Tachometer Compatibility Chart

[304065]

There are several different VDO tachometers out there. This thread is an attempt to classify the characteristics of each. Can anyone help by furnishing the VDO numbers from the back of any spare tachs they may have lying around?

1. 1965-1968 SWB Tachs. These may have VDO number 333.230/041/002, according to VDO, "this is a old version Tachometer with 24 Volt Square Wave signal from Ignition KL.1."
KL is German for "klemme" or contact, and #1 is the points contact, so this is a 24-volt square wave signal that comes off the coil. In another thread, Pelican 914GT tested it and found that the coil output was way more than 24 volts, but the spec came from VDO. Tach Thread These also say "12V 14000 Imp." which I think is the impedance value to make them work? I think all 914 tachs fit into this category as well.

2. 1969-1970 LWB Tachs. These have VDO numbers 333.230/041/007 to 333.230/041/010. According to VDO, these are driven by an "11 volt square wave." Now, we know that up until around 1971 the tachs were driven from an "intermediate unit" which was a silver can with Bosch part number 0 227 990 001. We know that this contains an inductor that is probably used to simulate the original coil to drive the low-impedance tach. Some of these say "12V 24000 Imp."

3. 1971-1973 Tachs. These appear the same externally but have been altered internally to be driven off the high-impedance circuit between the points and the Bosch CDI box. In the early 3-pin CDI boxes, there is a 33 ohm pullup resistor in the trigger circuit that would allow only around 425 mA to flow in the circuit. The tach must be sensitive to 11 volts and 425mA, which is why the later CDI won't drive the earlier tach off this circuit-- the earlier tach needs higher voltage.

4. 1974-1977 Tachs. I know these were driven off the CDI box, but I don't know when the CDI box changed from an "001" to an "008" or when the cutover from a 33 ohm pullup resistor to a 120 ohm occurred. With 120 ohms the input current is only 116mA so the tach must have changed somewhere along the line to match. If you send your tach to NH speedo to be recalibrated they want to know what CDI box you have, so this must affect the input value to the tach.

5. 1978-1983 Tachs. These are driven from the "TD" tachometer drive output from the six-pin CDI box because the points were eliminated in favor of a magnetic reluctor. According to the following thread, the SC tach signal is the same as the later Motronic:SC Thread

6. 1984-1989 Tachs. These are driven from the Motronic box. Chris Bennett posted the Motronic signal here: Motronic Thread

7. 1990-1998 Tachs. 333.240/006/001. The 964 and 993 tachs are also driven from the Motronic box, no?

If ANYONE has any information on the signal requirements of any of these tachometers, it would be very helpful to post it here. Also, can anyone post the numbers from the back of their tach? A helpful format would be:

230/041/002 12V 14000 IMP 6/72-- that way we can figure out what tachs are out there, when they were made and when the cutover to a different impedance was.

Thanks!

[randywebb]

Great Compilation!

[Gogar]

I have 2 tachs:

230/41/10 12V/24000Imp. 9/71
230/41/10 12V/24000Imp. 6/72

My CD is a 0227-200-001

[Early_S_Man]

John,

The 33-to-120 Ohm changeover occured around the time of the '72/'73 model changeover. My CDI is an early '73 model, date-coded '227' and the 120 Ohm resistor is original. The date code is stamped on the same side as the 'Vorsicht!' sticker near the lower edge ... with very tiny 2 - 3 mm high numerals.

You can easily tell which pullup resistor you have by measuring from terminal 'B' to terminal 'C' with a multimeter set to measure low Ohms [200 Ohm scale] ...

If anyone has the CDI unit off to clean the Faston connectors [a good annual procedure to put on your preventive maintenance list] ... you can check the date code, measure the pullup resistor, and take a closeup pic of the circuit board after removing the back cover.

It would be very useful to me and others if CDI postings were in the format:

Part number, Date Code, Pullup resistor value ... followed by a good closeup pic taken outdoors without flash ... 800 pixels wide.

[304065]

Gentlemen, thanks! This helps end the mystery.

Warren, I found a Bosch intermediate unit but I can't bring myself to cut it open for testing! But knowing what the early tachs want for a signal makes it easier to come up with something. I do find the inductor approach too inelegant, however, so I am planning on doing one or both of the following:

1. Fab a transistor buffer stage out of a single transistor, with 24V from a 12V-24V DC/DC converter to the tach /1 terminal and the collector and the current from the CDI's points trigger to the base, emitter to ground; or

2. Finish my LM2917 tach project (I was missing a few of the supporting components but I can get them from Radio Shack) and install the new circuit on prototype board in an old '66 912 Tach I have (I'll have to either have a new redline screened on it or, more interestingly, scan and reproduce my own gauge face using a color laser printer).

Other than those alternatives I don't think there are alternatives to a big, heat-producing inductor.

[304065]

Bosch has put the original part numbers up on the Internet here:

http://www.automotive-tradition.de/de/wissen/ausruestungslisten/index.htm

Schaltgerät, Zundung

9.77 8.83 0 227 300 004
9.75 8.77 0 227 200 008
9.73 8.75 0 227 200 001
9.71 8.73 "
9.69 8.71 "
7.68 8.69 "

The "Vorschaltgerät" (which, translates both as "Ballast" and "Control Gear") called 0 227 991 001 was used from 9.69-8.71. It does NOT appear on the list for the 9.71 and on cars, so that's the cutoff date for its inclusion in the car, therefore the tach must have been modified at that point as well.

