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Sun Oct 14, 2018 2:29 pm

Thanks for the info.

I’m wanting to use the W terminal to run my tach. I have an AAZ and no ecu.

Also hoping to do plasma gauges at some point. Otherwise that tacho face plate would be great.

In the mean time, I’m hoping to get this gas tach converted to run off the W terminal.

Was hoping more people had done this and could share their experiences with how well it works.

It would also be great to see more photos of a diesel circuit board if anyone has them.

Sun Oct 14, 2018 5:15 pm

That is apparently a later (post 1988) tachometer and it looks like they changed the circuit board. it may well be that the components and schematic are the same for both boards.

Sun Oct 14, 2018 5:37 pm

This is the post 1988 board? (Which I believe to be a diesel board)
And I think this is the stock board that they used as a template to produce a new board to be used for gas tach.

And the instructions at the beginning of this thread are correct for converting gas to deisel?

Sun Oct 14, 2018 6:41 pm

I don’t know what board you have, but if you can read a schematic and compare the component values it may prove to be either a later European diesel board from a JX engine or a gasser board. Have you searched for similar boards on T3 websites?

A post by ?Waldo? found a slight error I made with two capacitors. Apparently it was irrelevant since the tach has been accurate ever since.

As a last resort you could put it together and see how it runs.

Sun Oct 14, 2018 9:51 pm

I have the same board shown in the initial post. I’ll be using all the components listed.
Can’t say that i know how to read circuit diagrams. I’m sure this basic circuit is pretty straight forward, but it’s not something I’m familiar with.

Tue Jan 08, 2019 7:45 pm

So I did this modification to the board, as shown in the photos. This did not work. The tachometer does nothing. I used a cluster from a newer van, with gas tachometer. Modified the board. Changed the wires on the cluster connector to match the newer cluster. I put the W terminal wire in location 9, where the tach signal is supposed to go. The W terminal wire read ~16V AC, so I know it’s working.
I adjusted the potentiometermeter on the back of the tach. But it does nothing.
Everything else on the new cluster works.

Just wanted to share my experience as a caution to anyone else thinking of doing this. I know I’m not the only one who did this mod and didn’t have it work.

It’s too bad someone can’t come up with a reproduction of the diesel tach board.

Tue Jan 08, 2019 8:39 pm

The same basic circuit has with the outlined differences was used on VW gas and diesel tachometers from Mk1 through the end of Mk2. I know of many people who have made the outlined changes to the various gasser tachs from those years, including myself, and it has worked fine. Any chance you flipped a capacitor so the polarity is wrong?

Wed Jan 09, 2019 12:18 am

Hey Waldo.
It is possible. I thought I had taken a picture of it before installing, but I guess not.
Perhaps tomorrow I’ll pull it apart again and have a look. I’ll post the photo on here as well. Quite possible I made a mistake. I know how to solder and follow directions. But I don’t know much about circuits. So my troubleshooting is quite limited.

Wed Jan 09, 2019 12:53 am

Well it was sitting in my kitchen, so I took it apart now.

This is what I did.

So the three capacitors I have circled.

The red one, positive is towards the top of the photo, and negative towards the bottom. Assuming the long arm is positive.
-> Just realizing, as I look at the picture of the traces, and the earlier photo with a + stamped on the board; this capacitor is probably reversed from what it should be. I believe positive should actually be towards the bottom of the photo.
I can switch this around and see what happens...

The one circled in blue, doesn’t appear to have any markings for polarity.

Th one circled in green, also does not appear to have markings for
polarity.

Otherwise, did I use the right type of capacitors?

Thanks in advance for any help.

Wed Jan 09, 2019 3:46 am

I have a diesel board somwhere
I converted my tacho from diesel to petrol
Ill have a dig and get back to you

Wed Jan 09, 2019 8:24 am

I believe the values are correct although I am not sure about the one circled in red. If you purchased it new you should be able to confirm its value. Did the tach work as a gasser tach?

Wed Jan 09, 2019 9:43 am

The one circled in red is 3.3 uF.

I bought this tach as a Working Tach from a local parts shop. Only having a diesel Vanagon, I hadn’t tested it. This shop would not sell it as working unless it actually was. So I assume it to be.

I’ll reverse the capacitor in red and see what happens.

Wed Jan 09, 2019 11:02 am

I reversed the capacitor and still nothing. I’m now looking at the cluster foil with a multimeter. The good looking traces don’t seem to conduct the positive to the tach. And now I ripped the ground trying tying to pull the connector.
I will have to do some soldering and try it again.

Wed Jan 09, 2019 11:29 am

It works!!!

Fixed the traces. I’ll need to secure the wires down. I’ll use the same red Tuck Tape. It seems to work well.

Then I’ll reassemble it and have to calibrate the tach.

I really can’t believe this thing works!

Thank you so much for the input!

Wed Jan 09, 2019 11:52 am

Excellent!

Sun Feb 17, 2019 6:14 pm

So still having problems with my tach. It works, but I haven’t been able to calibrate it. I have adjusted the potentiometer with a tiny screw driver. It only seems to want to sit in two places. Idle at 1100 rpm. Or ~2000 rpm. Nothing in between or on either side of that. And it’s finding a sweet spot to make it 1100. Otherwise everything else makes it sit at the 2000 mark.

I’m pretty sure it idles at less than 1100. And didn’t make a difference if cold or hot. And the revs would go up past 4000 when it should have been more like 2500. And when it was showing I was idling at 2000. The rev counter would top out. I have now disconnected it.

I’m pretty sure I had used an ohm meter on the potentiometer previously and had checked it to be in range.

Any Thoughts?

Sun Feb 17, 2019 6:50 pm

I have a diesel pulse adapter and a timing light with tach feature for an accurate reading to compare to. There are some other options, including using a smart phone strobe tach app. It's been a while since I calibrated one of them but if memory serves, the adjustment on the back of the tach should make a fairly regular change to the needle and widens or narrows the reading across the whole range. If yours is not doing that, then I would suspect the pot itself. You might try spraying it with de-oxit or some other very high quality contact cleaner. The other adjustment is to remove and replace the needle at a different reading. Adjust the pot to get the needle off by a uniform amount through its range then adjust the needle position on the shaft to get it dialed in for all readings.

Sun Feb 17, 2019 8:11 pm

Mine does not do as you say. No adjustment range. Just either those two idle values I stated. It does sweep smoothly with rpms. It’s just way off. I don’t seem to have fine adjustment.
The ‘pot’ you are referring to is the actual mechanical mechanism that drives the needle?
Yes, I could reposition the needle. But Im not sure that will solve the problem. I can play around with it more though.
I can also try replacing the pot. I have one more with an intact shaft. The part where the needle attaches to likes to break off.
Not sure when I’ll get around to doing that. It’s at the lower end of priorities. But would definitely be nice to have this thing working and complete and off the To Do List.

Sun Feb 17, 2019 8:28 pm

beach_creature wrote: ...The ‘pot’ you are referring to is the actual mechanical mechanism that drives the needle?...

No, that is not what I am calling a 'pot'. I believe the little electrical component that you can adjust with the little screwdriver is a 'potentiometer' or 'pot' for short. It should give a smooth and gradual adjustment. If it doesn't, it may just need its contacts cleaned.

I believe the two little contacts ride on a carbon track and if making an sketchy connection would behave as you describe.

Sun Feb 17, 2019 8:30 pm

Ah. That makes more sense.