Tach wiring - Trouble Shooting? 1970

[bobswei]

Hi, I've been trying to get this tach to work for a while. It's a 70' with CD ignition. Everything seems to test "good". Except, the wire (black with purple) that goes to the "negative" on the tach, doesn't test with a flashing signal. I ran a separate wire (tested with light flashing) from dizzy to tach. and nothing. swapped rpm transducer from another car (if this is in the loop) and nothing. Is there a simple diagram that shows just what wires are involved and were they connect? It's gotta be between the dizzy and the tach and I'm not sure what is in between.... Thanks for any ideas.

[304065]

What month of manufacture? I guess if you have a silver can in the engine compartment on the electrical console that says "Einbau Senkrecht Klemmen Unten" on the side, that means your car was made before October, 1970. That means it still uses an old-style tach that takes probably a 24V square wave to trigger it.

One way to tell is to the look at the back of your tach, get the date code, that will tell you. There's a little code stamped into the metal and there's the QC code stamped with blue ink. You want the one stamped into the metal. Also, while you are laying in the trunk on top of the gas tank looking with a flashlight, note whether the terminal the black/purple connects to is labeled with "C" or "/1" My guess is that it will say "/1" given the old style, but I'm curious.

Anyway, the black/purple doesn't go to the negative. There are three terminals on the back of the tachometer: +12V, this should be a red wire with black stripe; - , this should be a brown wire to ground and /1 (or C). This is where the black/purple connects.

OK, that's out of the way. Now you say you think you have or you do have a silver RPM transducer with the "Install Vertically Contacts Below" written in German on the side? OK, here's the wiring diagram for that, a shot from an old InterEurope repair manual originally brought to these pages by the Late Early _S_Man (may he rest in peace).

Tach - oh - woe (pre '74)

Scroll down a couple pics for the diagram.

Porsche really made this confusing because the letters on the intermediate unit aren't the same as on the tach or on the coil. . . but you can see the diagram and wire colors anyway. Hook the intermediate unit up as you see there and give it a try. Intermediate units are somewhat rare (69 and through october 1970 only) and who knows what condition they are in. .

I don't know what you mean by "flashing signal"-- you won't be able to get the tach to register unless you can drive it with 24V (or 11V if the later type) square wave pulses, and that's pretty hard to do even if you are using a signal generator.

Let me know what progress you make and this problem is easily solved. A couple photos of what you've got would help-- a nice macro shot with flash of the back of the tach might save you the Yoga of climbing in. . .

[bobswei]

Wow! Lots of great info. Thank you.
I took a quick look at the tach and it's date says 10/69. Also, it has a "1" connection on it between 12v and ground. I wasn't sure if the tach was original to the car but this appears to be. Tomorrow, I will take photos that will show the strange stuff. I don't have a red wire w/black for 12V. Mine is blue with yellow stripe but does provide power. The "flashing signal" I mentioned is a test light/probe connected to the dizzy connection. Test light flashes very fast so I assumed the signal is good at the source. The RPM transducer, I believe had a decal on the side. Also, I will confirm tomorrow.
Thanks again. Update coming soon........

[bobswei]

OK, here we go. I'm still lost. I don't see writting that says, "Einbau Senkrecht Klemmen Unten" but my tach is date stamped 10/69. The RPM transducer is:

Here is the rear of the tach. It has the "1" and not a "c".

Also, not sure what the "intermediate unit " is? Is it the "Ballest unit" in this diagram you refered me to:

I'm not sure I have a ballest unit or what it looks like. There's an empty spot behind the RPM transducer. Should it be there?
Thanks!

[bobswei]

Another update: I found this "silver can" previously disguised in over spray so would this be the intermediate unit? I know it as a RPM sensor. Do these go bad? Is there a way to test it? It does have the purple and black wire coming from it going to the tach.

Thanks, anyone....

[304065]

Yes, that cylindrical object is called the "intermediate unit." You can see the remnants of "einbau senkrecht. . . "

The box you have circled with the part number 0 336 611 001 is the MFI "speed switch" which controls the solenoid on your MFI pump. Next to it, missing, is the defroster relay box, which is the same size and is easily confused for the speed switch, but it has a red label, not a black one.

I don't like the looks of those aftermarket splices and the zip tie on the back of the tach or the bulb lying in the bottom of the trunk, this looks like prior-owner hackery along with the black overspray. But as long as you verify that nothing is shorted out or going to start a fire, it's ok for the moment.

So you have the wiring diagram, that's a good place to start, I would begin by checking with an ohm meter for continuity on the wires back to the electrical console. The black/purple wire starts at /1 on the tach and goes all the way to the intermediate unit. Using a long jumper wire, you can verify that it's not cut somewhere in the car or shorted out.

[bobswei]

Thanks 304065. Good news, sorta. After checking for continuity on all wires, which showed good. There was one blue and yellow wire I tried to trace. Wiring diagram shows it going back to the coil (from the silver can). But the coil didn't have the wire. I added a jumper wire and the tach came to life but is all over the place. At idle it swings from 1000 to 6000 about once a second. Rev the engine a little (to maybe 2000) and it goes way past redline. Could there be another part I'm missing?
Thanks for getting me this far.
Oh, the PO made a mess of this car. No electrics worked and I've spent months getting all the interior and exterior lighhts working correctly. Tach is the last holdout.....

