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Gas tachometer to diesel conversion
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Gnarlodious Premium Member
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PostPosted: Wed Oct 22, 2014 4:24 pm    Post subject: Gas tachometer to diesel conversion Reply with quote

I sleep in my van, so I unplugged the clock years ago because it was so loud. That meant wasted instrument panel real estate, so I decided to put a tachometer in there. The 1.6TD diesel tach was not supplied to the USA market, you would have to buy one from Europe which is chancy, expensive, and requires case modification. Prices I saw were up around $300. The gasser tachometer will work with some mods, so that is what I decided to do. The gasser sweep isn't too efficient since your diesel will never hit 6000, and the colors are meaningless to a diesel driver. But its a tachometer and looks great:
Image may have been reduced in size. Click image to view fullscreen.


I got this cluster out of a burned '88 Vanagon, the only part I wanted was the tach. You need the plug, so get the circuit foil if you can. If not, a part listed below fits with modification. This page has some great instructions how to do the mod:
Gas Tachometer Conversion to Diesel W terminal - IDI Engine
That site can periodically go offline, so I am rewriting the procedure here with additions. There are no specific instructions anywhere on how to do the entire mod, so I was unable to predict how much work I was getting myself into.

You need to modify the gasser tachometer circuit board to downsample the AC coming in from the alternator's "W" terminal. Discrete components from salvage are getting hard to find, so good luck with that, and most modern devices are all surface-mounted components. I just ordered all the parts from Digikey:
R1: Do nothing
R2: Do nothing
R3: Delete
R4: Do nothing
R5: Delete
R6: Replace
# CF14JT10K0CT-ND
RES 10K OHM 1/4W 5% CARBON FILM
R7: Replace
# CF14JT15K0CT-ND
RES 15K OHM 1/4W 5% CARBON FILM
R8: Remove

D1: Bridge

C1: Replace
# 399-11522-ND
CAP FILM .01UF 100V 5% RADIAL
C2: Replace
# 338-3299-ND
CAP FILM 0.047UF 400VDC AXIAL
C3: Replace the old capacitor with a new one of the same value, observe the banded side!
# 4199PHCT-ND
CAP ALUM 22UF 16V 20% AXIAL
C4: Add a capacitor where there was nothing
# 445-8294-ND
CAP CER 3.3UF 16V 10% RADIAL

Components cost less than $5. If you don't have the foil and therefore the tach plug, you can get receptacle A19972-ND and carve it down to fit, along with 3 of the A32533-ND sockets.

The original board:
Image may have been reduced in size. Click image to view fullscreen.

The schematic to drive the signal from the gas engined alternator:
Image may have been reduced in size. Click image to view fullscreen.


Using the listed components, make your board look like this:
Image may have been reduced in size. Click image to view fullscreen.

The schematic modified for the diesel alternator:
Image may have been reduced in size. Click image to view fullscreen.


The circuit board is the easiest part of the mod, it took me about 90 minutes to solder up the board. Before you put it together, make sure you have a small screwdriver to fit the calibration adjustment on the board. If possible, save yourself work now and calibrate the tach while it is out and back at the engine. Otherwise, the calibration access hole is hiding under a piece of white paper tape.

The '88 tach fits into the '83 clock space with no modification. I just left the clock adjustment pin where it was for nostalgia's sake. I cut off the tach's plastic tab for the cluster connector, as I am using the diesel circuit foil. You will see below how I adapted to that problem.

The tach plug is wired like this, I soldered the #7 trace directly onto the W signal pin:
Image may have been reduced in size. Click image to view fullscreen.

Red #7 into the 14-pin had been driving the clock, so I recommissioned that circuit for the W signal:
Image may have been reduced in size. Click image to view fullscreen.

You can snap apart the connector to solder directly onto the pin. Donít just chop the red wire off and leave it dangling naked! Using this method, the entire cluster can be removed by disconnecting one 14-pin plug. BE CAREFUL with the connector, as the foils come loose and fold back easily. RTV Silicone seems to stick pretty well to both the copper foil and the mylar. Also, bear the the left side as you insert the plug, it takes pressure off the copper foils.

The power supply for the tach is your standard 12v DC, I took it off the DC-in pin of the regulator:
Image may have been reduced in size. Click image to view fullscreen.

Scrape the old metal pretty well with Xacto and use flux to get the solder to stick.

The '88 tach has the regulator positioned differently, so I rewired it like this:
Image may have been reduced in size. Click image to view fullscreen.

