repaired the CC as shown below, using the top spool shown at the upper right of the photo.

The next photo shows how I drilled the spindle for that #10-32 screw:

Notice the two-piece alignment jig that I used to center and align the drill and the spindle.  Also notice that large drill sticking out beneath the CC.  That was placed there to hold the CC at the proper angle; there really isn’t any practical way to clamp the CC in the drill press vise.

Remember that badly-worn top CC spool shown above?  Well, here it is again:

I’m using my Dremel tool to remove some of the worn and tapered end so that I can repair it.  The end result is shown in the next photo, along with some bits and pieces:

The steel bushings that I made from 8mm x 10mm tube are cut to a length of 0.47″ (12mm).  Three of those steel bushings are shown at the lower left, along with three equivalent brass bushings just above them.  At the top are 5 new “Spring Guides”, Volvo P/N 1257731, and at the lower right are 5 of the larger (10mm x 12mm x 12mm) bushings, Volvo P/N 947258.  The brass piece that I used to repair the worn CC spool is 1/4″ wide by 3/32″ thick.  I would have preferred a wider piece, but that is what I had.  There is no functional advantage to a wider piece, but it would be easier to install.

Notice the location of the 3mm x 0.5mm x 8mm flat head screws, one at 6 o’clock and the other at 3 o’clock in the above photo.  Those locations were strategically chosen to put the screws where they would not interfere with the CC cable.

The next photo shows how I used my drill press to steady the tap while tapping the holes in that brass piece:

The 3mm screws were treated with some Green Loctite, #290, penetrating grade, to ensure they would not shake out:

The bushing in this CC spool was badly worn, so I had to replace it.  Also, the top of the spool was badly worn, and at an angle, so I had to put it in my lathe and cut it flat and parallel.  As a result, the top of the spool is now recessed, and the bushing sticks out above the top of the spool, as shown in the next photo:

To  compensate for the missing material, I made a thick washer to fit down over the protruding bushing.  That thick washer is shown at the left of the “stack” in the above photo.  Items that go onto the top of the CC are as follows, from left to right:

  1. The thick washer, 12mm ID (present in this instance only to compensate for wear).
  2. An 8mm steel washer.
  3. A Teflon-impregnated Delrin thrust washer, reddish brown color.
  4. An 8mm steel washer.
  5. A compression spring, 7/16″ x 1-1/16″ x 0.041″, Century Spring Corp, #C-632, purchased at my Friendly Local Hardware Store.
  6. An 8mm steel washer
  7. Two 8mm x 1.25mm hex nuts.

And here’s the finished product:

I like this design better than either of the previous two.  It allows for easy adjustment of spring tension, and that spring tension is applied all the way around the spindle, not just along the contact lines of the wavy spring washers, so it should provide better damping of vibrations.  Then, there’s that super-nice Delrin thrust washer.  That’s just “tickety-boo.”*

* “Tickety-boo” (c) Estate of Beryl McPherson.  Used with permission.


Additions, made July 11, 2017 to show lessons learned while rebuilding the first dozen of these D24T CC spools:

First, I’ll show you that better way to remove the steel bushings that I promised you:

Use your Vise-Grips to flatten the entire length of the bushing along the flat side of the spindle.

Then, loosen your Vise-Grips a bit, rotate them 90 degrees, and re-attach them lightly:

Then, gently rotate and pull the bushing off.  A bit of lubricant, such as PB Blaster, or WD-40 may help, but probably won’t be needed.

You may have to file a burr off the E-clip groove on the spindle.  If you’re careful (and a bit lucky) and depending on how worn your bushings are, you may be able to re-use the bushing by rotating it 180 degrees and pressing it back onto the spindle.

To cut the 8 mm threads on the brass rod (shown earlier) I use this self-aligning die from Irwin/Hanson Tools:

Even though the die is “self-aligning”, I found it advisable to use my lathe center to steady and help start the die:

To drill and tap the other end of the brass rod, first use a center drill on it.

Then face the end off square.  I used the parting tool shown in the above photo.

Next drill the rod about 1/2″ deep using a tap drill (#21 for a #10-32 screw, or #19 for a 5mm x 0.8mm screw):

The rule lines on the lathe tailstock are a convenient means to gauge the depth of the hole.

