1980SC Speedometer Problem

[armand80sc]

I'd like to introduce my self to the forum as first time poster, and first time Porsche owner
I recently purchased a 1980 911SC. There was a known problem with the speedometer and odometer, which the previous owner had put down to incorrect wiring when a mechanic had worked on the car.

The problem:
When you turn the ignition to the first position, the speedo needle automatically goes to the maximum speed and stays there. After you turn the car on and drive, needle stays at 160 and does not budge. It goes back to zero once car is turned off. The odometer does not work.


When I looked at the wiring attatchments, and comparing to pictures and comments on other threads it looked like they had been wired incorrectly. It was wired the following way:
+ reb/black
2 empty
A red/brown
31b dual wire single connector brownbrown + red/brown
- empty

My car doesnt have cruise control, I changed the wiring connection to the following:

+ red/black
2 empty
A empty
31b red/brown
- dual wire single connector brown/brown + red/brown

Having done this, the speedo needle still moves to maximum speed on ignition turn and stays there. Odometer still doesnt work.
Is it a problem with the speedometer itself - broken gear etc? or something else?

Any further advice would be greatly appreciated

[schumicat]

do some searches, there are posts with photos of the correct wiring. the two brown/red wires (you usually list main color first and stripe color second) are from the sender and IIRC I don't think either should be grounded as yours is.

[Andy911sc]

First off welcome to the forum. Tonight after work I will post a picture on here. I just sent my speedo out for odometer fixing but I took a picture of the wiring before I removed it. All the spade clips are suppose to be full for your year. Plus I think from what I remember the big ground cluster should be on the spot second in from the right, but don't quote me on that. I just took the pictures two days ago and my speedo worked great, just not the odometer (they are notorious for the plastic gear inside breaking).

EDIT: I didn't see you mention no cruise control until now. I have cruise so that probably explains the one connection being empty. I will still post my picture in hopes that it helps you.

[schumicat]

I just looked at the wiring diagram on host site and your power (black/red) and grounds (brown) and sender (brown/red) wires look correct (I was wrong that one of sender wires isn't grounded, it is). I would take off the panel on the rear floor and look at the wires to the sender. unplug and see if that changes anything. touching them together quickly simulates motion and should cause the needed to bump up slightly.

[Scott R]

If you eliminate the sender connections and just plug in a ground and the red hot to the speedo does it do the same thing?

[OsoMoore]

Let us know how those fixes go.
Also, post pictures of the whole car! We are always eager to see another SC.
I just bought an SC last spring - they are the best.

[armand80sc]

Thanks for the replies.

ScottR : I disconnected the connection as you said and it still did the same thing - needle stuck at >160mph.
The only time the needle didnt move was when I removed the red/black wire from the + terminal.

[Scott R]

Wiring is good, looks like an internal fault. I have a few extra pcb board if you need one.

[armand80sc]

Thank you Scott, but I think dealing with repairing the internal components myself is probably going to be way over my head. I was hoping it was misplaced wiring issue or something relatively simple. Will look at taking it into a Speedo repair shop and see what they can do.

[Walt Fricke]

Someone once explained how these speedometers work sort of like this.
The sender is just a magnetic reed switch, which senses the 8 magnets inside the transmission, and closes (or opens - doesn't matter) each time one is near. This produces a square wave, because closing the switch grounds the signal wire. For some reason, Porsche and Bosch decided to have the sender ground back on the speedometer itself. However, this makes it impossible to miswire the sender, at least it doesn't matter if the two sender wires are reversed.

In the speedometer (which supplies some current to the sender, so when not grounded it shows a voltage), the square wave - basically a digital signal - is converted into an analog signal. Which means a voltage. The faster the wheel rotates, the higher the voltage, I believe.

This speed varying voltage then turns a small DC motor. Such motors run faster as the voltage increases (think model train).

The spinning motor now turns a set of gears, which ultimately operate a rather old fashioned counter system, where each time a wheel turns a full revolution, it turns the next wheel one tenth of a revolution. That's the odo part.

As to speed, I think it works like older VW speedometers do - there is a magnetic cup which the motor turns, probably directly. The cup exerts a magnetic drag on something whose axis places it inside the spinning cup. That something has, on a different end, the needle. There is a spiral spring like clocks use which holds the needle system closed. All of this is carefully calibrated so that as the drag increases, the spring allows the needle to rise to an equivalent speed position.

I can't extrapolate from this sketchy functional description to why this needle would peg. The oil pressure and oil tank gauges peg if there is a break in the sender wire (or inside the sender). But they are a different deal altogether.

At least in the pre-SC electronic speedometers, Bosch did something very peculiar. In order to allow them to be used in both negative ground cars (just about every car made at the time) and positive ground cars (what market were they thinking about?), the electronic guts of the speedo were electrically isolated from the case and the car's ground. There was one ground wire to the case, and another either from a separate car ground, or from the case ground, which provided a ground for the speedometer.

It turned out that if you failed to hook up the speedo guts ground, a diode inside the speedo would burn out. I figured that out, and repaired the two failed speedometers a previous owner had tried to make work (which is why I bought my '77 without a functioning speedometer). All it took was a common diode, which happily was right at the top of the PCB when I pulled things apart to look. Anything more complicated than that would have defeated me, anyway.