A few small screws, nuts washers and wire eyes is needed for this, all from Ace Hardware or your scrap bin.

The '88s regulator was screwed down to a steel plate that was grounded, possibly for blocking EMF to the tach board, not sure. I grounded it anyway, running the same ground wire to the temperature gauge small center pin:
Image may have been reduced in size. Click image to view fullscreen.

The above picture shows in the background the temperature gauge negative post that goes off to the sensor. The blue wire to the ground comes from the photo immediately above.

I had success soldering directly on the foil with the ColdHeat resistance device.
Image may have been reduced in size. Click image to view fullscreen.

Its not really cold heat, it uses the resistance of the solder to melt itself, so it is just less hot. Wiring the terminals of the tach onto the clock is a little challenging:
Image may have been reduced in size. Click image to view fullscreen.

Getting the 14-pin socket attached is up next. I hacked it off as much as possible and bolted it down using the fuel gauge negative post:
Image may have been reduced in size. Click image to view fullscreen.

The results were very solid, but I also had to carve away a chunk of the socket retainer to snap down past the post:
Image may have been reduced in size. Click image to view fullscreen.

It helps to have the spare brass nuts on hand from your donor cluster to double-screw the parts down. A 7mm wrench is a must-have when working under the delicate mylar sheet.

I would estimate 5-6 hours of work to get the cluster ready to install. It took me a lot longer since I had to take it apart 3 times to get it right.

I calibrated my tach using the $10 "Engine RPM" iPhone app which was way cheaper than buying a dedicated revmeter to do one job. Worked for me in real-time while sitting at the instrument cluster with hatch open so it offers a big advantage over pulley-based measurements that make you run back and forth. Note that you have to give the app some idea of a range to work in. I used known transmission gearing to get close, and the app zeroed it in for me.
Image may have been reduced in size. Click image to view fullscreen.

I did try a few different strobe-based apps but results were not satisfactory as the iPhone's light is somewhat weak and above 1400 flashes per second seems to lose its punctuality, even during night.

The result, I have a tachometer where the clock used to be:
Image may have been reduced in size. Click image to view fullscreen.


Oh, and I also installed a voltmeter while I was at it:
Image may have been reduced in size. Click image to view fullscreen.

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Last edited by Gnarlodious on Thu Oct 23, 2014 4:26 pm; edited 1 time in total
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Syncroincity
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PostPosted: Wed Oct 22, 2014 4:51 pm    Post subject: Reply with quote

Excellent write up, thanks for transferring the info here. Applause
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?Waldo?
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PostPosted: Wed Oct 22, 2014 7:40 pm    Post subject: Reply with quote

Great job. Thanks for the write up.
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Jeffrey Lee
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PostPosted: Wed Oct 22, 2014 8:46 pm    Post subject: Reply with quote

I'll add a 'great write-up' and ante up a "holy crap!"

I've been hoping to do this mod for awhile now and this will be invaluable.

Now to start looking for a gasser tach ...

Thanks, Gnarlie!
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Waldemar Sikorski
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PostPosted: Wed Oct 22, 2014 9:32 pm    Post subject: Reply with quote

Thanks. Might come in handy one day.
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PostPosted: Wed Oct 22, 2014 10:44 pm    Post subject: Reply with quote

Congratulations, what a great post!

I remember seing many years ago a similar thread, but I could never find it.

It'd be easier to start off a tachometer cluster model for sure, but you did it!

Can you take you up on your offer for the components?

PM on its way Wink
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PostPosted: Thu Oct 23, 2014 2:38 am    Post subject: Reply with quote

You could use the old clock adjust knob to mount an adjustable redline indicator, like many aftermarket tachs feature.
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PostPosted: Thu Nov 27, 2014 11:42 am    Post subject: Reply with quote

I'm glad this worked for you. I was looking at the schematics/description and it seems there are discrepancies.

Your description says to replace C3 with the same value capacitor (22uF) but the gasser schematic shows that capacitor as 47uf. There is no 22uF capacitor listed on the diesel schematic. The diesel schematic shows the value for the C3 capacitor as 47uF as well, not 22uF. In looking at the vwdiesel.net source threads, the value for C3 is consistently 47uF. Your pic clearly shows that you did use a 22uf capacitor there and apparently it works that way.

The value for R2 should be 72 in the diesel tach but is 82 in the gasser. You say to leave the 82 ohm. Maybe that's close enough, but I'd probably still replace it.

The diesel schematic also has a couple components labelled incorrectly. C5 is really C2 and C6 is C4...

