modifications converting a a1 tach for diesel use: note this can be done on any tach that is used in a vw solong as it is a genuie VDO gauge. here's what you need - an alternator with a "W" output terminal a Bosch 65 amp modeel off of an 84 jetta diesel might have a sutible doner. check the local junk yards and see what the have . - a tachometer out of a gasoline model you dont need the whole dash just the tach ( be freindly and dont destory the dask and the other gagues take what you need and leave the rest. you might need it latter......) any model from 81-84 should have one. youer fuel an d water gauge are still there in the new assy. - wiring stuff: about 15 ft of wire to run from the alternator w terminal to the fuse block. on my car I had to add it on . wiring was already existed from my fuse block to my instrument panel, though ( don't you love vw's?) A small groumment to get the wire through the firewall. This is the best way to go so that you don't short out your eletrical system.also some quick conncetors male/female and some wire ties to make it all look nice. - the electronics : a 1/2 Watt 1.3K-1.8K resistor (you need a !/2w becuse it might see a little power) , a multi turn 5k ohm trimpot (for fine-tuning your vehicles rpm gauge), a 1/4 or m1/8 watt 1k-5k resistor ( these varances are needed due to altatude and location of you vehicle you my need to try them to find one that works for you best.) - TOOLS : wrenches for alternator adj. blots, wire crimpers/stripers, drill and drill bits for grommet hole, a soldering iron, eletronic solder (NO ACID CORE SOLDER) screw drivers, flashlight. here's the install / setup process step 1 : Alternator install upgrade your altanator to the new "w" terminal model. Might as well check brushes when you have the alternator out ( brush length <5 mm =bad) step 2 : alternator test/ tachometer initial test do a quick external tach hook up to make shure the alternator and tach work. if you look at the circuit board on the tach (shaped like a caressent ) it will have a little "+" for positive, and a "-" for ground, and a "1" for w. do a quick hookup if you have some spare wire laying around. the tach will idle atabout 5,000 rpm minium. acceleration will cause bazarre behavior, but this test is to make sure your alternator works. step 3: modify the tach ******do exacly as explained ****** Unscrew the twoscrews that hold the metal plate on. This will make the metal plate loose. You dont need to remove the needle, just let the plate dangle on it, burt be careful not to damage the plate or the needle. now you can get to the circuit board. there are three (3) points of interest on your tach : #1 is where the blue wire connects on your circuit board. solder the 1.3k-1.8k resistr BETWEEN the BLUE wire and the the circuit board here. make sure the solder ends don't touch the othere traces, or the metal plate. Point #2 the trimpot. Replace this unit with your new 5k model. Take care to note how the old pot was in the circuit so that you can replace it properly. (the old one has too high a value / not enough precission for diesel setup.) Make sure it does not stick out of the side too far, and make sure the adjustable part is towards the side, for easy adjusting. point #3 is the resistor on the edge of the circuit board- it has a trace connecting it to one of the trimpot's leads. this resistor is wired in series with the trimpot. replace it with your 1k-5k resistor. (the old one once again has a value too high). when you are done put the PCB the plate and screws back how they used to be. step #4 : check it out (modified tach check and adjusment) do the same as instep #2 to make sure everything works, except this time with the modified tach. Adjust trimmer (the pot the you put in) to get the rev's in the right place. (if everything's working right you should be able to set the tach where you want it - at about 80-900 rpm idle) i have tested revving the tach up to about 4000 rpms, and it seems to work ok. step#5 install tach in instrument panel remove dashboard pannels as follows: Left speaker opening - 1 phillips (gets you access to the speedometer cable connection behind instrument panel.) dashboard panel - remove stereo, cold start knob (twists off) remove cigarette lighter flap cover, remove 2 hex screws (left and right) on bottom of panel by cigarette lighter, remove 3 screws at the top of panel, remove headlight switch the knob pulls off, remove panel (don't forget to turn off headlights!) remove four screws holding in instrument cluster. nowsee if you can reach in the left speaker hole and unscrew the spedo cable. if it is too tight, then go from thebottom and reach up into the cavity and unscrew it, it looks tight but your hand can fit in there be careful of the wires you dont want to pull anything loose. after thecable is unscrewed rotate the cluster towards you and remove the multiwire conector. now you can remove the cluster. remove the old clock .(you will need to cut the two traces that hang out and conect to the clock.) put the new modified tach in where the colck was. fule and water gauge hook right bak up (don't you love vw?) step 6 : run wire from fusebox to alternator remove fusebox covering panel (if you haven't already). the place you want on the fuse box is conveniently located. Find the "fuel Pump" relay loaction - its on the right, in the middle. There should be no relay here its only for gas models. the instrument cluster tach wire is connected to a slot towards the middle left of where this relay should be.(look on the back of the block and it should have a black wire with a red stripe connected to it.) a standard quick connect connector goes right into the slot. run your wire throught the firewall (drill your grommet hole and run wire through the grommet hole.) crimp the quick connector and plug it into the fusebox.(Thr slot that you found earlier) Working in the engine compartment, route the wire along with the other wires using wire ties, all the wat to the alternator. crimp on a female quick connect plug it into the "W" slot on the alternator. now start your moter and test final install. now is your last chance to tweek your RPM reading just get to the trimmer and turn it if needed. When all is good close your dash back up. Insallation is the reverse of removal. ( Now would be a good time to lube the speedo cable and get ride of any "needle flutter" ) well there it is I would rate this as medium difficulity- it takes time and involves soldering. if there is any questions and I can help I will me e-mail adderess is ixmerlinxi@netzero.net I have done 3 of these all worked fine. also i can do the modification on a tack and send it to you for a small fee for the time and parts. happy dieseling !!!!