gas tach conversion/cluster swap

[n_tensetuning]

hey guys,

just wondering how many out there are running vw a1 gas tachs on their diesel clusters ala mike's page  http://www.4crawler.com/Diesel/CheapTricks/Tachometer/vwdslfaq.txt

i actually tried out my tach, but used a 10k ohm trim pot.... and the tach idled perfectly and revved pretty accurately up to 2500rpms... after that it would bounce all over the place

i'm having trouble sourcing a 5k ohm (mini)trim potentiometer... i've tried everywhere around town....  

i guess if i can't get it to work right, i have an 85 a2 jetta turbo diesel cluster w/tach.... that i can splice in  :wink:


david

[n_tensetuning]

oh.... almost forgot.... anyone know if any Vw a1/mk1 diesel rabbit, etc... ever come with a tach on the cluster??

i wonder if some of the 1983-4 a1 jetta turbo diesels had tachs on there??

it would be a nice bolt on for the rest of the diesel rabbits



d

[81rabbit]

If you can't find the resistor part you need locally go to:

www.digikey.com

it's where we get most of the special parts we need at school. they just have that stupid shipping and handling rates.

-J

[fspGTD]

I am the inventor of this mod, a long long time ago.  Following some trial-and-error of the resistor values, I got it working properly for the stock revv-range of the 1.6lTD.  Although I didn't end up writing down the resistor values I finally ended up using.  The resistor values may need some tweaking compared to what is posted on that page.  I also have another set of instructions on doing this mod you can compare with (that I *KNOW* don't have the right resistor values... in case that helps ): http://home.comcast.net/~vwgtd/vwdslfaq.txt

Now that I'm revving close to 6000rpm, the modified GTI tach is past it's limit and it won't work up to that level.  So I've got it on my list to modify the tach circuit again.  The GTI circuit has a tendency to "poop out" duing heat in the summer.  I've heard from GTI owners who are running unmodified tachs that also experience this problem.

[fspGTD]

I just checked out the page on Roger's site.  It lists the same resistance values that I originally came up with, which are wrong.  No wonder people are having problems getting it to work well at high RPMS!  

Anyway, I can check the values I ended up using next time I have my dash apart.  Sorry for the confusion.  Or maybe someone else who has figured it out will post what resistance values they found to work up here.

[Hammy]

Hey guys:
 
 How would I test the 'W' output of the alternator for correct opperation. Would it show 12volts when running, or do I need to check frequency output? The reason I ask is, that I already have an alternator with the 'W' pin connected, and I tried a diesel tach cluster (used) and it didn't work. Maybe the tach cluster is bad, I don't know yet. That's why I want to check the output of the 'W' pin.

 This forum is great! There is some much good, trial and error info. It makes for great reading. I only wish I could help out some.

[BlackTieTD]

doh! i just dropped $25 at the electronics store last night for the resistors and other parts to do the GTI tach mod as well as your glow plug relay turbo timer jake... but i guess i bought the wrong parts. i can always return them, no big deal, i'll just start with the turbo timer.

please post the specs on the resistors and other parts you used jake and i'll make another trip up to the store... i'm just getting into this electronics mayhem, i've always been interested, but have never really taken much of a stab at it other than high school projects. i once turned a photo into a light-up-LED poster like that famous jimmy dean at the diner one  :lol:

[BlackTieTD]

i'd like to go the route you did, but (for now anyway) i'd like to keep things more or less 'stock' (appearing) under the hood. i also didn't really like the idea of using optics in a diesel engine bay... not a huge concern, but regardless...

[DieselsRcool]

5.375/2.625=2.047619047619047619047619047619

Roughly 2 to 1

[VWRacer]

Okay, the crankshaft pulley on my '85 Quantum TD is 5.5" diameter while the alternator pulley is 2.5".

That yields circumferences of 17.279" and 7.854", respectively, and a pulley  ratio of 2.2:1.

[fspGTD]

Here are the pulley ratios I've measured for 2 Rabbit diesels (note: there is some margin for error here because some measurements are actual OD of outermost pulley "lip" while others may be OD of V-belt riding in the pulley.  None are actual true diameter of center of V-belt "contact patch" which would be less.)

'84 Jetta TD w/ power steering no A/C:
  Alternator belt pulleys:
    crank: 5 7/8" OD (5.875")
    water pump: 4 9/16" OD (4.5625")
    Bosch 65A alternator: 2 5/8" OD (2.625")
    Denso "mini" alternator (this is the one I'm running on my Rabbit right now): 2 1/2"OD (2.5")
    calculated Bosch alt drive ratio: 2.238
    calculated Denso alt drive ratio: 2.350
    calculated water pump drive ratio: 1.287
  Power steering belt puleys:
    crank: 4 13/16" OD (4.8125")
    water pump: 4 1/16" OD (4.0625")
    power steering: 4 9/16" OD (4.5625")
    calculated water pump drive ratio: 1.185 (note: this should match the calculated water pump ratio of 1.287 above, but doesn't match perfectly probably due to measurement accuracy errors.)
    calculated power steering drive ratio: 1.055

'84 Rabbit 1.6lNA, no power steering, no A/C
  Crank: 6"
  water pump: 4 3/4"  (4.75")
  Bosch 65A alternator: 2 7/8"  (2.875")
  calculated Bosch alt drive ratio: 2.087
  calculated water pump drive ratio: 1.263

I would suggest having an adjustment feature for the alt-driven tach.  That way it can be "dialed in" to perfection.  Get the initial adjustment "ballpark" by using the pulley ratios, and then fine-tune the actual RPM adjustment when the engine is running.  Matching the engine sound to the frequency of a tuning fork works very well with high accuracy for doing final calibration (if you've got any sort of a musical ear).

