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Shimmy and vibration at highway speed

This is the single most-reported chassis complaint filed with the United States road safety agency for the later trucks of this generation. Owners describe a steering wheel that has to be held with both hands, shaking that arrives in a narrow band of road speed, and — the part that matters for diagnosis — a fault that survived new tyres, repeated balancing and more than one visit to a workshop. One owner report goes further and names the front upper control arm joints and their bearings as what was eventually found — one report, not a pattern, and the causes below are ranked on how this failure usually goes rather than on that single account. The pattern worth trusting is the broader one: many owners, several model years, one complaint that survived the obvious fixes. When a vibration comes back after the wheels have been balanced twice, the wheels are usually not the story: something in the front end has enough freedom to let the wheel steer itself a little, and the tyre then feeds that movement back through the linkage. This is a heavy independent front end with torsion bars, and every joint in it is decades old. It is not dangerous the way a separated joint is dangerous, and the truck stays perfectly controllable at ordinary speeds — but shimmy is a symptom of looseness, and looseness in a front end is on a path that ends somewhere worse. Keep the speed down and get the joints measured.

Gen 2Gen 2.51996–1999USWorks worseCommunity consensusSafety-critical

Drive gently — repair soonManeje con cuidado — repare pronto

Safety notice — Suspension.

This affects a safety-critical system. Reference material only: for safety-critical work, consult a qualified mechanic.

Esto afecta un sistema crítico para la seguridad. Material de referencia únicamente: en trabajos críticos para la seguridad, consulte a un mecánico calificado.

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

  • The steering wheel shakes side to side once the truck is up at highway speed
  • The vibration appears in a band of speed and eases off above and below it
  • Shaking felt through the seat and the floor as well as the wheel
  • It is still there after new tyres and after the wheels were balanced again

Likely causes

  1. Worn front upper ball joints
  2. Worn control arm bushings
  3. A tyre out of balance or out of round
  4. Worn universal joints in a driveshaft
  5. Worn-out front shock absorbers
  6. Worn steering damper

Diagnostic steps

  1. Note whether the shake tracks road speed regardless of throttle, or whether it comes in under power and eases when you lift offPoints toA tyre out of balance or out of round · Worn universal joints in a driveshaftRules outWorn-out front shock absorbers
  2. Move the front wheels to the rear and drive the same road again, and note whether the shake moved with them, stayed, or changed characterPoints toA tyre out of balance or out of roundRules outWorn front upper ball joints
  3. With the wheel raised and the suspension supported at the correct point, lever under the tyre and watch the upper joint for the ball moving in its socketPoints toWorn front upper ball jointsRules outA tyre out of balance or out of round
  4. Lever each control arm against its mounting and watch the bushing for the sleeve moving inside the rubberPoints toWorn control arm bushingsRules outWorn front upper ball joints
  5. With the driveshaft in place, hold each yoke and try to twist it against its partner, feeling for the click of a worn crossPoints toWorn universal joints in a driveshaftRules outWorn control arm bushings
  6. Bounce each front corner and count how quickly it settles, then look at the shock bodies and the steering damper for oil weeping down the tubePoints toWorn-out front shock absorbers · Worn steering damperRules outWorn universal joints in a driveshaft

Fix paths

Renew the front upper joints and the control arm bushings

$$–$$$Difficulty 4/5

Joints and bushings are done in pairs across the axle, and the truck is aligned afterwards because replacing them changes where the wheel sits. Torsion bar front suspension holds real energy in the bar, and unloading it wrongly hurts people — if you have not done one before, this belongs with a qualified mechanic.

Have the wheels balanced under load, or replace the tyre

$–$$$Difficulty 1/5

An ordinary spin balance cannot see a tyre that is stiff in one spot. A balancer that presses the tyre against a roller can, and it is the cheapest way to take the wheels out of the argument before anything gets unbolted.

Replace the driveshaft universal joints

$$Difficulty 4/5

Universal joints are replaced as a set on the shaft that has one worn, and the shaft is refitted in the same orientation it came out in. If the shake arrived with power rather than with speed, start here rather than at the front end.

Replace the front shock absorbers and the steering damper

$$Difficulty 2/5

Worn dampers rarely cause shimmy on their own, but they let it build instead of killing it. This is the cheap end of the list and worth doing while the front end is apart for anything else.

Sources

  1. NHTSA owner complaint database — 1997 Mitsubishi Montero — note the year's dataset also carries Montero Sport records, a different platform, which are excluded from what is relied on hereReference workarchived copy
  2. NHTSA owner complaint database — 1998 Mitsubishi MonteroReference workarchived copy
  3. NHTSA owner complaint database — 1999 Mitsubishi MonteroReference workarchived copy
  4. Luso Overland — Montero Gen 2 / 2.5 IFS upper ball joint (Mitsubishi MB860829 left, MB860830 right)Vendor cataloguearchived copy