Next, tap the hole, using the lathe center to steady the tap:In the next photo I’m making a new spindle shaft to replace worn ones:

The rod I’m using for the spindle shaft is 8mm oil-hardening drill rod, available from multiple sources.  I got mine from

Now, tap the hole:Tapping this drill rod material is difficult, but not impossible, provided you’re very careful.  However, after making several this way, I bought more drill rod material and made some spindles with a male thread on each end, 6mm x 1mm on the bottom end and 5mm x 0.8mm on the upper end.

The next photo shows use of a self-aligning 6mm die to cut the lower thread:

It’s much, much, easier to cut a male thread than a female thread; however, my very first attempt, with a brand-new self-aligning die, turned out like this:

I was able to salvage the above spindle by using a standard die and threading the rest of it.

But I was totally unable to cut a second thread with that self-aligning die; I had to replace the die.  The replacement die was just fine; I used it to cut multiple threads, with no problems.

Next, I’m using a 5-mm self-aligning die to cut a thread on the upper end of that same spindle:

And, here’s the resultant thread, after finishing the job with a standard die:

Next, I took my new replacement spindles to a friend’s machine shop, where he cut flats on them, using his Bridgeport:

Here’s a shot of my new spindles, some with male threads, some with female threads:

The ones with male threads do not yet have flats cut on them.

The male threads are 5mm x 0.80mm.  The female threads are #10-32.  A #10-32 screw is 0.190″ diameter, whereas a 5mm x 0.80mm screw is 0.1969″ diameter and has 25.4/0.8 = 31.75 threads per inch.  The two threads are so close that it’s possible to screw either type into or onto the other without significant damage to either thread.

In the next photo, I’m center-drilling the end of an old, worn, spindle so that I can drill it and cut it off:

Notice how the flat on the old spindle is strategically placed between jaws on the lathe headstock.

Next, I drill it out, using a Letter A (0.234″) drill:

The idea was to use a drill slightly smaller than 6mm (0.236″) to ensure the new spindle was tight in the hole.

Next, saw the old, worn, spindle off:

I couldn’t quite cut it all the way off using the hacksaw without cutting into the bracket, so I finished the job using just the blade:

Now, file the cut end off, nice and flat and smooth:

Here’s something that seemed like a good idea at the time:

I was intending to use the center spool (above) in place of a missing top spool, so I installed that brass piece, using a “J” hook fabricated from a piece of 1/8″ steel rod.  But, well, this idea was soon discarded and replaced by the design shown at lower right in the next photo:

Here’s the next step in the process:

If you look closely (through the slotted hole) you can see that there is a nut screwed up against the head of the bolt to provide more and better contact with the “arm” coming from the bottom spool and up through the middle spool.  On some of the repaired CCs, depending on just how much space was available between the top and middle spools, I also put a flat washer between the nut and the lower side of the slot, to move the head of the bolt downward and provide even more contact with that “arm.”

To make this work, you’ll be needing a left-hand spring on that top spool.  You can make one from the original right-hand spring using the process outlined in the next photo:

On the left is a standard-issue right-hand spring, taken from a D24T CC spool.  Next is a similar right-hand spring with the left end pulled up and over and onto the right end.  By continuing the process, the spring is transformed into a left-hand spring as shown in the third position.  After a bit of stretching, it will be just like the standard-issue left-hand spring (on the right) that comes with B230 CC spools.  This particular left-hand spring came out of a brand-new D24T CC spool that was mis-assembled at the factory.

It’s probably not necessary to stretch the spring.  Truth be told, I had a few of those incorrect (B230-type) springs, which I used for repairing the CCs shown in this post.

Next, you need to re-bend each end of the spring:

I re-bent each of my springs without using a torch, but I did manage to break a bit off each of two ends during the re-bending process, so you might prefer to heat the ends up a bit before re-bending.

In addition to the spring, you will also be needing a guide for it to wrap around:

I made mine from the above 3/4″ copper pipe plugs, which I purchased from Home Depot:

They did require a bit of modification.  First, I used my lathe to drill an 8mm hole in the end, then put them on a mandrel (made from a D24T exhaust manifold stud and a couple of 8mm flange nuts):

I cut the length down from 1.010″ to 0.816″ as shown in the next photo:

The length isn’t crucial; later ones were cut to about 3/4″ long.  Note that the upper end of the plug was left a larger diameter to provide better support to the top of the spring.

