Gas Tacho / Revcounter Conversion to Diesel W terminal

[regcheeseman]

Originally dealt with in this thread http://www.vwdiesel.net/forum/index.php?topic=3096.0, now condensed into the various solutions to convert all type of VW gas type revcounters to work from the W terminal on VW fitment altenators.

The original gasser schematic



Re-worked into diesel w terminal version


First up the VDO unit...

The gasser board before

replace R1 with 100 ohm
leave R2  
replace R3 with wire link
replace R4 with 191 ohm
replace R5 with wire link
replace R6 with 10K
replace R7 with 15K

 
replace C1 with 10nF
remove C2
leave C3
remove C4
replace C5 with 47nF
fit C6 3.3uF in vacant spaces next to tacho motor (red and blue) wires or desolder wires from board, fit cap and then solder wires to cap wires/board.

adjust RV1 to mid range

The final working board.


Note the two resistors joined together are because I could not find a 191 ohm resistor so joined a 180 and a 10.

C6's value is not critical, I used a 2.7 instead of a 3.3, it works fine without this cap altogether and I believe is only there to stop needle flutter (I didn't have a problem with or without this fitted)


Mods for westmoreland clocks...

First the schematic as is..


The annotated board..


A lot of similarities with the original VDO circuit, I'll wager the SAK chip is the same and converting to the diesel tacho circuit should work..

Make R1 = 100ohm
replace R2 with 72ohm
replace R3 with wire link
replace R4 with 191ohm
replace R5 with wire link
replace R6 with 10K
replace R7 with 15K

replace C1 with 10nF
leave C3
remove C4
remove C?


make RV1 = 22K
remove RV2 and replace with 47nF cap

If you can get a 3.3uF across the needle output wires then do so....it's not critical though.



Now the Motometer clocks...







Swap R1 for 15K
Remove R2
Remove R3
Swap R4 for 191 ohm
Swap R5 for 72 ohm
Swap R6 for 100 ohm
Swap R7 for 10K

Replace zener with wire link

Swap C1 for 10nF
Swap C2 for 47nF (nanofarad)
Swap C3 for 47uF (microfarad)
Swap C4 for 3.3uF

If replacing C3 or C4 with electrolytics then observe polarity as marked on board.

Leave the terminal marked ZA disconnected where appropriate.

[Trev0rbr]

this should help anyone doing a gti tach:

That is a very nice tiltisorie ... bad spelling I'm sure but it tilts the car up on it's side nicely. I've been eyeing a few on the vortex and this one looks great compared to some of them.
 I was messing with the '83 westmoreland tach, figuring what parts I need to source.
 It looks like there are a few resistors that can be moved from one spot to another.
 R6 is a 15K resistor and can be moved down to where R7 is. Then a 10K is used in the R6 spot.
 R3 needs to be a wire link and is a 100 ohm now. On the diesel tach, R1 needs a 100 ohm resistor so that can be used there.
  Actually I'm thinking the R1 resistor determines how far the needle swings so I might put a variable resistor in there and see if I can get it to work with the stock gas tach face and go to 7000RPM.
 The stock R1 resistor in the gasser tach is about 10ohm (it was 15ohm in my '91 Golf gasser tach) so I might even keep that one in there.

 ...so the parts list looks like:
10K resistor
 72ohm resistor
191ohm resistor
 and a 10nf cap (the '91 Golf already has a 10nf cap that is getting removed)
 4 little pieces

[Black Smokin' Diesel]

If I were to use a CE1 golf/jetta MK2 8v gasser tach to convert to diesel use, which schematic would I use?

[Trev0rbr]

try to find the one thats closest to the board you have, they are all so different and there are so many of them

[Black Smokin' Diesel]

Allright I'll grab one from the junker and go from there. Maybe I'll bring a print of these with me and try to get one as close to the ones in this thread.

Thanks a lot.

[RabbitGTDguy]

Another SUCCESS for this FAQ. I was able to convert an early Mk1 (round) VDO tachometer to W terminal with success in the last couple of days. I was even able to easily calibrate it on the b3 16v's 90amp alternator as it has an active W terminal.
No fluctuations and very little adjustment. All adjustment was able to be done at the potentiometer itself...

Good stuff...maybe I'll post a picture of the modified board here in the next few days. My soldering skills are rusty, but getting better I guess...some pretty creative wrapping and and installation a few components in a series.

