Balance shaft belt takes the timing belt with it

The 4D56 diesel runs two belts behind the same cover: the timing belt, and a smaller balance shaft belt sitting behind it. The inner belt drives nothing the engine needs to run, which is exactly why it gets forgotten — a truck can have had its timing belt changed on schedule for decades while nobody touched the belt underneath it. A buyer's guide written for this generation and an owners' club thread agree on what happens next, from different directions. The club's members describe a snapped balance belt taking the cam belt with it and describe the worn, chewed sections of tooth that mark where the shafts stopped turning. The guide states the outcome for a belt-driven diesel of this design in one sentence: a thrown timing belt usually means a replacement engine. **The fitment above is narrower than the guide's own sentence, deliberately.** That guide groups this generation's diesels together; this entry is scoped to the one engine the club thread is actually about and the one the belt pair belongs to. If your truck has the other diesel, do not assume the entry applies or that it does not — find out what drives your camshaft and your balance shafts before you plan a service around either answer. There is no warning light for this and nothing to feel from the seat. What you have instead is paperwork: if nobody can tell you when the inner belt was last changed, treat it as never. The same thread records that the club's own experienced members often remove the balance belt rather than replace it, and reports the engine running smoothly without it — that is a real and widely used answer, and it is also a change to how the engine was built, so it is set out below as its own choice rather than buried in the recommendation.

Gen 2Gen 2.51991–1999Damages other partsCommunity consensus

Do not driveNo lo maneje

Community consensus. This entry is not backed by factory documentation — treat its values and steps as a starting point, not as if they came from the factory manual.

Consenso de la comunidad. Esta entrada no se apoya en documentación de fábrica — tome los valores y los pasos como punto de partida, no como si vinieran del manual de fábrica.

Symptoms

No service record shows the inner belt was ever changed with the timing belt
A rattle or whine from behind the timing cover that changes with engine speed
A new vibration through the engine that was not there before
Oil pooled inside the timing cover and soaked into the belts
The engine stopped without warning and will not turn over properly or restart

Likely causes

Balance shaft belt worn, shredded or already broken
Leaking front oil seal soaking both belts
Worn tensioner or idler pulley
Timing belt already past its service life

Diagnostic steps

Ask for the service records and find out whether the inner belt was changed with the outer one, and treat 'the timing belt was done' as not answering the questionPoints toTiming belt already past its service life · Balance shaft belt worn, shredded or already brokenRules outLeaking front oil seal soaking both belts
Take the timing cover off and look at both belts, at the pulleys and at the inside of the cover for standing oilPoints toBalance shaft belt worn, shredded or already broken · Leaking front oil seal soaking both beltsRules outTiming belt already past its service life
Turn the engine over by hand and inspect the inner belt right around its length, looking for sections where the teeth are worn away rather than a single damaged spotPoints toBalance shaft belt worn, shredded or already brokenRules outWorn tensioner or idler pulley
With the belts off, spin the tensioner and every idler by hand and feel for roughness, and check each for side playPoints toWorn tensioner or idler pulleyRules outBalance shaft belt worn, shredded or already broken

Fix paths

Replace both belts, the tensioner and the seals as one job

$$–$$$Difficulty 4/5

Both belts, the tensioner, the idlers and the front seals come out together, because the labour to reach the inner belt is nearly all the labour of the job and doing half of it now means doing all of it again later. If a seal was leaking, the new belts will be ruined by the same oil unless the seal is fixed with them.

Remove the balance shaft belt rather than replace it

$$Difficulty 4/5

What the club's experienced members describe doing, with one member reporting the engine noticeably smoother without it. Stated honestly: this removes the risk of the inner belt destroying the outer one, it costs nothing in parts, and it is a deliberate departure from how the engine was built — the balance shafts exist to cancel a vibration, and opinions on what the engine feels like afterwards differ. It is a choice to make with your eyes open, not a repair.