Drivers almost never describe this failure as a symptom. They describe it as a count.

Lost half my gears. Only got the bottom five. Can’t get the top side anymore. Whatever the wording, the complaint arrives as arithmetic, and that is genuinely useful, because on a 13 or 18 speed box the number of ratios you lose is close to a diagnosis on its own.

That only works if you know how the box is built, and most owners do not, through no fault of their own. Nobody ever explained it to them. Ressorts Laval / C.T. CAM has been rebuilding heavy manual transmissions for decades, auxiliary units included. This article covers how these gearboxes are actually assembled, why the ratios you lose usually name the section at fault, and what that changes before anything gets pulled apart.

Multiplication, not addition

A number like eighteen puts the wrong picture in your head. Nobody managed to line up eighteen gear pairs inside a truck transmission case.

What is actually bolted under your truck is two gearboxes in series:

• The main section, sitting right behind your clutch. This is what your lever commands directly, and it holds a modest number of ratios. That is all it has.
• The auxiliary section, mounted behind the main one as part of the same assembly. Your lever does not reach into it the same way. Its job is to take whatever the main section hands over and run it through an additional gear set.

The result is arithmetic, and this is where the big number comes from. Ratios do not stack, they multiply. Every ratio in the main section exists once per configuration the auxiliary section offers.

A 10 speed is the cleanest example. Five lever positions, run once in low range and once in high range. Five times two, ten ratios. The driver works up through five, changes range, and repeats the identical lever path for the next five.

A 13, 15 or 18 works on the same principle with one more subdivision layered on top of the range. The exact combination changes from model to model, and we are not about to hand you a universal chart: the real count is built into the model designation and stamped on the transmission tag. The number is not the point. The principle is. Some of your ratios do not come from the gearbox your lever is holding.

Range and split are two different words for a reason

North American shop English gives you something here that is worth using, because it maps directly onto the hardware.

Range and split are not two ways of saying the same thing. A range change moves you between two halves of the gearbox and takes you across a block of ratios. A split is a smaller step that subdivides individual ratios, which is exactly what turns a base gearbox into a 13 or an 18. Drivers keep the two straight instinctively. They will say the box will not go to high side, and they will say the split is gone, and they mean two genuinely different things.

Keep those two words apart when you report a fault and you have already done half the sorting, because losing a range and losing splits do not produce the same pattern in your gear count.

Reading the section by what went missing

Everything below follows from one fact established above: your main section gears each get used more than once per full sweep of ratios, in different configurations. That repetition is what makes the fault readable.

When one position fails, then works again later

If a lever position refuses in one range but delivers correctly in the other, you just learned something specific. The main section gear tied to that position worked in front of you. It is a poor candidate for the blame.

A fault behaving that way points instead at what differed between the two attempts, which is the auxiliary section configuration at the moment you tried.

When a whole block disappears at once

This is the lost half my gears call.

If every lever position answers normally in one range and none of them answer in the other, your main section is likely fine. Five gears do not choose the same Tuesday to quit. The range is no longer changing, and the truck is parked on whichever side still works.

It is worth saying plainly that this symptom frightens people more than it deserves relative to what it usually is. Losing one ratio and losing half the chart are not different in severity so much as different in kind.

When it is every other ratio instead of a block

Here is the pattern most owners have never had described to them, and it is the reason the range and split distinction earns its keep.

If the splits stop working, you do not lose a block at one end. You lose the intermediate steps spread through the whole sequence, and the box behaves like a gearbox with fewer, wider ratios instead of one that suddenly ends halfway. Drivers usually report that as gaps rather than as a loss, and it is easy to under report because the truck still covers its route.

What to write down before you call

This does not replace a diagnosis. It does something else: it gets the truck to the shop carrying the information that matters, instead of a shifts bad that forces everything to start from zero.

1. Which positions answer, and in which range. Not a few are missing. Which ones, and which side. This is the only note that separates the two gearboxes without opening anything.

2. Whether the fault follows the position or follows the range. Same question as above, asked from the other end, and it carries the most weight of anything on this list.

3. Whether ratios vanished in a block or as gaps. Block points at range. Gaps through the sequence point at splits. Two different reports.

4. How it arrived, and who else drove the truck. Overnight or creeping in over weeks are different stories. And on a fleet with rotating drivers, somebody usually noticed it earlier and said nothing because it still worked. That date is worth real money.

