Seized Alternator

Mod Details

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On the 450 fortwo and the Roadster, due to the open design and the placement of the alternator, if you drove your car in wet conditions and then left the car standing for a few weeks, there is a chance it won’t start. This is because water gets in and creates rust and corrosion. This stops the alternator from spinning.

Symptoms

If you seem to have full power at the battery and you hear the starter motor engage but then the engine doesn’t turn, the alternator is probably seized.

If the engine turns over very slowly (not quick enough to start the engine) then your alternator is seized and your alternator belt is probably too lose due to stretching or incorrect tensioning. Partial seizing can cause the alternator to also spin slowly.

Belt Problems

Having a car that doesn’t start is a problem but it can be worse. If the belt tension isn’t tight enough, the belt can slip over one of the pulley wheels. Why is that worse? I’ll talk you through it.

The belt is driven by the crankshaft pulley at the bottom of the engine. If the belt is slipping over the seized alternator pulley, the car won’t charge. You could get 5 minutes down the road before the car died.

The bigger problem is if the belt is being held firm on the alternator pulley so it slips over the crankshaft pulley which is trying to drive it. This way, the belt doesn’t move at all. The problem there is that the same belt also turns the water pump. If that doesn’t turn, your car will overheat 5 minutes down the road.

How Do I Check?

If the engine is turning slowly, you need to jack the car up, take the wheel off, remove the arch liner and remove the belt cover. Try and start the car and look at the alternator pulley to see if it is turning or if the pulley is stationary and the belt is just slipping over it. If the engine isn’t turning, try freeing it using the info below. If it is moving but very slowly, still try the info below.

How Do I Know My Alternator Is Seized?

A seized alternator and a seized starter motor have the same diagnostics. The engine won’t turn. To work out if it is the alternator or the starter motor, try push starting the car. If you can push start it, it’s the starter at fault, if it won’t start, the alternator is probably seized.

How To Fix A Seized Alternator

Depending on what tools you have at your disposal, you may be able to repair it without lowering the engine at all. Have a look at this page for the full guide.

Other Damage

Continuously trying to start or run a car with a seized alternator can burn out the starter motor and damage the belts.