And the diameter was cut from about 0.872″ down to about 0.835″:

The diameter is fairly crucial; if it’s too small, the guide will not provide sufficient support to the spring; if it’s too large, the spring will tighten around the guide and prevent the spool from rotating sufficiently.

The next photo shows the spring and various other pieces, ready to be installed on the top of a CC spool:

If you have a really sharp eye and are wondering why the copper plug has solder on it, well, that’s because this particular one was salvaged from a very old piece of pipe that I’ve been carrying around for 33 or so years, and when I got it, it was probably at least that old already.

The spring anchor is fabricated from a 1/2″ x 2″ mending plate, purchased from my FLHS (Friendly Local Hardware Store).  The top spring keeper is made from a worn-out OEM one, such as was shown earlier.

And the finished product:

Except that the spring anchor was later cut off a bit shorter.  Note the multiple holes in the spring anchor.   Later versions have only one hole for the spring.

Notice that the brass rod has a 7mm hex filed on the end to allow use of a 6-point 7mm box end wrench.  That helps a lot when adjusting the spring tension.  BTW, the optimal amount of pre-tension on that spring seems to be about 1/6 of a turn, which is fairly easy to judge, given that the hex nut has 6 sides on it.

Why put the spring atop the CC, you ask?  Well… it serves multiple purposes:

  1. It allows me to use a center spool piece in place of a top one, and since I had several more center ones than top ones, I could repair more CCs.
  2. It allows me to use a worn out keeper (one with the D-shaped hole worn away) to retain the spring.
  3. I get to conserve my limited supply of new D-shaped keepers, which will permit me to repair more CCs, just in case someone should ever need them.
  4. If the top portion of the spindle is worn (but not too badly) then it may be possible to repair the CC without replacing the worn spindle with a new one.
  5. It makes it easier to ensure that there is axial compression force applied to both the top and the center spool, which helps to prevent vibration and premature wear.
  6. If the CC ever needs repair in the future, it’s bloody unlikely that it will be the top spring and keeper that need to be repaired.
  7. The 6mm bolt and nuts used to attach the spring anchor provide much more contact with the vertical “arm” coming from the bottom spool and up through the middle spool.  That permits me to use a bottom spool with a badly worn “arm” and still get secure engagement with the top spool.  So, there are suddenly lots more CCs that I can repair successfully.

So, given all those advantages, I decided to use the same approach on a top spool as shown in the next few photos.  Below, I’m setting up a slightly worn (but still perfectly good) top spool in my drill press.  I’m using a short, stubby (and very stiff) “center drill” to enable the drill to stay in place as I drill the hole:

Because of the way the center drill is machined, it appears to be a bit off-center above the slightly larger (roundish) area beneath it.  But, I assure you, that drill is centered, dead nuts, above that roundish spot.

We’ll come back to the above piece after the next photo, which shows a pair of brand-new, never-used, D24T CC throttle spool pieces:

In the above photo, the middle spool piece, on the left, measures 0.497″, and the top spool piece, on the right, measures 0.466″.  This information is for future reference, as we go back to work on the top spool piece shown in the previous photo.

Here’s that same top spool piece again, this time mounted in my lathe:

I’ve just finished facing off the worn portion of the top.  You can see a bit of the worn area left behind around about 10:00 o’clock.  The top and bottom of the spool pieces need to be flat and parallel to allow the compression spring to hold the spool pieces securely together and prevent vibrations.

Next is the same top spool piece, mounted the other way around in my lathe:

I’ve just finished facing off the bottom surface, and once again, I’ve left behind a smidgen of wear at around 10:00 clock.

The middle spool shown in the next photo also required facing off (on the lower side) to square it up with the spindle:

In this case, so much material was removed from the spool piece that I had to insert a Delrin shim between it and the bottom spool to ensure sufficient clearance between the two spool pieces.  The thickness of the spool piece, plus that of the Delrin shim comes to just about the 0.497″ thickness of the brand-new spool piece shown earlier.  I believe the shim itself was about 0.015″ thick.

