911 Tacho

[nickphillips]

I have a problem with my 1970 Tacho, it doesnt work, I traced the wiring ensuring there was no broken wires etc. So was it the Tacho or the signal to the unit. I found that there are various types of tacho’s and different ways of driving them.

http://forums.pelicanparts.com/porsche- ... chart.html

My unit is has a pin labelled /1 and is driven from a Ballast Unit (bosch) which appears to be an RLC circuit. I wanted to establish if it was my tacho or feed, my tacho works from a 12v square wave input, so I decided to build a square wave generator, lucky for me maplins sell a kit (Order Code: N41FL) for £6.99 . I had to change the linear pot to 220kΩ and feed the tacho input from the test point rather than the output of the fet. Connected it all up to a 12v source and it worked, I decided to take some measurements to establish how accurate the tacho is after 40 years.

The first picture is the ccrt board and tacho at 2000rpm


This picture shows the square wave output from the unit.


The table below shows the approx resistor values you need to produce the various square wave freq’s. As you can see my tacho varies slightly, at an applied freq of 100Hz this should produce 2000rpm, but it shows 2050, to obtain 2000rpm on the tacho I need to apply 99.97Hz



I hope this will be useful to someone who has similar issues as mine, most of my info came from pelicanparts forum, so have a look.

http://forums.pelicanparts.com/porsche- ... -74-a.html

I still have a problem with my tacho drive signal, so if anyone has any ideas let me know (or has a spare ballast unit i could try).

Cheers

Nick

[impmad2000]

Hi Nick,
If tacho triggers from the circuit you have made, giving simply a 12V square wave, why can't you just connect the Black/Purple tacho wire directly to the points on the car. With a CDI setup, you get very much this signal at the points. (12V pulled up in the CDI via a resistor pulled down to GND by the points)
It should work fine.
The Bosch Ballast unit (I think) was introduced when the cars switched from the points/condensor kettering ignition to the CDI. The kettering system gives a back emf kick off the points and the tacho was designed to be triggered by this. When the CDI was introduced, this bosh unit was added (To use up stack of the old tachos ?), but I think its just a coil affair to give this same back EMF kick, but having never seen one I don't really know. The later 911 tacho's have a different input resistor and so can take the trigger direct from the points of the CDI input. These don't need the bosch unit. I have changed the input resistor inside the tacho to get it to trigger directly off the points in the past, but you do need to split the tacho to do this. I can't remember the resistor values off the top of my head, but a few quick experiments and you'll get it, its not too sensitive to value (10K or 1K or something like that !)
Andy (Lightweight_911) is selling an electrical panel with a bosch unit on it. Perhaps he'll split it down for you ?
http://www.ddk-online.com/phpBB2/viewto ... sc&start=0
Cheers
Tim

[Jonny Hart]

I notice you have Picoscope. Hook it up to the drive signal and post the results here. Try grounding the scope signal in different places and see how the signal is affected. If that has a significant effect, run a temporary ground wire straight from the drive signal source to track the bad ground.


Jonny

(slightly rusty electronic engineer).

[nickphillips]

Hi tim/jonny,
thanks for the replies, this is what i get at the output of the intermediate unit 'C' and at the tacho.



This is the configurartion of the intermediate unit (pelicanparts forum)



It appears to but an RLC ccrt, is it just being used as a step down transformer (460v - 12v) ????

I have also changed the points to pertronix, but im sure this shouldnt have any impact as i think the tacho wasnt working with points fitted .

Cheers

Nick

[Jonny Hart]

Apologies for the generalisation, but I don't have a 911 (yet) so can only comment on experience fiddling with other old motors.

There are two types of tacho, older types can be current sensing (RVI) and newer tend to be voltage sensing (RVC).

My guess is that Porsche introduced a newer voltage controlled tacho like the one you have before the electronic ignition came in (you wouldn't be able to drive an RVI unit of your 555 timer circuit so it MUST be an RVC) and shipped the Bosch widget as a way of driving straight from the coil. Older RVI tachos probably had the transformer as part of the tacho itself. RVI units do not age well, and generally it is the transformer that is the problem. In your case, luckily, the transformer is in the Bosch widget so I would suspect that has failed. Looking at the trace, it's hard to tell how wide that rising edge is and there is a lot of 'ringing' so perhaps the cap has broken down as well.

