VOLKSWAGEN DIESEL FREQUENTLY ASKED QUESTIONS Newsgroup - Rec.Autos.Makers.Vw.Watercooled Current copy archived at - http://weber.u.washington.edu/~jaker/gtd/vwdslfaq.txt Additions or comments? Send e-mail to Jake Russell: jaker@u.washington.edu - Additions gladly accepted! (Come on, diesel-heads!) Date: Modification: 1/12/97 - Converting an A1 GTI Tachometer for Diesel Use This is it so far! ---Coverting an A1 GTI Gas. Tachometer for Diesel Use >From a-jaker@microsoft.com Tue Aug 6 12:53:37 EDT 1996 Article: 29701 of rec.autos.makers.vw.watercooled Path: taco.cc.ncsu.edu!gatech!newsfeed.internetmci.com!news.ac.net!news1.erols.com!hunter.premier.net!netnews.worldnet.att.net!news.u.washington.edu!netnews.nwnet.net!news.microsoft.com!news From: "Jake Russell" Newsgroups: rec.autos.makers.vw.watercooled Subject: Re: need help adding a tach to vw diesel Date: 6 Aug 1996 00:05:02 GMT Organization: Volt Comp Lines: 150 Message-ID: <01bb832b$3e5c4f50$62ac389d@a-jaker-office> References: <01bb80e6$583f56c0$d3aa389d@h4violet> <4u0j6r$l69@dfw-ixnews3.ix.netcom.com> <01bb82e7$4de489e0$d3aa389d@h4violet> NNTP-Posting-Host: 157.56.172.98 Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Newsreader: Microsoft Internet News 4.70.1141 > Well I did it! After a trip to the local electronics store and after > inhaling some lead solder fumes, I now have a tachometer on my diesel. I > first looked all over for a diesel tach, and I got two dealers saying that > they only had a listing for the gas tach, so I still don't know if VW even > made one for a diesel. Having a tachometer on a diesel is such an awesome > upgrade! I can't imagine going back to not having one! (Especially on my > stick-shift car.) With this tach, I notice how low the diesel produces > torque (1000-3000 seems to be the range) I'd say redline is somewhere a > little below 4000) Highway cruising is 2000 @65 MPH (right in the middle > of the range.) If anyone wants instructions on this conversion let me > know. > > Jake Ok here's everything you'll need to the the Diesel Tach add-on: (For A-1 Model w/ square-type gauges - 81-84) - An alternator with a "W" terminal output. I used a Bosch 65 AMP model off of an '84 Jetta Diesel. (The Jetta didn't have a tach, but it did have a suitable alternator! Check your local junkyard.) From the Bentley schematics, it seems that they didn't start using these special alternators until '82. Do not confuse the "W" terminal with an extra B+ terminal (which is intended to be used for the add-on alternator protector-suppressor thing.) If you measure AC voltage between ground and the W terminal you should get a positive reading. (The other terminals have just DC output.) - A tachometer out of a gasoline model. You don't need the whole instrument panel, just the tachometer assembly. (Connected with 4 phillips - just remove the flimsy circuit board first.) I got mine out of an '83 GTI, although any model from '81-'84 with the square instrument cutouts will work. It will replace the analog clock assembly. (Don't worry, water temp. and fuel level are still on the tach!) - Wiring stuff: About 15ft of wire to run from the alternator W terminal to the fuse block. On my car, I had to add this wire run in. Wiring was already existing from my fuse block to my instrument panel, though (Don't you love VW's?) A small grommet to get the wire through the firewall (unless you want to poke a hole through the stock grommet and run the wire with the others - can be dangerous, I tried and I poked into a wire and got an arc - not good. Go with the grommet. If you have an existing grommet (from a stereo setup maybe) you can squeeze the wire in there. Also a couple of quick-connect type connectors (one female for the wire to go to the W terminal, one male to stick into a receptacle on the fuse block) and wire ties to do neat wiring under the hood. - Some electronics parts: A 1/2 Watt 1.3K-1.8K resistor (Try to find 1/2W because this resistor might see a little power,) A multi-turn 5K ohm trimpot (for fine-tuning your vehicle's RPM gauge,) A 1/4 or 1/8 Watt, 1K-5K resistor. - Tools: wrenches for alternator adjusting bolts, wire crimpers / strippers, drill for grommet hole, a soldering iron, some solder suitable for electronics use, screw drivers, and a small hex wrench for dashboard screws. Here's the process: Step 1: Alternator install Upgrade your alternator to "W" terminal model. Bentley manual works well for this step. Pretty simple. Might as well check brushes if you have the alternator out (brush length < 5 mm = bad) Step 2: Alternator test / Tach initial