I'm on a posting roll!

The other problem I have with my classic is that the Rev counter reads too high. The axle ratios are right and the gearbox and transfer ratio's are standard.

The only thing I can think of is that it has an alternator pully that is too small in diameter and therefore reads incorrectly to the rev counter.

Does anybody know what size the alternater pully should be on a 4.2 serpentine belted car? Or indeed if I am missing something more obvious?

Cheers C

What about the speedo take off at the Gearbox????

since when has the rev counter been driven orf the gearbox?

Forgot to add that the alternator only drives the tacho. and is not connected to the speedo. electronically-input to the speedo comes from the gearbox, not the engine speed-think about it when you are doing 4000rpm in low range gears and moving like a snail!!!

Irishrover said:

Forgot to add that the alternator only drives the tacho. and is not connected to the speedo. electronically-input to the speedo comes from the gearbox, not the engine speed-think about it when you are doing 4000rpm in low range gears and moving like a snail!!!

Click to expand...

i thought the tacho would be driven by an output from ecu?

Oh Mad Hat- got me tacho. mixed up with the speedo. I've been driving all these years at what I thought was 4500 m.p.h.
Yes, the pulley size would make a difference to the TACHO. but I would have thought the serpentine belt tensioner would have been almost on the dead stop if the correct belt was fitted and the pulley was smaller. As I posted to Redrange ref. throttle pedal-come on guys get your ruler out and let us know what size (Pulley) you have

At a true 70 mph the tacho reads 3250 rpm. I beleive that it should read somewhere in the order of 2600 rpm. At a true idle of 900 rpm the tacho shows 1150 rpm.

Both are aproximately 25% out, its not a major problem (albeit that the alternator may be running 25% too fast) its just a phsychological one, when you are on the motorway overtaking you often see over 4000 rpm and think of the £1.10 a litre you could potentially be guzzling!

I think the only answer is to compare the size of my pulley with somebody elses! After a11 a 25% increase in circumference on an already small pulley wouldn't be a huge increase in physical size.

Cheers and thanks in anticipation

C