Smart Fortwo 450 Won't Start (Alternator Soft-Seize)
450 won't crank and feels like a starter or battery problem? On a 450 that's been sitting, the alternator pulley can soft-seize and lock the engine. Check pulley rotation by hand before suspecting starter.
Typical Symptoms
- Engine won't crank or barely cranks
- Battery is good, terminals clean
- Symptoms appeared after the car sat unused for days or weeks
- Sometimes a single click then nothing
- Bump-starting the car works (engine runs fine once started)
What it means
This one’s a 450 classic and it sends owners down the wrong path constantly. The car won’t crank. Battery’s fine. So you replace the starter. Nothing changes. Then you start eyeing the engine.
The actual culprit on a 450 that’s been sitting is alternator soft-seize. The 450’s alternator has an open design for cooling, which is great for thermal management but lets water in. If the car sits for more than a few days — especially in damp climates, especially without the plastic alternator guard fitted — the alternator’s internal bearings or freewheel can lock up. Because the alternator is belt-driven off the crank pulley, a seized alternator means the engine can’t turn either.
The starter is fine. The battery is fine. The engine is fine. The alternator is locked, and that’s holding the whole thing.
Evilution documents this thoroughly under "Unseize the Alternator" — the canonical reference for the 450 community.
Likely causes (cheapest first)
- Alternator soft-seize from sitting. The dominant cause if the symptoms appeared after the car was parked for a stretch.
- Missing or damaged plastic alternator guard. Owners often lose this during other work; without it, water gets to the alternator faster and the seize keeps happening.
- Battery dead or weak. Test it — should be obvious. If 12V at the terminals and fully charged, this isn’t the issue.
- Starter motor commutator damage. A real failure mode (Evilution covers it) but rule out the alternator first.
- Engine internal seizure. Last on the list. Vanishingly rare on a 450 unless something has gone catastrophically wrong (cooling failure, oil starvation).
DIY check steps
- Try to turn the alternator pulley by hand. Get under the car or remove the offside (passenger side) wheel for access. You’ll see the alternator on the side of the engine, with a belt-driven pulley. Try to turn the pulley by hand. If it won’t turn, that’s your problem — soft-seize. This is the diagnostic step every 450 owner should do before throwing parts at the car.
- Apply Evilution’s unseize procedure. A combination of breaking the seize loose with a wrench on the pulley nut, working some penetrating oil into the bearings, and turning the pulley back and forth until it spins freely. Many 450s recover without alternator replacement. Full procedure on the Evilution page.
- Once the pulley turns freely, refit and try cranking again. If the engine spins normally, you’ve solved it.
- Refit or replace the plastic alternator guard. Critical for keeping the seize from recurring. Worth sourcing one used if yours is missing.
- If the alternator can’t be unseized, replace it. Evilution’s "450 alternator change" page covers the full removal — lower the engine on the 450, which is how Smart designed engine-bay access (yes, really).
When to call a shop
If the unseize procedure doesn’t work and you’re not comfortable lowering the engine to swap the alternator, this is shop territory. A Smart-experienced indie can do it cleanly — price ranges $300-600 depending on parts.
If you’ve already replaced the starter, the battery, the cables, and you’re still nowhere — stop. Get under the car and try the alternator pulley. The 450 community has watched this exact diagnostic loop play out hundreds of times.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Alternator (replacement) | $150-300 | Search Google |
| Alternator pulley / freewheel | $30-80 | Search Google |
| Plastic alternator guard (often missing) | $20-40 used | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread; call a Smart-experienced shop for an actual quote.
Manual references
- Workshop / service manual PDF — full procedure with torque specs, hosted on smartcarmanuals.com.
- Browse Smart manuals on smartcarmanuals.com — pick your chassis code section on the home page if a specific manual isn't listed above.
Community references
- Evilution: Unseize the alternator (the canonical fix)
- Evilution: 450 alternator change
- Evilution: Starter motor repair (rule this out second)
Stuck on this one?
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