Scream under bonnet imediately after starting.

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Yorkshire Engineer
Posts: 19
Joined: Tue Dec 20, 2016 5:14 pm

Scream under bonnet imediately after starting.

Post by Yorkshire Engineer »

Noise lasts for 3 seconds.
Alternator fan belt very slightly too slack.
Background: An alternator in good condition can provide well over 200 Amps of charging current to the lead acid starter battery that has been partially discharged by use of the starter motor. This represents over 3 horse power of output that the alternator drive belt has to transmit and it tends to slip producing the noise.
Caution: over tight drive belts will cause failure of alternator bearings.
A happy compromise is a scream that lasts for half a second.
The alternator can replace the drain of a typical quick start within 5 minutes at a fast tick-over. Revving the engine is unnecessary and will lead to premature bearing wear as oil takes some time to fully float shafts. Ample current should be available for rear-screen heaters etc. at 1300 rpm. Avoid using lights for the first 5 seconds after an engine start. Alternator output voltage can be as high as 19V and this blows bulbs.
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