Tach help

[Cairns888]

Hi,

I have an early 911 VDO Tach dated 2/65 which does not work.
Upon opening up I found it had been upgraded. It has Circuit board PCB marked VDO and ...120 on it and that is it.
It is using and IC with markings SAK215 and I have found a shorted out glass encapsulated diode which I suspect is a Zener Diode.
Does anyone have a circuit diagram, part number, diode valve, or anything at all that can help.

I can post a photo soon if needed.

Thanks

Jim

[Merv]

Not sure if this helps Jim http://www.thesamba.com/vw/forum/viewtopic.php?t=497798

http://forums.pelicanparts.com/porsc...er-guts-2.html

I would give these guys at Loganholme a call. They do a lot of VDO stuff http://www.ableinstrument.com

[304065]

Jim,

I spent many, many hours reverse-engineering the old VDO tach to figure out how it worked. (That is the schematic linked in the two threads above-- with reference to my old mentor Warren Hall (RIP) included!)

But that is not the tach board you have. The IC is pretty common, it's called a "pulse shaper for revolution counter" and was made by a bunch of different companies-- you can see a datasheet here:

http://www.datasheet-pdf.com/datashe....php?id=489215

My guess is that you have a board from a later German car, it could be from an SC/Carrera although I never had one of those apart, could be a VW or Benz, hence the "VDO" label on the board. VDO used pretty much the same meter movement and if you buy a modern VDO aftermarket tach and take it apart you will find many of the same parts. How's that for amortizing tooling investment over decades!

Now, looking at the datasheet, it basically shows the IC as the main event, then a handful of resistors, etc. for conditioning the input signal. It does not show a Zener, nor do I see a zener on the board in many of the tach photos I can find that use the SAK215.

Which is not to say that it's wrong, just different. So we're left guessing about the zener value. I suppose the point of it is to regulate the input voltage to the chip-- I would expect to see the Zener in series with a high value resistor to step down +14v to the 5v logic level used by the chip. So try a 5V in the spot and see if it works.

Hope this helps-- some detailed, clear photos of both sides of the board would really help.

edit: somebody from Germany worked over on the Zuendapp site to come up with this, which I haven't spent time looking at but you can see a 6V zener on the input line. . .

[Richardnew]

Here's an article I wrote about gauges. It doesn't address this specific problem though. What's interesting though is that most of the damage to gauges is caused when we open them up in our home garage. Not that this is the case here though.



Richard Newton

[Cairns888]

Thanks for your help guys.
We have been working from that exact circuit diagram and also the manufacturers sheet for the IC.
That's all we could find as well.
It appears it is a 4 cyl Tach as when given 133 hertz the reading was 4000rpm and the correct frequency should of been 200 hertz for a 6 cyl. Replaced the unknown Zener diode with a signal diode as in reference to the data sheet for the IC the input trigger range is 1.5v to 20v.
It works fine when used with Tach Adapt for the 4- 6 cyl issue.
We will replace with a Zener diode 6.2v for not other reason apart from that is what appears in the circuit diagram.
I will take some photo's for reference and post them up.

Thanks again

Jim

[Cairns888]

[Cairns888]

No luck with the 6.2v Zener diode. So we put the other diode back in.
Works well after checking against varying frequency. Had to adjust slightly but can't get it spot on but it's close enough to show you the rpm.