Tacho options for a petrol dash?

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937carrera
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Re: Tacho options for a petrol dash?

Post by 937carrera »

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Re: Tacho options for a petrol dash?

Post by CJH »


Actually, I think that '19' is either a misreading of the resistor code or a typo. That circuit is for the petrol T3 tacho, so I think the 19 Ohms should be 191 Ohms. And the 602 Ohms resistor that I was concerned about earlier is shown as 802 Ohms in that diagram, which confirms that I'm probably wrong with my second value of 100 Ohms. In fact, I've pulled that resistor for now, as it may be the cause of my residual current - it links the coil signal input to IGN, albeit via very high resistance, but maybe the 100 Ohm value was low enough that there is a small residual current.
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Re: Tacho options for a petrol dash?

Post by Cobra88 »

Here is a google translate file for the German text that went with the diagram.

It might be useful, it might not

Rick

http://rickpiper.cbfsim.org/files/Diesel2petrolt.txt

original site and German text here http://www.t3-infos.de/t3-infos_a.html#bordcomputer
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Re: Tacho options for a petrol dash?

Post by CJH »

Some success!

Just a reminder of the circuit that I implemented on my PCB:
Image

Removing R8 was key - with it in place, either as 100 Ohms or 100+620 Ohms, the needle swung right over to the right. With it removed, both RV1 and my variable R4 allowed me to adjust the range. They seem to work slightly differently, so getting the right reading across the range meant adding a bit to one and taking a bit off the other. In the end I've got a setup that works fairly well.

Image

Image

The multimeter has a lag, whereas the needle moves much more quickly in response to changing rpm, so I had to hold the engine speed steady for a few seconds to try and get proper readings - not easy with a camera in one hand and leaning over to the throttle with the other. It's clear that the low end is 100-200 rpm too low on the gauge, but it's better at higher rpm. I could perhaps tweak it further, but I'm happy with this result.

I checked the variable resistor settings after adjustment. RV1 was at 1665 Ohms, so added to R6 that makes 21.7K. R4 was at 149 Ohms.
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Re: Tacho options for a petrol dash?

Post by bigbadbob76 »

That's great you got it sorted Chris.
191 and 802 ohms are precision values so are probably critical. VDO wouldn't use expensive precision resistors if common values would do.
I have a box of 1% resistors but not those values unfortunately.


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Re: Tacho options for a petrol dash?

Post by CJH »

bigbadbob76 wrote: 191 and 802 ohms are precision values so are probably critical. VDO wouldn't use expensive precision resistors if common values would do.

That's what I figured too, but I'm a bit puzzled that (a) my PCB works without that 802 Ohm resistor, and (b) that the value of the 191 Ohm resistor seems to be somewhat compensated by the 22k variable resistor.

I've been comparing the schematic that I implemented against the one that Rick posted. Here they are for reference:

Image

Image

There's a subtle difference. The top one has pin 1 connected to pin 2 via a 1k resistor, and pin 1 to pin 4 via an 803 resistor.
The bottom one also has pin 1 connected to pin 2 via a 1k (ignoring the direct short, which I did not implement), but it has the 602 resistor connected from pin4 to both pin 1 and pin 2 (I did not implement the connection to pin 1 - the direct short I referred to above). So it looks as though I corrected the wrong error in the version I implemented. That probably explains why the resistor between 2 and 4 has such a drastic effect on the needle - it shouldn't be there. I will look at whether I can modify the PCB I have to match the top schematic.

But I'm puzzled about the function of those two resistors. They're not present in the sample circuit shown in the SAK215 spec sheet (Figure 2, posted earlier), and certainly I have found that leaving out one of them doesn't stop the circuit working. Perhaps I should pull the 1K resistor as well.
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Re: Tacho options for a petrol dash?

Post by CJH »

CJH wrote:I will look at whether I can modify the PCB I have to match the top schematic.

This is possible - I can break one trace, and add a wire, but can anyone hazard a guess as to what these resistors do in this circuit?
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Re: Tacho options for a petrol dash?

Post by Cobra88 »

I can only think they smooth out the coil pulses

But i know jack about this level of electronics so it is just a guess.

I cant see why you would put shorts in for any other reason

Rick
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CJH
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Re: Tacho options for a petrol dash?

Post by CJH »

That's how much I know too!

Here's my interpretation of the errors in the schematic that I implemented.

Image
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Re: Tacho options for a petrol dash?

Post by CJH »

I went back to the photo of the '87 Vanagon board. I now realise that the 803 (or 602) resistor is the one to the right of the SAK215 IC, and indeed it looks like a 1% 803 (or 802). I don't have an 803, so I guess I could use another 1k variable for testing purposes.

Image
Last edited by CJH on 30 Jul 2018, 07:22, edited 1 time in total.
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Re: Tacho options for a petrol dash?

Post by bigbadbob76 »

Yes, I think you're right with omitting that connection, your diagram doesn't show a connection dot there wheras it does for all other connections, I think the mouse slipped when whoever traced it drew it up. :lol:
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Re: Tacho options for a petrol dash?

Post by CJH »

I've modified my board to remove the resistor between pin 2 and pin 4, and to add a 1K variable resistor between pin 1 and pin 4. I've pre-set it to 802 Ohms, and I've also set the the current limiting variable resistor R4 to 191 Ohms. It looks like a bit of a lash-up now, but it will allow me to test the sensitivity of the board to those values before deciding whether I need to order in some 1% precision resistors.

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Re: Tacho options for a petrol dash?

Post by bigbadbob76 »

Looks good for a test.
I re-drew the Polish diagram to match your components, makes more sense to me in this layout.
Works computer won't let me upload to flikr.

Image



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Re: Tacho options for a petrol dash?

Post by AngeloEvs »

Hope you don't mind some input but it is highly unlikely that the resistance value of 802 ohms and 191 ohms are correct. The need for a precision 802 ohms (unlikely to even exist) in a series circuit with a total resistance of between 16K and 38K would have no signifcant effect regards current or volt drop compared to say an 820 ohm. I suspect that the colour coding has been misread or measured.

The spec sheet for the chip is very poor but there are example application circuits that will probably serve you better than what for me appears to be some rather misleading diagrams of the VW petrol tacho. I'm on holiday at the moment with only occasional access to the internet but if I get the chance I will have a look at some.
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Re: Tacho options for a petrol dash?

Post by CJH »

bigbadbob76 wrote: I re-drew the Polish diagram to match your components, makes more sense to me in this layout.

That's useful, thank you. Yes, I agree, rather easier to follow. I'll spend a bit of time comparing this to what I implemented.

Some comments/questions:
1) You've labeled the gauge as 50mA - did that come from the discussion accompanying that drawing, or have you taken that from what I wrote? If the latter, then that may be a red herring. Since I found that my 10mA gauge works well with an R4 value of ~150 Ohms, and can be compensated by RV1, I expect 191 Ohms to be the right value for a 10mA gauge after all. That means my understanding of the current limiting approach is wrong!
2) Your 1K resistor between pin 2 and GND doesn't have a label: R5
3) In your diagram, C1 is the other side of the Zener diode compared to the schematic I was working from. Does that make a difference?
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