[Grady Clay]

John,

Re: Porsche 901.602.601.00 “Ballast unit for transistor tachometer (BHKZ)” Vorschaltgerat fur Transistor – Drehzahlmesser (BHKZ).

The Porsche (paper) parts manual and wiring diagrams show that change (delete this part) beginning with the ’71 model in 9/70. That has been my experience also.

The ’70 911E tach 901.741.302.04 uses the “ballast.”
The ’71 911E tach 911.641.302.01 does not use it.
Similar for the other tachometers.

I don’t think the later tach will work with regular Kettering ignition.

Best,
Grady

[911pcars]

Nice compilation John. Good contributions from Warren and Grady too.

Does the above information provide any helpful hints for owners seeking correct tach readings when used with an aftermarket ignition system (Crane, MSD, etc.)?

Thanks,
Sherwood

[304065]

Grady, thanks for the cross-reference!

I actually have two different tach issues that I'm working on:

1) How to make the '66 tach work with a Bosch CDI; and

2) How to make the tach in my '71E work with a Bosch CDI (the tach is actually from a '70 S.

Sherwood, the underlying reason behind all this research is I've been trying to figure out when the tach cutover to low impedance is. That way, I can source a later tach and have it work.

That being said, I'm also working on the substitution of modern tachometer movement and circuit for the original VDO at the suggestion of Pelican 914GT. I have all the parts, just have to spend a few hours fabricating. . .

[TargaT]

John, looks like I may have an earlier tach. My car is an early 73T with MFI

Original 12v/24000Imp 230/41/10 572
Replacement 12v/24000Imp 230/41/5 869

Will this work? Looks like you're a bit of a techy too? I work for Agilent Technologies formerly HP and sell all kinds of high end test equipment. Short of building your creative breadboard any next steps you would recommend. Have you ever looked into the tachs and what goes wrong with them? I have all kinds of test equipment available to me.

Thanks, Joe

[TargaT]

I did a quick check with a light probe on the black/purple wire. I got flashing at the distributor and at the connection to the tach. I guess I should assume the circuit is OK? Looks like my original tach is bad and the replacement I got is not compatible?

thanks, Joe

[jasam]

Are the 69 gauges good for 70's as well?

[Lorenfb]

Actually it's very simple:

1. Low impedence tachs are driven from the points and some require a 33 ohm
pullup resistor in the CDI versus the 100/120 ohm resistor.
Note: These will result in a bouncing tach if connected to a CDI output.
Also, a low impedence tach used with a 100/120 ohm type of CDI may result
in the CDI not working requiring an additional pullup resistor.
2. Some early tachs (high impedence inputs) are driven from the CDI output,
i.e. the coil wire.
3. Most all late, e.g. 911SC 6 pin CDI, tachs are driven from a signal output of the
ignition unit or the engine's ECM unit.
Note: These will result in a bouncing tach if connected to a CDI output.

[Jim Williams]

I don't know if anyone is still compiling information on the Tachometer numbers, but here is an input:

From a '73.5 CIS car --
230/41/10 12V/24000 Imp. 4.73

From a '77 --
231/1/1 12V/24000 Imp. 10.77

From a '78 --
231/1/5 12V/21000 Imp. 12.78

With a '79 SC engine transplant into the '73, the '79 CDI box won't drive the '73 tach. I suspected this would probably happen, but thought I verify that it wouldn't work before removing the tach from the dash. I didn't bother to check out the '77 tach to see if it would work, since I had an SC tach available.

As another data point, I can verify that an SC tach will work with the 3.6 liter DME. Or at least it did with the tach in the particular SC I installed the 3.6 engine in.

[304065]

Thanks Jim, more data is always a big help. I imagine the reason a six-pin box won't drive a late three-pin tach is because the signal is different.

[BlueSideUp]

John here is the tach in my '70 911T. I think it's an SC tach.

Unfortunately this tach bounces all over the place in normal driving. I've already checked the common problem of having bad voltage spikes but the voltage runs steady. The only thing I can think of is that this tach is not working well with the ballast resistor setup in my car as loren mentioned.





I'm almost embarrassed to post this picture but this was my electrical panel when I first bought the car. It had been sitting in a garage for a few years and obviously was in need of some clean up. I have since cleaned it up a lot but it's not quite up to the standards of Shaun 84 Targa's restoration.



I also have a '73 tach that I bought but it apparently reads low. Based on what you said in your first post it won't work in my '70 so I need to keep my eyes out for a good '70 T tach.

Oh and here's an in progress shot of the engine room...

[Superdave1]

Hello everyone.

What are my options/test suggestions if I have a 1982sc with a 1975 2.7 motor/cdi. The tach bounces. I swapped out the 82 tach with a 77(3 pin cdi) tach and it bounces too. Checked out voltage reg/battery/alt. /wiring/ grounds. Everything checks out ok. Just the bouncing tachs. Any ideas?


Thanks dave...

[austin552]

http://forums.pelicanparts.com/showthread.php?t=455139
The tach has never worked. The PO put a 71 2.2l T in the 1974 911S chassis.
This is the 1974 Tach



This is the 1974 3 PIN CDI


This is the original 1971 Bosch Coil


A black and violet wire is connected to the points lead of the Distributor. How should I trouble shoot this problem?

[304065]

The top photo shows a '71 tach, not a '74. A '74 tach has a black center button.

The bottom photo shows a tach dated 6.77.

What is in the car?

[austin552]

This is what is in the car. Thought it was a 74' tach.