But that's not helpful here, because the symptom was nothing moved no matter what. Here you have a pegged needle and nothing happening with the odo. Nothing happening is also the symptom of a failed sender, or a broken sender connection. Or no positive voltage to the speedo. But none of those lead to a pegged meter. And, if you somehow had a runaway voltage to the motor I think is inside, while it would peg the speedo, you'd also expect it to make the odo spin like mad. And it doesn't.

So you are on the right track - off the the speedometer shop, where they will have schematics for the speedo and know what they are doing.

You might see if, when they have fixed it, they would give you a technical explanation of what went wrong, which you could then share with us.

[armand80sc]

Thanks for that helpful explanation Walt. I will post the what the final diagnosis and hopeful treatment ends up being.

[Andy911sc]

Sorry, but my photos were too blurry to make out when I went to look at them to post for you.

[armand80sc]

Just as I was about to send my speedometer to get tested I realised I hadn't checked underneath the cover in the back passenger area. And lo and behold I came across this sight:


I tried fishing around for where the other ends that these two wires are supposed to join to...but couldnt find anything Where should the ends from the sender unit be coming from, and what should they look like??
(With the help of some Pelicanites who posted pictures) I managed to find the speedometer sender unit at the transmission and it appeared to be in its correct place with silver wire piece holding it in position. I identified a black cable running from the sending unit which went in an upwards direction, but I couldnt trace it's course any further.

[Walt Fricke]

Send it off to the repair shop.

I looked at wiring, and my spare speedo, and am pretty sure miswiring caused internal electrical parts you aren't equipped to find and repair to fail.

As you surmised, it looks like whoever wired your speedometer really messed it up. The positive was in the right place, but the ground wires were connected to the signal (31b), and the signal wire was connected to A, which sends some kind of information to the catalyst mileage counter. This means that the speedometer had no proper ground connection for any of its functions, including the lighting. And it means that its speedometer and mileage functions were always seeing a sensor ground, not an intermittent ground as they should.

When this kind of thing happens, the voltage usually finds a ground, only it is through components which can't take the current, and they fry.

You rewired it correctly, but the damage had been done, and rewiring wasn't going to repair failed electronic components.

Looking at Pelican's wiring diagram for your era SC, both wires from the speedometer back to connect to the sender are brown with a red stripe. You can see those at your speedometer. The wires you picture in the tunnel don't look to be those wires. The wires from the sender, for some cars, are a green and a brown, though I think for some years they may both be the same color, as it does not matter which is the signal wire and which a ground.

So those don't really line up with anything I am that familiar with. You can check to see if they are the wires from the sender by attaching your ohmmeter (if you don't have one, buy one - they are inexpensive and a critical tool for dealing with simple electrical issues) to them and rotating the rear wheels. If they show an poen closed open etc reading, they are from a working sensor.

You can also perform the same test by attaching your ohmeter to the two brown with red stripe wires you have removed from the speedometer. That will also tell you if you have both a working sender and wiring continuity. My guess is that you do (unless the miswiring screwed up the sensor reed switch by burning it up).

If your ohmeter has a continunity checking function, that would be what to use. It buzzes or chirps when an electrical connection is made, and you are looking for eight chirps per rear wheel revolution.

So what else might those stray wires be? The other wires which I can think of offhand which emerge from the main wiring loom inside the tunnel are for the handbrake on warning light, and for the heater blower. You might look at the wiring diagram to see if their color codes match either of those.

But your speedometer is fried, which is why it pegs whenever it has voltage.

[schumicat]

those wires are almost certainly the sender wires, they should be wrapped (off camera) in a black sheath and run outside the car to a plastic disc on the passenger side of transmission secured by a piece of spring steel. that is the sender. if your sender is dead I have one that you could have for cheap.

awesome color of the car BTW.

[armand80sc]

I found a picture on the forums of what I should be looking at:


So Im guessing the white and green wires are from the sender unit, and they come through the small hole just to the left of the coupler tunnel. In my case that hole is empty, no wires. I couldnt locate them while underneath the jack stands, so I think i'll have to get this baby raised on a proper garage hoist to really find where those wires are hiding. I just remembered the PO said they had replaced the clutch plate recently...maybe they had disrupted the sender wires during that procedure, who knows.
It would be nice to find the sender wires and connect the system up as it should be, then and if the speedo is likely fried as Walt suggests I can get that fixed. Will def look at getting an ohmmeter also.
Shumicat : Thanks, I also liked the colour. It def needs a paint job though, some areas look rough. That will be last my project list lol

Also what is this thing that I spotted in the coupler tunnel?

[schumicat]

original photo you posted shows where the sender wires connect under the panel in rear floor. but it shows no wires coming from sender. I was looking at it backwards, the wires shown run from the speedo to rear floor (not from sender to rear floor).

second photo you posted is example of how sender wires connect under the panel (spade connectors).

third photo is the reverse light switch (thicker green/light grey wires with bullet connectors), not sender wires.

you need to look under car passenger side trans and see if you have a sender in the first place. it would be in a round recess secured by a metal clip with a thick wire (actually 2 sender wires wrapped in black plastic sheath) coming out of it.

[schumicat]

duplicate.

[armand80sc]

Thanks shumicat, yes I identified sender unit sitting in its correct place. Traced the wire back and up, but couldnt follow it any further. Going to get her under a hoist tomorrow see if I can find where those ends are.

[schumicat]

there wire should have an accordian rubber boot around it (plugs the hole where it goes into chassis). it may be missing.