speedo stopped, need wiring help

[7783911]

I was running wires to hook up a voltmeter and when i pulled the speedo from the dash, some wires must have come undone

can any one post me a picture of the back of their 77 911 speedo showing which wires go where?

Every other instrument is fine including the tach which I also had to pull too...but ZERO reading on speedo

Arrrgghh....one step forward, one step back

[proporsche]

hi..it is very easy if you use search option or even google

Ivan
Here
Speedometer Wiring

[7783911]

I did that and so far nothing i ve found matches..

re are the ones i used from internet..not sure if 77 but looks similar..but i dont seem to have green white wires shown except for one running to a lamp





However..this was a redone car when i got it and frankly..a bit bodged as far as wiring

so..i removed lighting wires and moved aside

here is what i have as my speedo



here are wires that appear to have been used (i also found others but not in use from what i could tell

pair of red/black combined



2 brown and brown red combined





single bown red



Here are unused wires from the loom..god only knows what they came from...
the AC was removed but i dont think that accounts for all of these, , also power mirrors deleted

red blue white red and paired red with a female socket





Last findlings...red blue and red blue paired together




If you can provide a photo or if you indicate what goes where, lets assume 1 is bottom left and 2-x are to the right of that in order

Pictures would be best, i know its a PITA but i am really stuck...


Last stupid question..the speedo isnt fused in anyway is it..i checked all fuses and were ok? Maybe the pulse sensor has come disconnected in the dash?

Thanks

[tdw28210]

My 85 has wires in the coupler access area. Take out the 4 screws of the cover and check if you see disconnected wires there.

[Alan L]

If you can find a wiring diag for the year car ( I can check one in a day or so for you) you will find the wires to the speedo are marked for their positions. And the marks are on the white plastic housing on the speedo. And the wiring diag will tell you which bulbs go in which holes.
Alan

[7783911]

Thanks Alan...the problem is i have a 77 Targa but that doesnt mean the instraments are original, this is a 260KPH VDO

911-641-516-06

Being a cdn car, it required KPH. But looking it up on line..i wonder if this came from
a Carrera 3,2?

if so, now i need to match wiring between old loom and newer speedo...what burns me is it worked before (albeit out by 8%) but now its dead cause I wasnt careful.


anyone with suggestions please

[Alan L]

There is a lot that carries over between models in the wiring loom, and the bulk of the wiring in the speedo is probably common.
If no one omes up with a diag I will look in a couple of days. If the wire colours match with the terminals on the speedo, they will be correct. That may leave you guessing for one or two, at worst.
I have wiring diags for the 3.2 Carerra also.
Alan

[timmy2]

Here is a ‘77 factory current flow diagram for you to download and print.
https://www.dropbox.com/sh/jxno8o6qae3roun/AADW1qBcRc53LoEG0TPbRwFva?dl=0

[snbush67]

See bottom pic;

Originally Posted by porwolf

I think you have the two brown/red speed sensor wires connected to the same terminal on the far right of your picture. In my '79SC the two speed sensor wires come out of a black sleeve together with the black/red 12V power wire. I am not absolutely sure but for my '79SC speedometer the correct wiring should be as shown here:




For comparison, my '79SC speedometer wiring:

[7783911]

i spent some time going through various manuala, checked the speedo for internal wiring (looked ok) and have come up with a combo of wiring diagrams to try today. Does anyone know if there is a simple way to test output at the various wires to make sure the sender is still sending and to narrow down to the speedo or if it is wiring..ie put a voltmeter across which connections to see what exists where?

otherwise a picture of the back of of a 77 or 83 speedo would do the trick

[7783911]

Thanks everyone for input...essentially redid all those wire connectors and now its working again...

[DG624]

When you say "redid" did you change the connectors or clean the connections? I have a similar problem with intermittent speedo cut out.

[7783911]

I redid the connectors and the spades and made sure the fit was tight
Send me a pic of your back of speedo and wires attached to confirm the sequence is right.

[DG624]

I have a wiring diagram for my 86 Carrera...i got it from North Hollywood Speedometers.

[DG624]

I can get my diagram. Don't guess it could destroy your speedo.

[Walt Fricke]

You have the speedometer for a later model, in fact an '88 or '89 3.2 Carrera.

In a '77 Pin 3? +12 is a red/black wire from the top of fuse S13 on the fuse panel, which means it is switched on with the ignition, but is not itself fused.

