Rev Counter

[shadowcaster]

The rev counter went south on my beloved Discovery so being a bit boracic at the moment I got a second hand one from E Blag. The old unit was a 100amp output and I have replaced it with a 120amp unit , since doing that the rev counternow reads about 500rpm more than before. So whats going on and will it do any long term damage the the electrics.

[Bonzo]

I don't think I can be of much help to you here Rich as I don't fully understad how the rev counter gets its signal from an alternator.

Is it possible that the replacement alternator had a larger or smaller pulley !!??

Personally, I doubt that you would do any long term dammage to the electrics. The signal changes will most likely be very minimal.

[flyerncle]

2 pence worth Rich,you have the same regulated voltage of 13.6 + volts its only the output that is higher and lets you use more load.

[Chris_]

What year is the discovery? Petrol or diesel?

I believe a lot of them take the rev counter pickup from the 'W' terminal on the back of the alternator.

The only reason i can think of for the 500 rev difference would be a different sized pulley, so either swap it for the old one and it should be all sorted.

Leaving it as it is shouldn't cause any problems, will just read a little high.

Chris

[shadowcaster]

Thanks chaps I can rest a bit easier now, the Disco in question is a 94 V8 Auto, she may be old but I love her. Will check the pully size and report back.

[Chris_]

On the V8 they have been known to do funky things with seperate coil packs for each bank and as such you cannot take the tacho feed from the coil.

Cheers
Chris

[shadowcaster]

Mornin all

Quick update, indeed it is a smaller pully fitted to the new alternator. Jusy spent most of the morning trying to get said pully off, even after applying heat and a lot of sweat it ain't gonna budge , so I guess I'll live with a slightly over reading taco .

[Bonzo]

At least you know the cause now.

Not likeley, but have you checked to make sure that it is not a left hand thread that you are dealing with

Those pulley nuts can be a real pain to undo

I have found it easiest to remove them with an impact driver, that said, not a lot of use if you don't have one available.

Hope you manage to sort it Rich

[shadowcaster]

Hiya Ronnie

The big problem is holding the shaft while trying to undo the nut, theres a hex hole for an allen key in the end of the shaft but most of that is now rounded off. Think the only way now is a 24mm deep ring spanner and hope theres a enough of the hex bit to hold the shaft. Annoying thing is the old pully came off so easy it was'nt true.

[Bonzo]

I know the feeling Rich

If you have some way of getting some decent amount of heat on the nut. I have often found it usefull to heat the nut up as much as possible. Cherry Red is nice, bright red even better. Need'nt be the whole nut, part of it at a decent temp will do the trick.

Once hot. Quench the nut in cold water........this will often loosen even the worst of heavilly corroded stubborn nuts.

Obvously, take care not damage the thread or immerse the alternator itself into the water