tacho / rev

on a petrol you connect it to the coil... so... assuming the 2.25 Diesel has a coil as well on there...

a diesel duse not have a coil !! duse it ??

and yea its the diesel 2.25 engine i have and want to fit the tacho to

hmm.. i'll take your word for it... i only know bits (and not many bits) to do with my 2.25 petrol so far!
When i finally find a 200tdi i'll start learning about diesels

I presume you have a tacho with a converter box that allows you to translate the RPM of the alternator into the RPM of the engine. You might have a problem here - the signal wire has to go onto an ac terminal on the alternator - usually marked "W" - and you're not likely to find one on an alternator fitted to a series. You can get at the wiring pre diode, but you have to get inside the alternator to do it. Which alternator do you have? (Sounds like a Lucas ACR type...)

Some tacho's will connect to an alty or to a coil, esp later ones (theres a switch to select coil or alty and the number of poles in the alty).
If your rev counter is a diesel type then the maximum rev reading will be lower, 5 or 6 krpm or less.
You can make your own connection on the alty if you are slightly electrically minded, I have done this a couple of times, its a case of finding the rectifier unit and soldering a wire to one of the field windings, a fairly simple job.
I have seen articles about doing this on a lucas acr, which is popular on landys, do a search on google, otherwise you'll need another alty with a w terminal, or you could take your existing alty to an auto leccy's and they'll fit you a w terminal for £££££'s.
Another thing I've seen was to fit a sensor to the crank pulley on a bracket so that the sensor 'sees' 2 markers per rev of the crank, citroen diesels have such a sensor, and these can be used with a petrol rev counter.

thanks very much Dr pepper

the rev counter i have goes to 8 ( x100) and it has a switch at the back for 4,6,8 cylinders
i think that the first thing i should try i to fit a w terminal to the alty i have a new Lucas alty on my desk atm come this morning if you would be willing to give me some help as to where to put the cable i am very good with electronics so it shouldn't be to hard

if its ok with you i will open it up take some pics and post them so u can tell me where and maybe what to do

thanks again

ok i have found this web site http://mk1ultra.com/mods/w_terminal.html
hear are 2 pic i took after taking the black cover off i think the wires i have mark are the ones that he talks about in the web site duse this look right to you i dont what to do it all put it in the car and find out i did it wrong

Yes you have that right, any one of the three wires with arrows pointed at them would give you the 'w' connection.
But there is a problem, it sounds like your rev counter is a petrol only.
Rev counters that do either have a switch for the number of cylinders, and another switch for petrol and diesel and the number of poles on your alty.
The only way to get you rev counter to work by the sound of it is to fit a pick up to the engine so the counter see's 2 pulses per rev of the engine, or if you like your lectronics then its still possible to adapt a petrol rev counter to an alty, you see from memory the lucas is an 11 pole alty, and so gives 22 pulses per rev, your counter expects 2, so it would drastically over read, you could divide the frequency from the alty using a cmos chip like a 4040, or maybe a cmos phase locked loop, or nowadays a little 8 pin pic micro.
You can get diesel tacho's on ebay cheap, I got a new one for my boat for a tenner, watch out for the supply voltage though, a lot are 24volts.
For a pick up you could use an industrial proximity switch, or you could rob a pick up from a citroen or pugrot diesel engine, like the xud9 used in a lot of the 90's 1.9 diesels, it might work direct, or you may need an amplifier, you could even transplant the guts from the donor cars rev counter to yours.

Don't forget that the engine rev to alternator pulse ratio also depends upon the diameters of the engine and alternator pully - alternator driven tachos should have a calibration adjustment but you need to measure the engine revs to do it!

Why?

OK, the tachometer measures electrical pulses produced by the alternator. These are produced at an exact multiple of the alternator RPM, depending on the construction of the alternator. The alternator RPM is usually faster than the engine RPM by a factor determined by the diameters of the engine pulley and the alternator pulley. I haven't measured the pulleys on a land rover but it looks about 2 to 1, but most vehicles won't have an exact ratio. The tacho therefore will be receiving pulses at a faster rate than the engine RPM but not an exact multiple of the engine RPM. The tacho has to divide this rate by a figure which will be an integer number plus or minus a bit to get the engine RPM. Hence the need for a fine calibration adjustment.