The signal wire (pin2?) is red/brown, and goes into the wiring loom which goes through the tunnel, ending at a connector in the back of the tunnel, where it connects to the speedo sensor wires.

Pin 1? is a ground.

These speedometers, at least the right ones of this era, were constructed so they could be used on positive ground cars, as well as the more common (and every Porsche) negative ground. So the speedometer ground is also a red/brown wire, and connects here, and goes through tunnel and connects like the similar TD wire.

What the speedo sensor does is connect these two "ground" wires 8 times per rear wheel revolution. That creates the pulse the speedo sees inside.

However, the speedometer electronics need an actual chassis ground. So a regular ground should connect to this terminal also. That comes from a ground for one of the speedometer illumination lights, which connects to a brown wire (always ground).

The '77 speedo ground is not internally connected to the speedo metal case, and thus not to the light bulb grounds. But the later speedos are internally grounded, so any chassis ground to the speedo ground pin will do.

So, you need positive, you need ground, you need the signal wire to go to the sensor, and you need one (doesn't matter which) of the sensor wires to have a chassis ground somewhere.

Later speedometers for our cars aren't much different.

You can test the sensor with an ohmmeter. Clip onto the TD wire and ground, and rotate the right rear wheel. You should see it go from open to closed.

You can test the speedometer by intermittently grounding, by hand, a wire clipped to the TD pin. The needle should jump, and even show a speed if you are quick.

An '83 has the same wire colors for +12 and signal. It has a black/red, a green (ground for the clock), and a brown from the ground terminal. The lights are connected internally in this model speedo to the ground terminal, so you can't screw it up as easily.

So, any ground to the ground. Be sure the signal wire is the brown red (either one, the other needs to be connected to a chassis ground). Any ignition switched +12. Test the speedo. Test the sensor. Should work.

The 3.2 Carrera looks similar for wiring/function, and that, from the date stamp, is what you have. It has an extra brown/red going to the upshift indicator, which you don't have. It has a white/red from terminal A to the automatic heater control, which you most likely don't have. Otherwise same wire codes and functions.

Should be easy enough to check the various wires you have for these functions and figure out what kludges, if any, were done. So the signal connector only goes to the speedo sensor in back. You need a chassis ground. You need +12. And the other speedo sensor needs to end up in a ground somewhere.

Get out the Ohmeter.

[JayMac]

curious - if you connected the wires incorrectly would it damage the speedo and if so does anyone have experience with that issue/fix?

[Walt Fricke]

The standard fix for a speedo which doesn't work is to send it to an experienced speedo shop. There are a number of those.

On the '77 and maybe older speedos, if you don't connect chassis ground to the speedo there is a diode inside which will blow. You can replace this with any common diode of the same type - it is soldered into the PC board. But to get at it you have to remove the bezel, which is tricky, and reinstalling it often looks like a hack job (though when in the dash you can't see that side of the bezel. But not for the faint of heart, or the electronically challenged. Still, you don't need to be an EE for that specific fix.

Later speedos have the chassis ground, case ground, and speedo ground all internal, so just one real ground does it. But there could be other problems.

Hooking positive where negative should be certainly can cause problems sometimes. A positive to the signal line might be an issue. Whether there would be damage might depend on which wires were hooked where, so there is no cook book answer to this question.

The positive is well marked on the speedo, isn't it? and easy to check, so this ought never to be an issue. Same for chassis ground. The two red/brown wires back to the connector for the tranny sensor can be reversed without causing a problem as long as the one to the signal lug doesn't also go directly to chassis ground, though doing that would probably not cause a problem other than that the speedo needle would never move.

[JayMac]

I made sure the positive+ was in the correct place and then tried the only other two combinations (two brown wire pairs) and no luck, so I took the speedo apart in hope that I would be able to easily determine if there was a bad diode - inside the speedo there is a small circuit board with more on it than I was expecting, would anyone know which to check and/or how to confirm that it is a speedo problem vs a sensor?

[Walt Fricke]

see post 15 for sensor test. If testing at the speedometer suggests the sensor is bad, it would be best to do this again at the 2 wire connector under the cover and upholstery at the rear of the central tunnel so you just check the sensor. In case of a wire problem between there and the speedo.

To test speedo, +12 with ignition on at speedo, speedo grounded, and a bare ended wire connected to the speedo signal terminal. Rapid tapping of the wire to a ground should make the needle move, even show a slow speed.