Since fitting a Ring power socket distributor to the dash of Naomi, my 1996 300Tdi Defender, I’d noticed that the handy little voltage LEDs were revealing that the alternator wasn’t kicking out much of a charge at tickover. This would explain why my modest array of lights looked dim when stopped at night on the Mac 4x4.
The penny dropped when I looked under the bonnet of a later 300Tdi and noticed that the alternator pulley was smaller than mine. Research revealed that in 1996 the pulley changed from 61mm diameter (STC 1796) to 49mm (STC 3202).
Going back to my O-level maths I worked out that the S-belt would turn the alternator about 25 per cent faster with the small pulley, a speed where it would be a lot happier to chuck out some volts. The easiest way to check yours is to look at the S-belt. If it has a 1595mm ERR3287 belt, it’s a big pulley; if it is a 1580mm ERR5911 it’s the later small one.
When the shiny new pulley arrived it was clear that a very specific spanner would be needed to tighten up the nut that sits in the deep central recess, so a deep-cranked 22mm ring spanner was on order.
Access to the pulley is a tad fiddly on the Defender and it’s arguable that it would have been easier to remove the alternator to do the swap. However, with the assistance of an extra-strong arm pushing the tensioner fully clockwise via a 15mm socket and breaker bar, and with an 8mm brake caliper Allen key stopping the centre shaft of the alternator from spinning, it freed up nicely.
Heaving back on the tensioner released the S-belt, which has to be changed to the smaller 1580mm size in conjunction with the pulley swap. Pulley and belt swapped, I now had happy LEDs – a cheap, simple upgrade.
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