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Author Topic: Tach  (Read 102264 times)

February 09, 2006, 06:59:25 am

kuggkrans

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« on: February 09, 2006, 06:59:25 am »
For all of you that dont have a tachometer and dont want to buy a converter for a gasoline tach.


This is done on a volvo 240 VDO tachometer, a vw VDO tach is nearly the same.

First remove the tach from the cluster
http://img156.imageshack.us/img156/5062/tach1by.jpg

Then remove the tach from the holder
http://img301.imageshack.us/img301/4283/tach13wq.jpg

After that remove the circutboard
http://img301.imageshack.us/img301/5595/tach25sh.jpg

Then change the capacitator to 22nF
http://img156.imageshack.us/img156/7720/tach33an.jpg

Then remove the other two capacitators, when you have done that you can put all together and calibrate the tach on the trimmer
http://img156.imageshack.us/img156/8025/tach45xu.jpg

This tach will work on the signal from the W terminal on the alternator
The Bosch alternator that i used gave 6 pulses on the W terminal and the ratio between the cranck and the alternator is 1:2,38.  When you calibrate it do it on the higest RPM possible you get a better reading on lover RPM:s.

If you calibrate it on 5000 RPM the input signal is 1190Hz (5000RPM/60)=83,3r/s

83,3*2,38*6=1190

i think vw have another ratio betveen the cranck and the alternator but that should not be a problem, the range you have on the trimmer is quite large[/img]



Reply #1February 09, 2006, 02:28:51 pm

MattRabbit

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« Reply #1 on: February 09, 2006, 02:28:51 pm »
If this works, it'd be a really easy thing to try out.  Is anyone brave enough to try it on their VW?
'81 Rabbit Diesel
'01 Golf TDI
http://matt_lisa.home.sprynet.com/

Reply #2February 10, 2006, 04:04:24 am

fatmobile

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« Reply #2 on: February 10, 2006, 04:04:24 am »
Things looks different in my  '91 Golf gasser tach and my '85 Jetta turbo diesel tach.
 No rectangular caps for one thing.
 The SAK 215 (not visible in the links) pulse shaper chip is in both of them but the other components are different.
 I've been messing with this but it's would be easier to figure out if I had a signal generator/frequncy counter.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #3February 10, 2006, 04:20:55 am

kuggkrans

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« Reply #3 on: February 10, 2006, 04:20:55 am »
I have seen other capacitators in older tachs, it can be other shapes than rectangular. i have gotten my hand on a gasoline cluster from a vw -89, dont know witch model. i shall try to modify  that one also.

Reply #4February 10, 2006, 07:25:20 am

RabbitJockey

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« Reply #4 on: February 10, 2006, 07:25:20 am »
the one guy has a nice right up on his website about converting a gas tach to work a diesel, but his is done through using a resistor and a variable resistor for fine tuning...

http://www.4crawler.com/Diesel/CheapTricks/Tachometer/index.shtml
98 Pontiac Grand Prix GT:stock daily
 81 Jetta: in progress 1.6td, ported head, holset turbo.
81 Rabbit:AAZ, hybrid T3 turbo, Giles pump, lightened fw, and yellow clutchnet clutch.

Reply #5February 13, 2006, 06:58:38 am

regcheeseman

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« Reply #5 on: February 13, 2006, 06:58:38 am »
I was looking to do that to mine, and followed his instructions to the letter - they didn't work. :cry:

I reverse engineered the board and the suggested mods and there was no reason why they would ever work. I emailed the originator of the gasser tacho mod and received no reply.

So  figured - do it myself (I'm an electronics design engineer)

If I recall correctly the w terminal kicks out 25 pulses per rev, the gasser tacho needs 2 pulses per rev.

I used a 4017 divide by ten chip wired in series with the tachos normal input to reduce the W signal down to 2.5 per rev and then used the trimmer in the tacho to get it spot on.

It works perfectly on the bench testing with 'scope and signal generator but unfortunately doesn't like the 'noise' of W terminal when installed in the car. I guess a bit of signal conditioning may sort it out? And if so it has cost me less than 1 to convert my tacho from gas to W terminal  :D

Reply #6February 13, 2006, 11:39:02 am

fspGTD

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« Reply #6 on: February 13, 2006, 11:39:02 am »
I came up with this mod a few years ago.  Many have gotten it to work OK on Westmoreland '83-'84 GTI tachometers, but it may not work at all for other kinds of tachometers.

The resistor values will also need some fine-tuning to allow the tachometer to work in the higher RPM area.  It may not be possible to get it to work right at extremely high RPMs.

Relevant threads:
http://www.hostboard.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4037&t=483

http://www.vwdiesel.net/phpBB/viewtopic.php?t=671&start=6
Jake Russell
'81 VW Rabbit GTD Autocrosser 1.6lTD, SCCA FSP Class
Dieselicious Turbocharger Upgrade/Rebuild Kits

Reply #7February 14, 2006, 04:54:42 am

regcheeseman

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« Reply #7 on: February 14, 2006, 04:54:42 am »
Thanks for the link, unfortunately the solution is similar to the previous one suggested.
The various solutions I have found either limit the input signal or vary the resistance of the needle coil or both.

Neither seem to work well with my particular tacho at high revs, the chip becomes unstable just above 1Khz (equivalent to a w signal on a 3000 rpm engine) and shuts down shortly after.
It recovers when the supply is removed.

