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Alternator Testing

Mod Description
Find out if the alternator is charging the battery
Mod Details
PremiumYes Difficulty Mod ID732 Creditevilution Cost£ï¿½7 For450 Fortwo451 Fortwo452 Roadster454 Forfour Link Copy to Clipboard

Testing Conditions

Start the car and turn on all of the electrical doo-dads. Cabin blower, rear screen heater etc. This ensures that the alternator is requested to output as much power as it can. Some smarts regulate the voltage from the alternator so with no extra accessories running, a healthy alternator can output a much lower voltage if the car isn’t using very much.

The test is ideally done from cold. 60 seconds from then, the car is no longer classed as being cold. So check quickly.


With the car running you should see in excess of 14 volts.
On cars with stop/start like the 451 MHD and 453, they tend to use a much higher voltage to quickly top off the battery. On these models, it’s not unusual to see over 16 volts.

But what happens if you see 12v or less when it’s running?

This page tells you how to check the alternator and how to diagnose the problem causing the low voltage.

Checking The Alternator


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