Tach - oh - woe (pre '74)

[304065]

Warren, you are correct-- the translation of the manuals from German often results in bizarre component names such as "intermediate unit."

For the -853 and -852 parts, however, it's called 'series resistor' in the catalog. I believe you are correct- they would have used both Bosch and Beru and then superceded one to the other when supply ran out.

The "ballast resistors" from the old cars are not the same as the "intermediate unit" that we have been dissecting in this thread, of course. The parts book lists one from Bosch and one from Beru, inside identical rectangular metal boxes with an integral thumbscrew to secure the lid. While I haven't been able to find detailed photos on the Internet (a friend has one on a '66 that I am hoping to obtain when he breaks the car) the Mercedes Ponton enthusiasts have a good web site with a similar resistor.


photo credit: http://www.mbzponton.org/valueadded/maintenance/ignition/coil.htm

In any event, my search for the "intermediate unit" Bosch can continues. Eventually a 69 E or S will be broken and one can be obtained for testing, reverse-engineeering, and installation in my '66.

A fun, but time consuming alternative would be to go the LM2917 route. I have been looking at the datasheet and it is NOT that difficult, even for a novice like me. Also, I discovered a source to print PC board masks using a laser printer, which are then "ironed-on" to the board, the board etched, and the toner removed using acetone!

The other alternative is of course to send my '912 tacho out to the California speedometer shops and have the board replaced, the face silkscreened with the higher redline of the 911, etc. However, it is far too soon to give up: to paraphrase The Maestro, good things happen if you keep the long hood faith!

[304065]

Well, until I can find an intermediate unit here's what I'm going to try to make the tach work. Basically you are charging an inductor when the points are closed and when they open a big charge dumps into the tach signal wire. The 914 guys wrote to VDO, VDO said it was a 24 volt square wave signal. The later tach uses an 11 volt square wave signal.

In the Kettering ignition when the coil breaks down I suspect a high voltage is generated which is sensed by the tach. The Intermediate unit's choke (or NI resistor), cap and resistor probably serve a similar function.

Can't wait to try it! This is what works on my '71.

[Early_S_Man]

Well, time for an update ... until someone disects an intermediate unit and measures the inductance of the air-core inductors we may be lacking, still, a bit of data on the mystery beast ... but we can at least look at the circuit and make a few observations about the mystery inductor/transformer.

The enameled magnet wire seems to be very fine, say, between 30 ga and 40 ga.

The primary and secondary seem to be the same gauge wire.

Primary winding has ~ 36 - 39 Ohms DC resistance.

Secondary winding has ~ 250 - 260 Ohms DC resistance,

Turns ratio would appear to be ~ 1:6.5 based on reported resistance measurements.

[Hebster52]

I tried to read through this thread a few times...

I have been trying to find out if I can use my -65 912 tacho for my 2,0l boxersix from a -69 911T? What is needed to do? (Sending it to a shop for rework is out of question)

[RWebb]

rather than use a 4 cyl. tach on a 6 cyl car somehow... I think you are far far better off to find a nice used tach for your current motor.

[304065]

Easier to buy a 69T tach than spend any time on the rework.

[Hebster52]

Well the pre-69 911 tachs aren't that common around here so if anyone have any clue where to find an early tach with green digits that would work for my flatsix I would be interested I can trade it for the 912 tach too..

[304065]

Hebster,

What car are you seeking to install this tachometer in? The 911T came out in 1968, so there never was a 911T with green numerals (like in my avatar). In 1968 the gauges were black and white with silver needles and stayed that way through 1973. Of course the mounting studs on the back went away in 1970, but that's not a big deal.

Are you saying that you have a 65, 66, or 67 911 normal with a 2,0 liter engine with 7000 redline, and you need a tachometer? I have a few but I can tell you that I wouldn't sell them any cheaper than you could have yours converted by www.nhspeedometer.com. Call them, they are nice people, they can convert whatever you have.

[Hebster52]

Thanks John!

If you check this thread in the VW conversion forum over here:
The Rennsport Doppel.

You see there what I'm working on for the moment.

As speedo I have a 356 speedo so the 912 tacho would look perfect together with the speedo...

[Donzenac]

Hi guys,

I come back on this post, I hope you can understand my English, as I am writing this Message from Bordeaux in France.
I am looking for advice about my tach, because the needle is absolutely not stable when I start the Engine. then , after about 20 or 30 kms, the needle of the tach is becoming stable, and all is perfect.
So i am looking for the cause of this problem, Why the tach is working well when "warm" and is not stable when "Cold"
I have her a small video of my tach to understand how it doesn't work .
Thanks for yout help !
my car is a 911 2.2E from 1970
https://www.youtube.com/watch?v=4mRSmAjHT44

[Bob Ashlock]

Please confirm that your distributor points are correctly adjusted. If they are barely opening the tachometer is affected.

[RSTarga]

Yes, start by replacing your points and checking the green wire coming off the points. Check that the grounds on your tach are good and that your alternator is providing a stable charge.

[Donzenac]

Ok I will check this tomorrow , and come back to let you know , thanks

[Donzenac]

I have checked everything you all guys told me to check
alll is correct, alternator is ok , regulator is ok, distributor points are ok

Now I have a very "strange " good new:

I received today a new Bosch intermediate unit 0227 990 0001 , (the little alu ballast cylindric)
I have installed it instead of the old one ,
and then , the tach is now absolutely STABLE !! BUT it indicates the double of the real rounds p minutes
for exemple , at iddle I know the engine is turning at 1000 rpm; now with the new Bosch intermediate unit (The new alu ballast cylindric) , the tach indicates 2000 rpm !! But is now perfectly stable ..

oh I dont understand at all this total enigma

i took the impedance of both ballasts ' ( new one and old one have almost same figurs 1% dif)
A to B 188 ohm
B to C 445 ohm
A to C 250 ohm

[petro206]

Hi to all,
My Tachometer was a little instable, at costant speed the needle come down.
I've a 2.4 T 1973 and I found a 2.2 E Tach, at reasonable price.
I connected directly to the points, as original scheme, and it doesn't work --> ok, it's a PRE-71 Tach and I should have intermediate unit, hard to find.

So, i connect directly to the + 15 of the coil and it's work correctly.


Can I leave the connection as is, or i have to insert some resistor to avoid the damage of the tach? I know that CDI give a very high voltage (400 V) output

This kind of link, could be reduce the efficiency of spark advance? The coil has a resistor connected in parallel.

Marco