My speedometer still reads with sensor disconnected!?

[jwakil]

I've been trying to diagnose my speedometer on my 930 for several weeks now (reading too slow, never going above ~60mph). In my attempt to diagnose the sensor that is attached to the side of the differential and the magnets and connections to the sensor, I disconnected the wires to the sensor on the differential completely and went for a ride. To my surprise the speedometer needle was still reading and moving and changing with speed of the car(although still reading incorrectly)! I was expecting the needle to not move at all, or be stuck at some constant speed. How can the speedometer be getting any kind of reading if the sensor is completely disconnected at the differential?

[Mick_D]

was the tach working?

[jwakil]

Originally Posted by Mick_D

was the tach working?

Yes, tach is a totally different gauge and working fine. Odometer is also unrelated and working fine. Only the speedometer has issues. Rather than trying to find out what is wrong with the speedo or sensor, now I'm wondering how a speedometer can move and sense some kind of speed if the sensor is totally disconnected.

[JJ 911SC]

Originally Posted by jwakil

... now I'm wondering how a speedometer can move and sense some kind of speed if the sensor is totally disconnected.

GPS version...

[mreid]

The tach question was a valid one. Tach pulses can cause erratic speedo behavior. Has anyone been behind your dash? Has it always done this or did it just start recently?

[fanaudical]

That's a weird one.

Did you disconnect the sensor wiring at the transmission/tunnel or did you disconnect it at the speedo?

Can you post a picture of current wiring to the speedo?

[Ayles]

My speedo showed activitiy while it was disconnected at the tunnel. Almost followed the tach and bounced a bit. When i reconnected the wires in the tunnel all was normal again.

[jwakil]

Originally Posted by fanaudical

That's a weird one.

Did you disconnect the sensor wiring at the transmission/tunnel or did you disconnect it at the speedo?

Can you post a picture of current wiring to the speedo?

I disconnected the pins from the sensor at the transmission/diffy itself. My sensor is a little different in that the pins are connected right into the sensor rather than having a length of wire and separate connector. I don't have a picture of the sensor handy but it is the one on the side of the transmission/diffy with two wires connected via pins into the body. After removing the two wires I was expecting zero needle movement at the speedo and it is still reading like before. I would think it would either read zero, be stuck at some speed, or be bouncing wildly. It seems to read and goes up with speed, but not accurately.

Here are some pics of the wiring at the speedo. I have checked all the connections at the speedo and they are fine (removed and reattached).

The speedo readings were fine before, just a few months ago started showing slow speeds and maxing out at about 50-60 mph. I thought it was a sensor issue or the magnets that spin, but this reading with the sensor completely disconnected has me puzzled.

[wwest]

The wiring to the sensor was already open somewhere?

Electronic pulse "counters" will do, can do, strange things with an open sensor circuit.

[mreid]

Which pin of the sensor inputs is grounded? This sounds like a ground issue or lack there of on the sensor inputs to the speedo. Check it with a multimeter with the sensor inputs disconnected at the speedo.

[timmy2]

Disconnect the Red/Black wire (Gauge Power) and it should stop moving.

The 2 Brown/Red wires are from the speedo sensor.

No idea what the white wire with a non standard connector on it in your photo is for. Any idea as to where it goes/coms from?

[fanaudical]

I recommend disconnecting the 2 brown/red wires from the speedometer and then checking continuity to ground on each with the speed sensor disconnected. Also check continuity between the two wires. Any continuity indicates a short somewhere, which may affect the counter circuit of the speedometer.

[mreid]

That white wire, as Dennis pointed out, is suspect.

[brianlay]

I think you might have disconnected the backup lights not the speedo

[Ronnie's.930]

Originally Posted by jwakil

I disconnected the pins from the sensor at the transmission/diffy itself.

Originally Posted by brianlay

I think you might have disconnected the backup lights not the speedo

I was wondering the same thing as the speedometer sensor pucks do not have pins. The ones on Turbos look like this -

[IMG]http://www.*****************/uploads/images/product/ABS%20sensor/91160620901lr.gif[/IMG]

[timmy2]

Originally Posted by brianlay

I think you might have disconnected the backup lights not the speedo

This^^
Speedo wires are disconnected at the rear tunnel access hatch inside the car.

[jwakil]

Originally Posted by Ronnie's.930

I was wondering the same thing as the speedometer sensor pucks do not have pins. The ones on Turbos look like this -

[IMG]http://www.*****************/uploads/images/product/ABS%20sensor/91160620901lr.gif[/IMG]

I'll take a picture tomorrow, but mine is slightly different. I'm pretty sure it is the speedo sensor. What else could be stuck to the right vertical side of the transmission, with two wires going into it. The only difference between the above picture and mine is the two wires are pinned directly into the round body, and that is where you disconnect them.

I have also never messed with the wires on the speedo itself. The white wire has always been there and the speedo worked fine for 3 years.

[jwakil]

Ok, maybe I did disconnect the wrong thing. Here are a couple of pictures of what I disconnected (the white round thing with hex nut and two wires attached, the lower one is a white wire, and the upper which only shows the blue connector is a brown wire).




After searching some more, I found what I think might be the actual speedo sensor (below):
If this is the case, then what is in the above two photos?

[Ronnie's.930]

The first two photos are the switch for the reverse lights.

And yep, the last pic is the speedometer sensor. The wire connectors for it are inside the car, beneath the cover plate for the shiftrod coupler (in front of the rear seats).

[jwakil]

Originally Posted by Ronnie's.930

The first two photos are the switch for the reverse lights.

And yep, the last pic is the speedometer sensor. The wire connectors for it are inside the car, beneath the cover plate for the shiftrod coupler (in front of the rear seats).

Crap, I wasted a lot of time looking at the wrong thing. Is the actual speedo sensor just press fit in? After checking the wiring, the next step will be to remove that and clean the magnets behind there. Do you just pull it right out?