What you are seeing	Where it points
One position bad in one range, fine in the other	That main section gear demonstrably works. Attention goes to the auxiliary section.
Same position bad in both ranges	Main section is involved. The fault follows the gear, not the range.
Every position fine on one side, none on the other	The range is not changing. Main section gears are not the story.
Ratios missing throughout, spread evenly	Reads as a split issue rather than a range issue. Report it as gaps, not as a loss.

One honest qualifier before going further. This reading orients, it does not conclude. It tells you which side to look at, which is already a lot, but it does not name the part that needs replacing. Nobody can tell you that without opening the unit.

Why it changes what we open

Both sections live in the same assembly. Knowing which one is at fault will not spare you pulling the unit when it has to come out. What it changes sits elsewhere, and it matters more than people expect.

It changes what gets looked for once the unit is open, and therefore how long that takes. It changes which parts are on the shelf before work starts rather than ordered midway with a truck sitting idle. And it changes the scope of the rebuild, because an auxiliary section and a main section are not the same job.

There is also a case every fleet owner should know about before paying for a full teardown. When an entire range disappears at once, the fault sometimes is not in the gearing at all, but in what commands the range change. The gearbox is then mechanically sound on both sides.

You will hear that from us straight when it happens, including when it falls outside what we repair. Tearing down a healthy transmission serves nobody.

What this article leaves out, and why

Better you hear it here than go looking.

Shifting technique. Working up and down an unsynchronized box cleanly is the entire subject of our article on double clutching, and it does not get summarized here. This one is about a component, not about how the truck gets driven.

Transmission wear symptoms. Grinding on the change, leaks, burnt smell, running noise and gears jumping out already have their own coverage in when to repair a truck transmission. None of that is repeated here.

Lubricant intervals and grades. Separate subject with its own rules, handled elsewhere. Deliberately absent so component mechanics and a maintenance program do not get blended together.

Power loss. Power going missing while every ratio is still available is a different problem, and the possible causes are covered in our article on the heavy truck losing power.

The system that operates the range change. We tell you it needs checking before drawing conclusions, because that is true and it sometimes saves you an unnecessary teardown. We do not describe it and we name none of its components: that system falls outside our six specialties, and we are not writing procedure for something we do not repair.

Bus transmissions. Transmission symptoms on a fixed route vehicle have their own articles, including one on a bus that jerks under power. This one is heavy truck territory.

Common questions about two section transmissions

What is an auxiliary section, exactly?

A second gearbox mounted in series behind the main one, inside the same assembly. It multiplies the ratios the main section offers rather than adding a handful to them. It is what lets a truck gearbox reach thirteen or eighteen ratios in the first place.

How do I know if my transmission has one?

The speed count is your first clue. Anything advertised as a 13, 15 or 18 speed is necessarily built in two sections. The exact model is stamped on the transmission tag, and we can read it with you.

I lost half my gears. Is the transmission finished?

Not necessarily, and correcting that assumption is most of what this article is for. Losing a complete block of ratios in one shot points at a range that is no longer changing, not at several gears failing on the same day. The main section is frequently untouched in that scenario.

What is the difference between losing a range and losing splits?

A lost range takes away a block at one end of your chart. Lost splits take away intermediate steps throughout the sequence, leaving a box with fewer and wider ratios. Reporting which one you have is genuinely useful information.

Can I keep running it in the meantime?

Driving it to the shop and finishing the week are two different propositions. A truck stuck in the low half has no place on the highway, and one stuck in the high half should not be pulling away loaded from a stop. Still moving is not the same as still fine.

Can just the auxiliary section be rebuilt?

Yes, that happens. Auxiliary units are among the components we rebuild, and the call between component repair and a complete unit gets made after opening, based on actual condition.

Does the transmission have to come out of the truck?

Usually yes, since both sections belong to the same assembly. What the symptom sorting changes is not whether the unit comes out, it is the search time once it is open and having the right parts ready instead of ordered while the truck sits.

Does the auxiliary section wear faster than the main one?

We are not going to invent a rule that does not exist. Both sections are working through every sweep of ratios, and a fault can turn up in either. The point of this article is different: there is no reason to assume the trouble is in the main section simply because that is the gearbox your lever is holding.