Hopefully fatmobile's schematics will help me out here.

I phoned the yard that is supplying my engine to get the GTD tacho sent to me as well - "sorry mate, car went in crusher yesterday...."  :x

Reply #8February 14, 2006, 05:01:25 am

regcheeseman

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« Reply #8 on: February 14, 2006, 05:01:25 am »
Quote from: "kuggkrans"
This tach will work on the signal from the W terminal on the alternator
The Bosch alternator that i used gave 6 pulses on the W terminal and the ratio between the cranck and the alternator is 1:2,38. When you calibrate it do it on the higest RPM possible you get a better reading on lover RPM:s.

If you calibrate it on 5000 RPM the input signal is 1190Hz (5000RPM/60)=83,3r/s

83,3*2,38*6=1190


I've just re-read though and it may seem that the 25 pulse per engine rev quoted elsewhere may be a little excessive and 14.3 is nearer the mark....

Reply #9February 16, 2006, 02:34:19 am

fatmobile

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tach
« Reply #9 on: February 16, 2006, 02:34:19 am »
Here's a picture of the '85 Jetta turbo diesel tach:


Here are some hand drawn schematics of the '85 TD tach. The one on the top right is the first draft. The one on the top left is probably the more accurate (notice the 47 uF cap).
 The one on the bottom is of the gasser tach, highlighting the differences between the 2.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #10February 16, 2006, 11:13:18 am

regcheeseman

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« Reply #10 on: February 16, 2006, 11:13:18 am »
Working my way through your schematics...

Where is pin one on the sak215 on your diagram? Is it the same in gasser and diesel?

Any chance of getting a single scan of the gasser sketch, it's difficult to see and doesn't like being enlarged much.

Thanks!

PS - 'ZE' is signal line right?

Reply #11February 16, 2006, 12:21:46 pm

regcheeseman

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« Reply #11 on: February 16, 2006, 12:21:46 pm »
Does this look right to you? It's the diesel tach.

I was unsure of the cap C4 value - just marked as 3.3 on your sketch.



No I have it modelled in proteus, I can knock the gas one up pretty quick.

Just need those SAK pin orientation...

Reply #12February 17, 2006, 03:46:58 am

fatmobile

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« Reply #12 on: February 17, 2006, 03:46:58 am »
Pin 1 ... refresh me on this. That's the pin that if you are looking at the chip from above and the indent is at the top end .. #1 would be top left correct?
 Basically in the schematic, you are looking at the chip from underneath. If you flip it right to left you'll be looking at the chip from the top and #1 would be on your top left. As it sits I do believe #1 is top right. Same on gasser and diesel
 The 3.3 cap (C4) is the small green one at the bottom right of the picture. It only says +3.3 16V on it.
 I'm not sure if ZE is signal line, I'll check. I'm thinking it is because of the AC line under it on the gasser schematic.
 Everything looks right on your diesel schematic.

Quote
modelled in proteus

 Speaking latin at me? ha.
Just took a picture of the gasser tach scheme.
 Trimmed hosted and posted:

 It looks like I went over the gasser tach trying to compare it to the diesel components. Everything in red is diesel.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #13February 17, 2006, 10:37:06 am

regcheeseman

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« Reply #13 on: February 17, 2006, 10:37:06 am »
OK, I cleaned up the schematics, first of a re-work of 'fatmobile's' 91 golf gasser tacho schematic...


I'm really unsure about this one, there are a few bits that seem odd, possibly incorrect? It seems too far removed from the 83 version and the TD version to be right?


Now we have the 85 TD Jetta...

The line from pin 7 coupled to groung via C1 is causing me concern...


My 1983 GTI gasser tacho...- notice the similarities with the 85 TD tacho... :D

I'm 100% sure on this except 1 resistor value (R5) because this has been removed by me during previous modification.

Modified 83 GTI tacho as per instructions from 4crawler link mentioned above

Notice that it is starting to replicate the TD schematic but  misses crucial capacitor changes.

The load resistor R8 limits the needle swing, I intend to keep full needle swing and modify the tacho face so that 5000 rpm is now where 7000 is - I have some nice template files to create your own custom dials if anyone is interested?

If I make a small change to the TD schematic (added red line it would now match the layout of the gasser schematic, could this line have been missed on your original sketch?

Reply #14February 18, 2006, 03:43:12 am

fatmobile

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red line
« Reply #14 on: February 18, 2006, 03:43:12 am »
Yeah, that red line was drawn in but changed when you flipped the chip.
 The gasser tach is in the car I'm driving so I can't check to see if I got that right. The tach wasn't a seperate circuit on that one it was part of the entire dash circuitry.
 I also noticed there was a diode in series with a resistor between pin 1 and 2 on the gasser tach, that you didn't draw like I did.
On your updated scheme of the '85 TD tach you said:
Quote
The line from pin 7 coupled to groung via C1 is causing me concern...

 That pin  is hooked up to alot of components. the cylindrical golden 47 cap (C1)is one of them. One end connects to pin 7 the other connects to pin 1 (ground).

Nice work. I'm going to have to get them printed out (my printer doesn't work right now). Having the chip flipped is messing with me ... along with scrolling in order to follow the whole thing.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

 

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