If designing the circuit so it's capable of being "fine tuned" is not possible... it would be best to measure actual pulley ratios by marking pulleys say with a splotch of "white-out", and then while hand-rotating the crank, counting the revolutions of both pulleys.  Without enough counting you should be able to measure some pretty accurate pulley ratios... should be more accurate than measuring pulley ODs.

[Otis2]

I would suggest having an adjustment feature for the alt-driven tach.  That way it can be "dialed in" to perfection.  Get the initial adjustment "ballpark" by using the pulley ratios, and then fine-tune the actual RPM adjustment when the engine is running.  Matching the engine sound to the frequency of a tuning fork works very well with high accuracy for doing final calibration (if you've got any sort of a musical ear).

How would this be possible?  With extra resistors or something?  I have heard that the diesel tachs had some kind of fine-tuning adjustment on them, but I don't see anything obvious looking at the back of mine (taken from a 1985 jetta diesel).

My diesel tach is definitely over-reading, although I haven't yet figured out by exactly how much.  The alternator (& engine) is from a 1997 Golf AAZ, and the tach from a 1985 Jetta.  So I'm pretty sure the alternator pulley diameters were quite different in those 2 cars (1985 and 1997).

At idle, the tach always shows 1200 rpm, and at 60 mph in 4th gear (ie. top gear) it shows 5000 rpm (!).  Yeah, sure, the Vanagon is geared really low, but that's got to be about 1000 rpm higher than actual engine speed.  

Any suggestions on "dialling in" a diesel tach?

[fspGTD]

I would suggest having an adjustment feature for the alt-driven tach.  That way it can be "dialed in" to perfection.  Get the initial adjustment "ballpark" by using the pulley ratios, and then fine-tune the actual RPM adjustment when the engine is running.  Matching the engine sound to the frequency of a tuning fork works very well with high accuracy for doing final calibration (if you've got any sort of a musical ear).

How would this be possible?  With extra resistors or something?  I have heard that the diesel tachs had some kind of fine-tuning adjustment on them, but I don't see anything obvious looking at the back of mine (taken from a 1985 jetta diesel).

My diesel tach is definitely over-reading, although I haven't yet figured out by exactly how much.  The alternator (& engine) is from a 1997 Golf AAZ, and the tach from a 1985 Jetta.  So I'm pretty sure the alternator pulley diameters were quite different in those 2 cars (1985 and 1997).

At idle, the tach always shows 1200 rpm, and at 60 mph in 4th gear (ie. top gear) it shows 5000 rpm (!).  Yeah, sure, the Vanagon is geared really low, but that's got to be about 1000 rpm higher than actual engine speed.  

Any suggestions on "dialling in" a diesel tach?

Trimpot.  The stock tachs probably don't have one, but it's something I put in my circuit.    To change the RPMS, just twist the trimpot a bit.

Suggestion for dialing it in: do you have a musical ear?  A tuning fork?  Take the tuning fork's frequency, (in cycles per second), multiply it by 60 to get cycles per minute.  Then divide by 2 at many iterations as is necessary until you find a suboctive that is in the usable RPM range of your diesel engine.  Then revv the engine to that frequency, and ring the tuning fork and press the non-vibrating end of the turning fork against your earlobe so you can hear it loudly enough.  At the same time listen the the sounds of the motor.  You'll know exactly when your engine hits the suboctive (with extremely high accuracy) when the "beating" interaction of the 2 frequencies dimishishes.  And use a little common sense to you aren't tuning on on the wrong harmonic by accident (4-cyls idle speeds are usually set to around 900-1000 rpm.)  If common sense says you're close, the tuning fork will get you the rest of the way exactly.

No musical ear?  Maybe find one of those fancy special tool tachs shown in the bentley manual that sit on the valve cover and convert vibrations to RPMs.  Or do some gear ratio calculations and tire circumference calculations and use your speedometer to test what rpm your engine is turning in a particular gear at a given road speed.

[fspGTD]

Edit: actually, I think the stock tachs (at least the '83-'84 GTI VW tach) might have a trimpot.  IIRC it has what looks like a phillips-screwdriver head adjustment "knob" on it.  I can't remember for sure if it even has one.

I know for my circuit I recommend a precision multiturn trimpot.  I use a 10-turn trimpot that way you've got a lot of precision in setting the exact rpm desired.  Of course it's got to have resistance values ranged correctly to be useful.

[fspGTD]

Also, for a quick and dirty tach install, just buy a G-tech pro competition.  It works off the alternator ripple signal through it's cigarette lighter power supply, and works great with VW diesels!  It does need to be calibrated though.  But once set up, it is quicker-responding than any factory tach I've seen.  http://www.gtechpro.com/