The gas tach does not run off the gas alternator. It runs off the ignition signal. The diesel runs off the alt because a diesel does not have an ignition signal.

Regardless, good work on getting it up and running.
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PostPosted: Thu Nov 27, 2014 12:44 pm    Post subject: Reply with quote

Oh youíre right, very good spotting! I put C2 and C3 in the wrong places, yet somehow it works. Probably because 22uF and 44uF are not very different values.

I didnít look at the schematic. Anyone is invited to update any information here, as I am not too good with electronics. My worry was that if the other page went offline permanently the knowledge would be lost.
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PostPosted: Thu Aug 27, 2015 10:22 pm    Post subject: Reply with quote

Would a cluster up to 89 work or should I stick with 88 or lower?

Thanks!
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PostPosted: Fri Aug 28, 2015 6:24 am    Post subject: Reply with quote

Havenít heard of any changes in the years. They should be all the same.

Now that this topic popped up Iíve been looking at the tachometer dial for a while and thought up a new mod to improve it. Someone needs to print up a new dial from 0 to 4000 RPM more suited to the diesel engine. The electronic components could be adjusted to speed up the motorís spinning. That would eliminate 2000 RPMs of wasted space up top, and make for a more accurate needle.

The job is almost half done:
Image may have been reduced in size. Click image to view fullscreen.

I suggested Chris from T3Technique make some diesel faceplates but got no response. This is part of the Plasma gauge set. It is probably as simple as making up some vinyl stickers.
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PostPosted: Fri Aug 28, 2015 6:54 am    Post subject: Reply with quote

I can see limiting it to 5,500 for a 1.6/1.6TD or 5,000 for a 1.9/1.9TD/1.9TDI but I would not want less than that. 4,000 is well below redline on all of the vw diesels.
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PostPosted: Fri Aug 28, 2015 7:23 am    Post subject: Reply with quote

Nice attention to detail, Gnarl and Libby. Thanks for taking the time guys. they have an ap for everything now, very cool write up.
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PostPosted: Fri Aug 28, 2015 5:11 pm    Post subject: Reply with quote

Image may have been reduced in size. Click image to view fullscreen.

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PostPosted: Mon May 29, 2017 2:24 pm    Post subject: Re: Gas tachometer to diesel conversion Reply with quote

I just ordered all the parts for this swap to accompany my ALH conversion. Are there any lessons learned or gotchas I should be aware of since this thread was originally authored? Thankfully I have a spare tach to experiment with, since soldering circuit boards has always been a challenge for me. I already modified my alternator for the W signal.

Many thanks to Gnarlodious and Orbitald for documenting this mod.
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PostPosted: Mon May 29, 2017 2:29 pm    Post subject: Re: Gas tachometer to diesel conversion Reply with quote

Good going. Andrew Libby pointed out a few discrepancies in my procedure. Haven't noticed any funny operating but you should take his corrections into account.
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PostPosted: Sat Oct 13, 2018 8:39 am    Post subject: Re: Gas tachometer to diesel conversion Reply with quote

Has anyone else had success with this modification?

I found these photos of a diesel circuit board here:
http://forum.club8090.co.uk/viewtopic.php?f=39&t=165030&start=15


Image may have been reduced in size. Click image to view fullscreen.



Image may have been reduced in size. Click image to view fullscreen.



Seeems to be set up quite differently than the instructions for this conversion.
I donít know how to read the schematics at all, but it appears to be the same board, with the same tracings on the bottom.
[/img]
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PostPosted: Sat Oct 13, 2018 7:43 pm    Post subject: Re: Gas tachometer to diesel conversion Reply with quote

Bump.
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PostPosted: Sat Oct 13, 2018 8:23 pm    Post subject: Re: Gas tachometer to diesel conversion Reply with quote

Yup, at the bottom of this post. I'm using it to read the tach output signal from the ECM though. https://www.thesamba.com/vw/forum/viewtopic.php?p=6274391#6274391
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PostPosted: Sun Oct 14, 2018 12:13 am    Post subject: Re: Gas tachometer to diesel conversion Reply with quote

great write up. i know this is an old post but if someone is interested in this mod then i believe this would help. itís available from foreign auto and supply. itís a tach for a tdi with adjusted redline. itís probably closer to what the 1.6 redline is, if not the same. hope this helps.

http://www.foreignautosupply.com/parts-accessories/108-diesel-tachometer-decal/#.W8LrkBYpBR4
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