Next, that same top spool piece that required facing off earlier.  Notice the hole drilled through it for the 6mm bolt and nuts used to attach the L-shaped spring anchor to the top:

This CC required a Delrin shim about 0.040″ thick to provide sufficient clearance between the top and the middle spool pieces.

I found these thick 8mm stainless steel washers (used under the exhaust manifold nuts on D24 and D24T engines) to be great for use as “thrust washers” between the top spool and the compression spring that I installed atop these CCs:If you look carefully at the washers, you will see that they’re slightly convex on one side and concave on the other side.  Now, if you install the concave side down, against the top spool of the CC, and the convex side up, against the Delrin thrust washer, and if you do everything else properly, I guarantee you that this washer will turn with the top spool piece, and the Delrin washer will remain fixed to the spring, so the thing spins where you want it to spin.

Among the “everything else” that you must do properly is to ensure that the stainless steel washer sits on the top spool piece and not on the top of the 8mm x 10mm bushing that that top spool piece turns on.

In the next photo, you can see that the 8mm x 10mm bushing projects a bit above the top of the spool: If I’d been paying more attention when assembling this CC, I’d not have let that happen.  But it did happen, and rather than remove the bushing to shorten it, and thereby destroy it, I simply machined a bit of a recess into that stainless steel washer and threw the thing together.  Good enough!

And here are a couple of finished products:In the “stack” atop the left CC, from bottom to top, we have:

  1. Thick Stainless Steel washer from a D24 exhaust manifold stud, installed concave side down.
  2. Delrin thrust washer.
  3. An 8mm steel flat washer.
  4. A compression spring, 7/16″ x 1-1/16″ x 0.041″, Century Spring Corp, #C-632, purchased at my FLHS.
  5. Another 8mm steel flat washer.
  6. Two 8mm nuts.

In the “stack” atop the right CC, we have all the same items, plus:

7. A guide, fabricated from a 3/4″ copper pipe plug.
8. A left-hand spiral spring.
9. A spring keeper, fabricated from a worn-out OEM keeper
10. An 8mm steel flat washer.
11. An 8mm nut.
12. And projecting above the nut, we have a bit of 8mm threaded brass rod, with a 7mm hex filed on the end.

Finally, we have a “class photo” of the “teacher”, a brand-new D24T CC, and her eight “students”:That brand-new D24T CC has actually been repaired to correct for the aforementioned mis-assembly at the factory.  And as for the “students”, well, in my opinion, they’re all, every one of them, superior to their teacher.   They will all perform better and last longer than the originals ever did.

Although two of the “students” are already spoken for, the other six are for sale, on an exchange basis only.  You must return a core.  Three have just a compression spring, and three have both the compression spring and the torsion spring.

Now, which is better, the ones with just the compression spring on the top, or the ones with both the compression spring and the torsion spring?  Well, in my opinion, the ones with both springs are superior; that is, they should perform at least as well as the others and last at least as long.  But they have the advantage of being more easily rebuilt if that is ever needed.

I would estimate that I have at least one full day of labor, plus some parts and tooling expense, invested in each of these CCs.  Because of the time and expense involved, I have to get a substantial amount for these things.  The last time I bought one of these was back on 6/28/2011, at which time:

  1. The list price was $200.48.
  2. I paid $158.91 each for them.
  3. They were all defective and required repair.

Accordingly, I am offering the 6 that I can spare at $200.00 each, exchange, plus shipping.  If you’d rather that I repair the one you have and return it to you, I’ll do that, if possible, at the same price.

Please state whether you prefer the one-spring or the two-spring variety.  My preference will always be to conserve what few “D”-shaped keepers I have and to make and sell as many of the two-spring variety as the market will bear.  So, unless you’re adamantly opposed to the looks of the two-spring variety, that is probably what you will receive.

Although I don’t currently have any D24 or early (1983) type D24T CCs for sale, I can rebuild them for you at the same price if you send me a core.

See my “About” page for contact information.









This entry was posted in Volvo 700-900 Series, Volvo Diesel Parts for Sale. Bookmark the permalink.

One Response to Volvo D24T Cruise Control Throttle Spool Repair

  1. Derek Smith says:

    A very well thought out fix ! Tickety -boo !

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