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300tdi Alternators


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Greetings all......

Is there anything that can be done to adjust the rev-counter pulses on a 300tdi alternator???

I've got a '96 300tdi Disco, with a dead alt. So, I swapped the alt off my (currently broken) '95 Disco. They're both 100amp A127's. However, the rev-counter now under-reads by about 1000rpm.

Looking in the Disco parts catalog, and lo and behold they changed from a Lucas A127 to a Magnetti Marelli A127im, which presumably is what's putting the rev-counter out?

Now in typical fashion the Marelli alt is even more expensive than the lucas one (~£170 inc VAT) and appears to be specific to LR's (so no trip to the breakers to find a donor car that has the same unit), so I'm trying to work out what my options are:

1. Put up with the rev counter being out

2. Try to get hold of a s/h Marelli alt

3. Do 'something' to the Lucas one to make it read right

Any suggestions? There's a 'wanted' ad about to go in the classifieds sections



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are the pulley sizes the same?

Pulley size will almost certainly be the problem, they changed around this time so you probably have one of each. There are two types, later vehicles have a smaller pulley (49mm instead of 61mm IIRC) and a different belt (ERR5911 or PQS000040 on later vehicles, ERR3287 or PQS000030 on early vehicles) the idea being for the alternator on later vehicles to spin faster at a given engine speed and therefore charge better when the engine is idling. So when you put an early alternator on a late vehicle, it spins slower (because of the bigger pulley) and the rev counter will be wrong.

I'd try fitting the pulley off your old alt, that should cure it and is free :)

Also it is important to note that the belts are different lengths, you can get away with using the longer (early) belt on a later vehicle but if you do it the other way round it is probably a bit too tight hence more strain on all the bearings, and the tensioner. Better to have the right belt fitted for the alternator.

If this isn't in the Tech Archive already Tony, might be worth putting it in there as I have provided the above info a few times in the past.

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