Also, I did not install the 3.3uF inline of the tacho output wires... haven't seen any problems as of yet.

Joe

[X@V]

Could someone help me please? I need to know what to do for this:

http://www.vwdiesel.net/phpBB/viewtopic.php?t=11404

[RabbitGTDguy]

More Proof!!! Posted in my thread as well...but this is good here too!!!

Finally,  got a chance to start up the mTDI Rabbit again here today. After fixing the dowpipe, etc. etc. and getting the exhaust hooked back together all is well. Just hadn't had the time
Also, I was able to full intergrate the tach into the early dash, I even used the factory german cluster plug and was able to change out the connections needed for a "factory" hookup and look so that I didn't have to tape off terminals, add connections, etc. The early MK1 cluster plugs are actually really nice to work with, They come apart EASY which makes swapping connections really easy as well. I ended up switching to the motormeter circuit sheet that I had as the VDO one was pretty bad. After comparing them though I didn't see any major differences, etc. and everything is good to go!

Idles a bit high. at about 1.5k and I don' t think the engine is actually idling that high so I think that accounts for the difference in between the pulleys on the test/calibration car (my 16v b3 gasser ) and the mTDI engine running serp belts and the 2.0 90amp alternator.
Easy enough to fix though...

Here are a few pics...as promised...



Had that second one revved quite a bit higher...but my digi cam recycle time SUCKS.... either way. No flutter, etc. I'm going to need to find another "VDO" parts tach for a good needle (as this ones paint is damaged and/or do something with the needles. The biggest drawback I think with the older gauges is the illumination itself...
Hmm...maybe an LED project in the future!!!
Either way...WORKING CONVERTED GASSER TACH!!! WAHOO!!!

Joe

[coalminer16]

I just got two tachs wired up for both of my rabbits and neither of them are the correct rpm. They both idle at 2000 rpm. Any pointers I used the correct resistors. I may have the wrong caps in the system (due to converting). Would this be the problem or do I just need to get a variable resistor. Can I put the variable resistor on the input line if that is the only way to correct it. Any suggestions let me know please. I did everything exact but the optional caps on the two wires going to the needle. Maybe I need a higher resistor value somewhere. Doesn't jump around though (the needle). Thanks

[prothe]

This is really good stuff.  Thanks!

[coalminer16]

I posted that a 5.6 k resistor makes the rpm correct at start up but changes by about half if I drive interstate speed. I don't think it does it in town but I haven't driven it long enough to know. I asked my Electrical Engineering friend/previous teacher and he said that it might have a "cold connection" is what I think he put it as (I am Mechanical Engineering so he has the better understanding on this part). He explained that as a person solders the resistors and caps on, if the ends of the wire moves before total cool down of the solder then it won't have a perfect connection. So after a while the connection will heat up because of this and the resistance of the solder actually changes. Long story short he said if you can get the metal flowing really good and have the wire end in the pool of metal and then take the heat source off and not drag it off (in a sweep motion, like welding with wire or arc almost) then it will prevent some of the cold connections. But because this is a small circuit board that is the old style with solder connections this will be hard anyways. Bottom line I have something that if I factor if the engine is warm or cold I know roughly the rpm and it looks nice and factory installed looking. Plus now I can see the temp gauge better if I am leaning forward on the steering wheel on some of those long drives (not uncommon I drive 24 hr strait-Spring break, SD to Vancouver to Mexico and back in 9 days with 4 days skiing). Hope this helps.

[rallydiesel]

Good thread.  :oops:

[Trev0rbr]

i just got another one of these put together last night, its a north american gti tach in my 81 rabbit,  works great! and looks great. 1 thing that i found is that if you use a permanent marker to color say the 5000rpm mark and all the marks after that red, it makes it pretty much impossible to read at night since the paint vw used must have some kind of reflective *** in it.  oh well it looks good during the day haha, i'll post pictures tomorrow.  oh yeah the other thing that pissed me off about it was that i put a gti 120mph speedo in it since my old odometer didn't work, and of course my new one goes to roll over 985999.9 and gets stuck just like my old one haha, i guess i'll be ripping it all back apart soon enough.


i may stick the old speedo back in... since it has the right mileage

[Krazy]

Do you have an easy way to get the cluster out of the dash?  I have trouble getting the cable unhooked from the cluster.

[stuvy]

Once the tach is moded, is it a straight put in the dash ordeal. Or do I need to run new wires?
I am working on a 82 rabbit FYI.