test Do a quick external tach hook up to make sure the alternator and tach work. If you look at the little circuit board on the tach (shaped like a crescent) it will have a little "+" for positive, a "-" for ground, and a "1" for W. Do a quick hookup if you have some spare wires laying around. The tach will idle at about 5,000 RPM minimum. Acceleration will cause bizarre behavior, but this test is just make sure your alternator works. Step 3: Modify Tach Unscrew the two screws that hold the metal plate on. This will make the metal plate loose. You don't need to remove the needle, just let the plate dangle on it, but be careful not to damage the needle or the plate. Now you can get to the circuit board. There's three points of interest on your tach: #1 is where the blue wire connects to your circuit board. Solder the 1.3K-1.8K resistor between the wire and the circuit board here. Make sure the soldered ends of the resistor don't touch another trace or the metal plate. Point #2 is the trimpot. Replace this unit with your multi-turn 5K model. (The old one is too high a value / not enough precision for diesel setup.) Make sure it doesn't stick out of the side too far and make sure the adjustable part is towards the side, for easy access. 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 the 1K-5K resistor. (The old resistor is too high a value.) Bridging the leads of the resistor is another option, but some settings might be weird on the trimmer. When you're done, put the PCB, the plate, and the 2 screws back how they used to be. Step 4: Modified Tach - Adjust and test Do same as step #2 to make sure everything works, except this time with your modified tach. Adjust trimmer to get rev's in the right place. (If everything's working ok, you should be able to set the tach right where you want it - at about 850-900 RPM idle. I have tested revving the tach up to about 4000 RPM's, and it seems to work ok. Step 5: Install Tach in Instrument panel Remove dashboard panels as follows: Left speaker opening - 1 phillips (gets you access to the speedometer cable connection behind instrument panel.) Dashboard panel - remove stereo, remove cold start knob (twists off,) remove cigarette lighter flap cover, remove 2 hex head screws (Left and Right) on bottom of panel by cigarette lighter, remove three phillips screws at top of panel, Remove headlight switch by pulling panel loose and pressing knob release button on switch mechanism. Remove panel (don't forget to turn off your lights.) Remove four hex head screws holding in instrument cluster. Now see if you can reach into the left speaker opening and unscrew the speedometer connection. If it's too tight, remove the Fuse box covering panel reach up from down there (Have to first remove the lower dashboard panel - Two hex screws then pushes up and towards you.) After the speedometer cable is disconnected, rotate instrument cluster down and remove multiple-wire connector on top. Remove cluster. Now remove the old clock. (Cut the two traces which hang out and connect to the clock) Put in your new diesel tach. New fuel and water temp gauges hook right up (don't you love VW?) Connect all of the electrical connections and put your instrument cluster back in. Step 6: Run wire from fusebox to alternator Remove the fusebox covering panel (if you haven't already.) The place on your fusebox that you want to install the connection is conveniently located. Find the "Fuel Pump" relay location - it's on the right side, in the middle. There should be no relay here (It's only for gas models.) The instrument cluster tach wire is connected to a slot towards the middle left of where this relay would be. (Look on the back of the slow and it should have a black wire with a red stripe connected to it.) A standard quick-connect type male connector goes right in the slot. Run your wire through the firewall (Drill your grommet hole now and run it through the grommet) Crimp the quick connector to the wire and plug it into the fusebox. Working in the engine compartment, route the wire along with the other wires using nylon ties all the way to the alternator. Crimp on a female quick-connect and plug it into W on your alternator. Now turn on your motor and test final electrical install. Now's your last chance to tweak your RPM reading - just get to the trimmer and turn it if needed. When all is good, close your dashboard back up. Installation is the reverse of Removal. (Now would be a good time to lube your speedometer cable to get rid of any existing "needle flutter" on your speedo if it's needed.) There it