SC Tachometer wiring - what's this black wire?
 

Where does the black wire go? What does it do?

Background:
- - My Tach is dead unless I input a signal generator pulse stream. :-/

- - The CDI (tested) runs the tach in another car.

- - Jumpering the Black/Violet TD signal wire does nothing.

So, I'm trying to understand the wiring, but the Bentley diagram is no help. It simply shows the black wire going (off the page) to "Engine control systems." (?)
...cruise control? (which I have had disconnected for years)

Black wire out of the actual tach gaugepod? I don't think there is a black wire coming out...

Tach is uber simple stuff:
power in (RED/BLK)
ground out (BRN)
signal in (CDI box) BLK/VIO
signal out (RPM kill switch) WHT/RED

and some lighting stuff.

Thanks Mike. Not all 911's have this.

This is what I'm talking about. (shared on the TD lead)

http://forums.pelicanparts.com/uploa...1407790940.jpg

From Bentleys? I'd flip a coin that that is actually an 84+ thing, since they seemed to switch back and forth randomly between SC and Carrera info...

Both the 78 and 81 wiring diagrams here on Pelican show no black wire, I'm 90% sure I don't have one one my 80, but I'll check when i leave the office tonight.

Just like you, I can't think of a place where the tach signal would go, unless it went to the Motronic brain in an 84+.

Does the BLK/VIO wire have continuity? Are you feeding the signal generator wave right into the tach, or at the far end of the wire near the CDI box? Could it be a simple wiring break?

Island, check the Bentley on page 970-9. There is another reference to the two wires from the tachometer:

http://forums.pelicanparts.com/uploa...1407792477.jpg

Yeah, I saw that a while ago. But it only confuses the issue more. ..being that it shows the RPM limit wire sharing the TD terminal. :-/

Driving the car, the rev limiter does work. (Even though the tach is not.)

Also, I'm fairly certain that the black wire was on the TD terminal when I first took the gauge out for testing. (after it stopped running in-car)

Could there be an additional, non electric, centrifugal, rev-limiter working in the distributor?

Yeah, that's what I was thinking. So I made a set of jumpers such that I could run a jumper from the CDI TD to the Tach TD - still no dial movement.

As I understand it, the CDI TD ground-pulses the tach to produce signal. My thinking is that the black wire is interfering with this. Yet, removing the black wire, from the tach TD terminal, doesn't seem to change anything.

I suppose that there could be. Good thought - I'll check.

Do you have the WHT/RED wire coming out of the TD terminal as well (so, three wires there)? I bet someone replaced it at some point with a generic black wire. The RPM switch is a small box with a 6 pin connector that looks just like what is plugged into the CDI box, and it is found right behind the gas gauge. Only 4 spots are used, and it only has 4 wires, the WHT/RED in, a BRN ground, and two that go to the fuel pump relay directly. Find that connector and see if it has the WHT/RED wire in, or a BLK wire.


I bet someone accidentally broke the WHT/RED wire at some point, spliced a BLK wire in, and that splice is now grounding out behind the gauges somewhere.

Although, you said things don't change when the black wire is removed...
possible that you got a voltage spike somehow and you have an actual blown out tach? Internal damage?

Island,

Measure the voltage on the TD wire with ignition on, engine not running. Should be around 6.8 volts.

Now, measure the TD pin on the back of the tacho with the TD wire disconnected. Should be a little below battery voltage ~12v.

So if the wire is reconnected and you get ~12v it points to a broken TD wire. It you get 0v, probably means the CDI TD output is toast.

Cheers

Jonny

http://forums.pelicanparts.com/uploa...1407796138.jpg
This the tach terminal wiring.

The black wire seems to be a loose cover - like heat-shrink tubing.

My guess is, this covering is over a White/red wire, that goes to the over-rev limit box.

That would explain both of the (conflicting) wiring diagrams. (one showing black, the other showing Wht/red).

Of course, I still don't know why the tach isn't functioning. I'll look at the CDI ground path next.

Could be. I can, however get the tach needle to bounce up, by tapping the TD terminal to ground.

Another really great post. --I read your description of how the tach ground-pulses thru the CDI, on another thread. (very useful)

I'm going off memory, from a few days ago, and remember ~6V off of the TD terminal (when bare). I'll check these again.

TD terminal Voltages: (this doesn't look good)

ign off - 0.25V

ign on - 7.77V

ign on & TD terminal wireless - 4.6V (that can't be good.)

system voltage ~12.8 (on battery tender)

Your voltages look ok to me. I tested the TD tacho terminal (I have a CDI box test kit) with the wire off and it was around 6 volts so "little below battery voltage" was perhaps not the most accurate info.

The fact that your tacho 'bounces' when grounded is a good sign. I'd start looking at grounding issues. You could try the following:

With CDI connector pulled out and the ignition ON.

1) Ground TD to chassis at the CDI connector? Tacho still bounce?
2) Momentarily connect TD to 31/1 at the CDI connector? Tacho still bounce?

^ Failure of either test and you have a grounding problem.

With CDI connector pulled out and the ignition OFF.

1) Check 31/1 resisance to ground. Should be close to 0 Ohms.
2) Check tacho ground pin resistance to ground. ditto 0 Ohms.

Some ASCII art, CDI Connector pinouts (at cable):


/-----------------\
| 31d   TD   31/1 |
|  7    15    A   |
__---------------__

Great ASCII art!

I did chase grounds, even going so far as jumpering 31/1 to the ground on the coil, and then from the coil ground, to the engine, and from the engine, a long wire up to the tach. Nothing, really changed; other than a bit better tach jump on start - then nothing on the tach (engine running fine).

I did take your advice on testing the TD wire at the CDI - that works to ping the tach, and the resistance is next to nothing. - so that's good.

I'm thinking that the tach must have a component gone bad.

I opened it up, but nothing obviously bad (no burnt smell, or evidence of fried components.)

hmmm..

Did you ever figure this out?

Tach on my 82 quit today.

Thom.. Found this dangling thread when searching for something else... Here's what happened:

I went thu every internal tachometer component - every resistor, every capacitor.

Turned out to be the Pulse Shaper SAK215

$3.50 on ebay.

http://forums.pelicanparts.com/uploa...1465359166.jpg


Some hard-won knowledge there.

BTW, I now have lot's of spares of every resistor and capacitor. :-/