rev counter telling porkies!

hi, can anyone tell me how many revs my engine should be doing at 70mph on a 300tdi coz my rev counter is saying 4500rpm? after 2500 revs the rev counter seems to fly round and will easily show 5-6000 revs without any problems. ive checked the wire on the back of the alt and seems to be fine. i would appreciate any help anyone can give.

Think mine's on 280 - 3000 ish ...

Re-check the alternator wire .. the connection to the spade connector often feels good, but check the wire hasn't corroded inside the crimp, thus not making a connection to it's own connector, before it even gets to the alternator ..

cheers will check connection now and see it looks

hi, just checked connections and everything is good so what tells the rev counter how many revs its doing or could it just be a knackered rev counter?

Slipping/grabbing fan belt?

regards

Dave

Number of rotations of the alternator converts directly to engine rpms, so possibly a slipping serpentine belt.

Is that right? I never knew that. So the alternator pulley must have a quarter of the circumference of the crank pulley?

That's how it was explained to me, and I believe it to be right ... which doesn't necessarily mean it is, but it sounds about right .. though I've never checked it.

Similar thing with a lot of cars which use camshaft rotations to work out engine rpms, cam rotates at a direct multiple of the engine so can be easily converted.

Yeah tis obvious re the camshaft - if it didnt u would have some major valve probs. Still not convinced bout the alternator tho.....

Obvious 'cos the cam's definitely, directly connected to the engine .. but the alt also is connected to the engine crank too, and to take an electronic reading is relatively easy to do.

Dunno how 'directly' the alt rotations match the engine rpms, but I'd bet it's a definite number or easily worked out multiple of them.

tiz a pure function of pulley sizes - if both are the same size its a 1:1 ratio.

Yeah granted. But does the alternator have a sensor to detect each RPM? Didnt think so?

some do - some dont.

In older vehicles, the Rev Counter is driven by the RMS Voltage waves from the low tension (LT) side of the ignition coil, while on others (and nearly all diesel engines, which have no ignition system) engine speed is determined by the frequency from the alternator Rev Counter output. This is a special circuit inside the alternator to convert from rectified sine wave to square wave, and the electrical potential difference is directly proportional to engine speed.

I dunno the workings of it, just what I've been told .. that it's a direct indication of engine rpms. How it might do it is beyond my electrical knowledge.

would a slipping belt not give a low reading due to one of the pulleys not turning as fast?

As I said, I dunno how it does it but the revcounter gets it's inoput from the alternator, so the 'connections' to the alternator are whatI'd check first .. the actual wire connector and the belt that turns it .. If all are OK then you've got to look further and maybe think it's either the wiring to the rev counter, the rev counter itself or something else, such as a poor earth somewhere on that system.

wot it does is put out a pulse every complete rev of the alternator - these pulses can be counted by the rev counter. anything that alters the pulse rate will alter the reading - a slipping belt, wrong size pulley (either crank or alt). check the connections first and if not sure remake them.

does anyone know what size the alt pulley should be as this sounds like what the problem could be because the connections are good and the rev counter does not jump or flicker.the rev counter works smoothly but seems to rise very fast. tickover is just under 1000rpm and 50mph is just under 2000rpm but 70mph is a good 4500rpm any faster and rev counter is showing nearly flat out!

Do the maths n that dont add up. We talking a manual box here???

wot he sed ^^^^^ are we talking in the same gear - coz 50mph:2000rpm = 25mph/1000revs - so 4500 revs shd be 112MPH!