Alternator pulley size affecting revs

I swapped my alternator ( it was attached to the engine I was swapping) a while back on my serpentine 3.9. The new alternator has a significantly smaller pulley. I'm convinced this is giving an artificially high rpm readout on the rev counter... Why are there 2 sizes of alternator pulley? Am I going mad?

sidewaysste said:

I swapped my alternator ( it was attached to the engine I was swapping) a while back on my serpentine 3.9. The new alternator has a significantly smaller pulley. I'm convinced this is giving an artificially high rpm readout on the rev counter... Why are there 2 sizes of alternator pulley? Am I going mad?

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sounds like you might have a diesel one-normally diesels have a smaller pulley as don't rev so hard

hmm, it was attached to a V8. How do the diesels generate the tacho signal?

From your description, I would say even though the alternator may be from another v8, it may be of a different rating hence its smaller pulley which allows it to run faster to compensate for load at any given engine speed. To recalibrate your tacho readout, swapping the smaller pulley with the larger one from your original alternator should give you more accurate readings.