is! Enjoy! I would rate this project medium difficulty - it takes a little time and it involves soldering. Happy dieseling! And remember, solder contains lead, so be sure not to eat it. Jake '81 Diesel Rabbit - soon to have a turbo-diesel ========================================================================== To: pdenz @ raleigh.ibm.com ("'Peter Denz'") @ RTPSMTP cc: (bcc: Peter Denz) From: a-jaker @ microsoft.com ("Jake Russell (Volt Comp)") @ RTPSMTP Date: 08-14-96 01:30:02 PM Subject: RE: FW: FW: you made my day!!! Security: Peter, one of your 1.5K resistors is wired in series with the needle coil (The thing that heats up and makes the tach needle move.) The bigger this value is, the less your needle will read. You could try different values here, but I think the problem is probably with the trimpot because it's not adjusting like it should. All pots have three connections, one is wired to one end of a resistor medium, one to a brush which touches the resistor medium in an adjustable location, and the other is wired to the other end of the resistor medium. There are two connections soldered together on the circuit board. Study the old trimpot and make sure the new one is connected in the same way, in particular the wiper connection. There has to be some reason that your revs are not adjustable. There is a control current going through the trimpot and that other 1.5K resistor (this resistor "tunes" your trimpot.) When there is less resistance (because you turn the trimpot) the revs go down. More resistance makes the revs go up - until it hits a ceiling, it will recycle down to a lower rev (as you've discovered, at 3700 RPM.) So basically these are the two areas in the circuit to work with - if you get your trimpot working it will make things a lot easier (It could be that there is a solder bridge of improperly wired trimpot.) I'd focus on the trimpot first. The important resistor is that 1.5K wired in series with the needle (connected to the blue wire.) When you get your trimpot problem figured out, you can fine tune this value so that you have maximum "ceiling" (In your case, you said it's now at 3700 RPM) A lower value will raise your ceiling. Try a 1K. Lowering this value will also raise your idle, and you might not be able to get it adjusted low enough if you have too low a value here. I used a multimeter to diagnose how the circuit works and made a partial schematic. I studied the tuning circuit and made a mod which I'm happy with. I might have that 3700 RPM problem like you do, But my diesel doesn't feel good that high so the rev range that I use works on the tach. Anyways, good luck, and I hope you can get things working. Let me know if you can get your trimpot working and which resistor value you use for the resistor wired in series with the needle. Good luck, Jake >---------- >From: Peter Denz[SMTP:pdenz@raleigh.ibm.com] >Sent: Monday, August 12, 1996 2:54 AM >To: Jake Russell (Volt Comp) >Subject: Re: FW: FW: you made my day!!! > >Jake, > >I located an '84 GTI since I emailed you last week, and I underwent the >modification >for a diesel. My car is already pre-wired for a tach since my upshift light >indicator logic uses >the alternator's W terminal as input. Your instructions were very >straight-forward. I am having >a few problems with the tach, though. The tach seems to read too low >(400-500 >RPMs at idle). >I tried adjusting it with the 5K trimpot, but it does not change anything. I >went back to the >electronics store & bought another trimpot, installed it, and still I cannot >adjust the reading at all. >The trimpot seems to have no effect on the guage at all. >Are the trimpot legs "standard"? (ie, I installed it in the same orientation >as the old one was >installed--the two close-together legs in the back and the one single leg up >front.) It also bothered >me a bit that you had ranges for the two resistors (1.3K - 1.8K and 1K - 5K). > >Will any resistor in these >ranges work fine? For the first, I installed a 1.5K and for the second I >believe also a 1.5K. > >My other problem occurs at high RPMs. Just before the rev-limiter kicks in >(the tach reads about >3700 RPMs--seems a bit low, but may be related to the above problem), the >tach >jumps down to >1500 RPM even though the engine speed does not change drastically. > >Any help/pointers you can give me to fine tune my diesel tach would be >greatly >appreciated! >How did you figure out how to do this conversion? Did you study the circuit >board & use an >oscilloscope to study the input signals and make appropriate mods? > >Thanks, >Peter >