Non Operating Tachometer

[69S-S/R]

I am seeking information and ideas re. the above. I have done a search and found some interesting tips but I need additional help.

The restoration of my 1969 911S is almost complete and the tach worked perfectly before the restoration. The engine was removed and the wiring in the engine compartment (relay board) was cleaned but not disturbed, I think. The gauges were restored by North Hollywod and reinstalled. The CD is the original Bosch 0227 200 001. The tach worked but had the infamous floating or irradic reading. Thinking that there was a problem with the gauge, the tach was returned to N H and after it was returned and installed there was no reading, as in dead. In an attempt to isolate the problem I installed a 1966 tach which does have a reading but it floats as before. I then took the original tach to a local speedometer shop and they reported the restored tach had normal readings. I believe the tach is wired correctly since there are only three wires. The parts book shows a "ballast unit", #901 602 601 00 which Stoddard says is NLA and there is no superceded #, I thouhgt perhaps this could be a problem. I have cleaned the points but not replaced them.

Any ideas or suggestions greatly appreciated.

Thanks,

Tom Scott

[kenikh]

MSD tach adapter works in lieu of the ballast. What year is your tach? Does it have a date on it? Mounting studs on the back?

[69S-S/R]

Hi Kenik, The tach is original to the car and has date stamp 1968, car production date 11/13/68. The tach has two mounting studs.

Thanks,

Tom Scott

[kenikh]

Weird, weird, weird - almost identical date to my car's tach. Same month! I don't need the adapter, nor should you. Still, if the early tach works, there is something going on with the signal wave pattern/frequency. Maybe the ballast failed on your tach? I know the shop said it worked; maybe they tested it for a non-CDI signal?

[304065]

I am seeking information and ideas re. the above. I have done a search and found some interesting tips but I need additional help.

The restoration of my 1969 911S is almost complete and the tach worked perfectly before the restoration. The engine was removed and the wiring in the engine compartment (relay board) was cleaned but not disturbed, I think. The gauges were restored by North Hollywod and reinstalled. The CD is the original Bosch 0227 200 001. The tach worked but had the infamous floating or irradic reading. Thinking that there was a problem with the gauge, the tach was returned to N H and after it was returned and installed there was no reading, as in dead. In an attempt to isolate the problem I installed a 1966 tach which does have a reading but it floats as before. I then took the original tach to a local speedometer shop and they reported the restored tach had normal readings. I believe the tach is wired correctly since there are only three wires. The parts book shows a "ballast unit", #901 602 601 00 which Stoddard says is NLA and there is no superceded #, I thouhgt perhaps this could be a problem. I have cleaned the points but not replaced them.

Any ideas or suggestions greatly appreciated.

Thanks,

Tom Scott

Tom, let me explain in non- EE terms what is going on. I will also refer you to my thread in which I explain in greater detail what these things are.

From 1964 until approximately October of 1971, the VDO tachs were driven off a 24 volt square wave signal. The tach wire came directly off the coil positive terminal until 1969 and ran up to the /1 contact on the back of the tach. When the points closed, the coil charged. When the points opened, the charging current broke down, inducing a current in the coil secondary, creating the spark, but also sent one MOTHER of a voltage up the tacho wire to trigger the tach. VDO says 24V, try a couple hundred!

Now, in 1969 they introduced the CDI. The CDI was driven off of the points terminal again, but the amount of current flowing from the CDI box through the points to ground was very small, on the order of 420 milliamps, which is a very small amount of current, at about 11 volts. We know this because we know the size of the "pullup" resistor inside the CDI box, in case you were interested.

Now, the problem was, in 1969 the factory didn't change the tachs. Maybe they didn't want to pay VDO to change them or VDO had already built a huge run of the boards, nobody knows. So the factory came up with a workaround, called the "intermediate unit." This is a little silver can with three terminals on it and the language, "einbau senkrecht klemmen unten" which means "install vertically, contacts underneath." Inside is another coil that is basically a little mini version of the big coil- it charges when the points are closed, and when they open it fires a MOTHER of a charge up to the tach.

Now, in October 1971 Porsche changed the tachs, they connect directly to the points terminal and don't use the "intermediate unit."

So you see why the '66 tach works. The floating is probably because the '66 tach has old capacitors, either that or your system voltage is too high. All these tachs are a circuit known to EEs as a monostable multivibrator or "one-shot"-- the arriving tach signal switches a transistor off, which causes another transistor to turn ON for a certain amount of time, charging a capacitor which then turns the first one ON and the second one off and the system returns to its stable state. While the capacitor is charging the circuit sends current to the needle, which is really a meter movement with a fixed magnet and little coil of wire-- the coil gets magnetized and the needle moves away from the fixed magnet. The pulses from the ignition bounce the needle higher and higher the more frequently they arrive, corresponding to RPM, and they are "integrated" by the weight of the movement to look like a smooth sweep. Too high a system voltage will bounce the needle all over the place.

Anyway, to fix your problem you need the little silver can. Can you see where Chuck Stoddard put one on his Perma-tune to drive the old tach in his 67S? They are not all that uncommon but be careful they haven't rusted out inside. Here is a wiring diagram for when you find one.

Does that help? Here are a couple threads that give more info (as if that were possible)

Good luck!

http://forums.pelicanparts.com/showt...st+unit&page=2

http://forums.pelicanparts.com/showt...hreadid=252299


http://forums.pelicanparts.com/showt...ght=monostable

http://forums.pelicanparts.com/showt...ave+tachometer

[304065]

Chucks car. See the little silver can. Not the capacitor, the one that says "einbau senkrecht. . ." that does NOT have its "klemmen unten!" You need that for your tach to work.

[tfiv]

You are the man John. Very informative as always.

[kenikh]

Yeah John, isn't that what I said???

[304065]

You are the man John. Very informative as always.

Tom, ask me what time it is and I'll tell you how to build a watch! Let me know if you need help hooking it up- I did the same thing in my '66 when I went to a CDI.

[69S-S/R]

Hi John, thank you very much for the incesive information. How wonderful it is to be associated with authoritive friends who know what they are talking about. I will try this solution and report.

Thanks again,

Tom Scott