Tach Swap 79->72

[dirk07]

I would like to swap the 79´ SC Tach 911.641.303.01, which had been in the 72 viperngreen when I bought the car, and like to replace it with an 72´ 911.641.303.01.

The HKZ is a 77`0227200008 that came with the 77´3,0 Carrera engine. The 79 SC tach is working, but bounces. Might be the condenser inside the tach.

Connected a 72 T and a 72 S version. Both did not work.

Resistance between B and C of the HKZ is 120 Ohms. The earlier ones up to 74 had 33 Ohms. I thought that a parallel resistor of 45 OHMS (B-C) should be fine, but

Mr. Gerberding, specialist on HKZ in Germany, told me that the resistance is unimportant and that the tachs should work with the HKZ.

Specialists?

[Jim Breazeale]

I would like to swap the 79´ SC Tach 911.641.303.01, which had been in the 72 viperngreen when I bought the car, and like to replace it with an 72´ 911.641.303.01.

The HKZ is a 77`0227200008 that came with the 77´3,0 Carrera engine. The 79 SC tach is working, but bounces. Might be the condenser inside the tach.

Connected a 72 T and a 72 S version. Both did not work.

Resistance between B and C of the HKZ is 120 Ohms. The earlier ones up to 74 had 33 Ohms. I thought that a parallel resistor of 45 OHMS (B-C) should be fine, but



Mr. Gerberding, specialist on HKZ in Germany, told me that the resistance is unimportant and that the tachs should work with the HKZ.

Specialists?

Do you have a 6 pin CD in the car? If so, a Tach from a 73 will not work

Jim

[dirk07]

Hi Jim,

008 is the late 3 Pin version.

BR

Dirk

[edmayo]

Contact Bob Ashlock of "Tach-Adapt" he can tell you. Don't have his information handy,,,should be able to google him.

[304065]

Dirk,

The 79SC used a six-pin CDI box with the tachometer driven from the "TD" (tachometer drive) pin. The signal is a 5V square wave.

The early and late three pin CDI boxes drive the tach with an 11 volt square wave off the points. The size of the "pull-up" resistor changed from early to late, from 33 ohms to 120 ohms, which only had the effect of altering the current in the trigger circuit-- it did not affect the tach itself.

The "bouncing" tach is always a sign of improper voltage-- most frequently encountered (when the correct tach is used) when the voltage regulator malfunctions, sending too high a voltage into the tach. In your case, the same thing is happening, you are sending an 11 volt signal to the tach, which expects a five volt signal. Hence the bouncing because the higher electrical "pressure" is charging the capacitor faster, causing the needle to move more quickly than it should, then it drops back when the capacitor discharges.

They used an IC in the later tachs, but the early ones were a "monostable multivibrator" consisting of two transistors, a capacitor and a few resistors. (Of course this is a gross oversimplification, see my other work where I reverse engineer an original VDO tach for more on this topic) One transistor is OFF the other is ON. Then a pulse from the input line arrives, it switches the ON transistor OFF and the OFF transistor ON, which begins to charge the capacitor. During the time it takes for the capacitor to charge, the needle is swinging upward, and the multiple pulses arriving from the tach are "integrated" by the meter movement and the weight of needle into a smooth motion. The later IC replicates this basically.

Anyway, that explains your issue. Use the right tach or send yours to North Hollywood Speedometer to have it modified with a modern board that can take any signal. Or use Bob's box, I have one, it's great but for show you want the tach mod to be internal.

More here:

http://forums.pelicanparts.com/porsc...ity-chart.html

[keith]

enjoying this one.

[dirk07]

Thanks 304065,

so the bouncing of te SC Tach connected to a late 3 PIN Bosch 008 is clear.

But why does no one of the 72 tachos work with the 77 Bosch?
I shouldn't need an adapter.

BR
Dirk

[dirk07]

The car is finally back from the body shop and I had some hours to test the tach issue.

Looks like the 008 CDI output current is to low (110mA) to drive MY72/73 tachs.
The tach will only show 1000rpm, while the engine also sticks at around 1000 rpm without any reaction on throttle movement.

Without a connection between CDI and tach the engine runs smooth.

Before I m going to change the CDI to a 001 unit I would like to start a discussion about:

1. Why does rpm stick at 1000?
2. Who drives a 72/73 tach with a parallel resistor of 45 OHMS (B-C) on a 008 CDI? And what are the disadvantages. Heard only that the 008 might work better with old distributor caps.

[304065]

Dirk the amperage of the drive circuit should not be an issue. The early BHKZ used a 33 ohm pull-up resistor 14V/33ohm = .424A or approximately 420mA. This was enough to trigger the input signal.

I guess the later version used a 100 ohm resistor for 140mA.

Anyway you would put the pull-up resistor (10Kohm) between +12v and the tach lead. I have never done it but long ago others reported this practice. Careful don't burn up your tach or BHKZ!

A=Achtung 350V
B=Batterie +12v
C=Contact (the points)
D=Earth