[304065]

No, that's what the intermediate unit does when it's worn out. Mine did the same thing, the old bouncy tach thing. It has to do with the fact that it's passing too much voltage to the tach. Are you absolutely certain you have it wired correctly?

[bobswei]

I double checked all the wires with two different wiring diagrams. With the new patch wire, it all looks good. Today, I made a permanet wire to the coil. Also ordered a new intermediate unit from the P dismantlers. I believe I'm finally on the right track. I will update when the new part arrives.
I don't like taking cars to shops and this time, I was starting to think, I'd have to. Thanks again for all your help.

[304065]

Ok. You know what I would do, even for concours? I would send the tach to North Hollywood speedometer for a new board and cleaning and bypass the intermediate unit entirely. It can be left in place on the electrical console, but the connections are concealed beneath the horizontal part, so not even the most fastidious concours judge can see it. That way you retain the appearance of originality while at the same time knowing what your revs are at.

I kind of doubt that a "new" intermediate unit will be any better than the old one-- inside there's a big coil and an electrolytic capacitor-- electrolytics tend to fail over time, so the "new" one probably isn't any better than the old one (cosmetics aside.) I'm interested to see how it works out for you.

Thanks for sticking with the repair-- one final tip-- be sure to use identical color wire to replace anything, to make diagnosis easier. I couldn't have helped you in the slightest way if the factory wire's weren't in place-- doing electrical diagnosis over the Internet is impossible unless everyone's got the same setup to begin with! Good luck and I can't wait to hear how it ends up.

[bobswei]

Well, I already have the part ordered and it just arrived this evening. So, this will be my first try at making it work correct. I do have a spare tach I should have tried but didn't think about it before ordering. It will probably be Thursday of Friday before I can install it. Life just get in the way of important things. But I will let you know, as soon as I know. Also, thanks for sticking with this repair issue....

[bobswei]

OK, the new rpm sesor is installed. No difference on the tach. It still swings back and forth. So, maybe the tach is gthe problem? I will try my spare tach when I get a chance. If that works, I can sell my old tach as a Metronome.
Thanks for any future updates.

[bobswei]

Next update. The extra tach is now installed. Same as the first tach. Now I have two metronomes. Maybe I should try the rpm transducer from my 72 since when I tried it the first time, I had a missing power wire? Is there anything left to replace?
Thanks.

[304065]

Ok, here's more info than you asked for about how the tach works, but it will explain what's going on.

The early tach design is called a "monostable multivibrator " or "one-shot. It consists of two transistors, let's call them A and B, a capacitor and at least one resistor. They are wired up such that when one transistor A turns ON, transistor B turns OFF.

Ok. Now, a pulse of current arrives on the black/purple wire at the back of the tach, this is caused when the points open and there's a big whammo of energy that comes up the wire. In the old days this was caused by the coil itself, in 1969 and 1970 through October, it was caused by the intermediate unit, which basically reduces the 460V output from the CDI to something more like 24V.

The 24V pulse hits Transistor A that's turned OFF and turns it ON. When A turns on, B turns OFF. Meanwhile the capacitor begins to charge, and the speed at which it charges is controlled by the resistor. Eventually, the capacitor charges up the point where its voltage is high enough that it turns B back ON. This resistor-capacitor circuit is connected to the meter movement in the tach, so that when current is flowing as it charges, the tach needle rises. Once the capacitor is charged, the circuit returns to the stable state (hence monostable) until the next pulse arrives.

This repeats itself with every tach pulse, with each pulse turning one transistor off, the cap charges, the other one turns on. The needle bounces higher when the pulses come faster-- think about one of those ball and paddle games, the more frequently you hit the ball, the further away from the paddle it goes. That's how the tach works.

Here's the problem: when the system voltage in the car is too high, the needle bounces too high, or all over the place. If you search here for "bouncing tach" you get 100 threads of guys complaining that their tach is wobbling, 99% of the time this is caused by too high system voltage, because their voltage regulator is failing.

The other problem is that the capacitor in the tach is an electrolytic type. These are known to fail over time, and it's certainly been decades since they were new. So that could be the problem also.

Anyway, what is your system voltage, measured at the battery, with the car running, and not running?

[bobswei]

Lots of great info that I kinda understand. I do remember, when I had an SC quite a while ago, the tach at one time would "float" and wasn't accurate. At the same time, the battery was over charging and sizzled with acid overflowing. It was the voltage regulator. This tach seems similar from what I remember.
This morning, I tested voltage. Car off, battery measured 12.3 volts. With the car running at idle it was up around 12.7 to 13.5 depending upon idle speed. Hard to tell with this tach. If you rev it a little, I would get 14.6 volts at about 2000 rpm (or 8000 rpm per the tach). To me, this seems normal. Now I'm gussing the next step is to subsitute another tach to see what happens? I have a 72' T and a race car with 72' S engine and electrics so those may not match up? Since neither car has an intermediate unit. The two tachs I've been testing with are stamped 1/69 and 10/69. Could both be bad? I'm, once again, lost....
Thanks for the great info. Where did you get all this knowledge of old electrics?

[thor]

Hi,

I ve rebuilt my tach to bypass the intermediate unit on my 69 "
How do you advice me to connect the black purple wire ? From coil ? From CDI ??
If you have pictures how to do it Properly ��
Thanks,

Jerome

[thor]

hello ,

finally , i've connected the black purple wire to the distributor.
And it is ok ..my tach is working

thanks