Jonny

[Jonny Hart]

Sorry, just read "I have also changed the points to pertronix". So does that mean you've got electronic ignition?

If so, there should be a feed of of that to drive the tacho directly. Time to get the Picoscope out again...

[impmad2000]

The tachs are all voltage triggered.
There's a lot of information here...
But read through carefully as the circuits "develop" as the thread grows !! A good start point though.
http://forums.pelicanparts.com/porsche- ... uts-2.html
The pertronix is a "points replacement" and is used to trigger the CDI.
The output from "C" shows clearly the large pulse from the CDI. This would trigger a mono-stable pulse shaper circuit on the tacho. At 50V It should be big enough to trigger at the tacho end.
Nick, do you still have this signal at the tacho ?
The output from "C" is also showing a lot of ringing (Probably a function of the high voltage DC-DC step up oscillator used in the CDI) suggests that the capatitor C1 in the intermediate is shot (It could be that the frequency of the ringing oscillation is too high to be filtered out by the value of the capacitor fitted) . Capacitors do tend to die over time... Any output from the Output of the CDI will always be electrically noisy. Taking the tacho feed from the points, or even better the Pertronix, will give the tach a cleaner signal.
Let me get this straight
1) The tacho driven by the Maplin circuit works ?
2) You have a good +12V and GND at the tacho ?
3) You have the dirty tacho signal at the tacho ?
4) The cdi trigger now comes from a Pertronix ?

... I think you could reconfigure the maplin unit as a replacement intermediate unit, triggered from the pertronix. I'll have a look into this...
Tim

[impmad2000]

Reconfigure the 555 to be a monostable, remove the flip-flop. Drive the tacho direct from the 555. Connect the whole lot to the Pertronix output (Where it connects to the CDI should be fine. A Pull up resistor (10K) can be fitted on pin 2 of IC1, but this is effectively fitted in the CDI.

Just thoughts, I've not tested any of this...
Circuit may be subject to noise etc, I think site the electronics behind the dash rather than in the engine bay.
Tim

[nickphillips]

Hi Guys,
Really appreciate you staying with this thread, some answers.
The tacho functions with the maplin ccrt no problems.
There is a good +12v and gnd at the tacho head
The cdi trigger comes from the pertronix as per diagram, only just realised I had this diagram.



Tried using the feed as I shows in the diagram, the tacho does now work,
the diagrams below is from the tacho head and shows 2000 rpm (191Hz) on the tacho



this diagram is from the tacho head and shows 3000 rpm (317Hz)on the tacho



so the question now is why does the tacho not appear to display the correct rpm if compared to the table i originally had???
How do i check the rpm of the engine compared to what is displayed on the tacho??.

Again thanks for all the help

cheers

nick

[impmad2000]

Hmm, approximately double !!
The tacho is being triggered by the spike on the Leading and falling edges of the square wave from the pertronix I guess.
Using the monostable 555 circuit to trigger the tacho should fix this. Perhaps a bit of filtering instead ?
You'll sort it
Tim

[Jonny Hart]

"How do i check the rpm of the engine compared to what is displayed on the tacho??"

Go back to the coil trace. You've got a six cylinder engine so it'll fire the coil six times in 2 complete revolutions (unless it's a two-stroke . ) . So in other words, 3 ignition pulses per crank rotation, therefore:

RPM = ( Freq(Hz) / 3 ) * 60

or more simply

RPM = Freq * 20;

[steve wright]

holy cow, this is mostly chinese to me - I am humbled by technical knowledge out there

[Mick Cliff]

holy cow, this is mostly chinese to me - I am humbled by technical knowledge out there

Me too! It's all complete gibberish to me

[nickphillips]

Hi Guys

just thought i would keep you up to date with my problem . I contacted pertronix and explained the problem, they recommended "
You may want to install a 0.01 microfarad 1000 vdc capacitor; install capacitor in series between tach and Ignitor" . Tried that, still low rpm display on the tacho.
A very kind ddker, Andy sent me an intermediate unit to try, fitted this with much anticipation only to have the same output as my intermediate unit, no reading on the tacho and same output via the scope.
Could it be 2 intermediate units exhibiting the same fault ?????, is there anybody who would try these intermediate units for me if i post them.
Could it be the output from the coil.

Again thanks for all your help.

cheers

nick

[impmad2000]

Nick, Is there a date code on the back of the tacho ?
Cheers
Tim