[VaSteve]

I was driving in to work today and my speedometer got up to around 40 - 50 mph and then the needle dropped to zero (the car was still moving). It also appeared that the odometer stopped working as well (but I'm not sure). A couple of miles later, after a stop, it came back and worked the rest of the way to work!

Any guesses as to what to look at for such strange performance? I though the speedo/odo were two sperate connections.

[Dave O]

Others with more knowledge can confirm/deny this but I believe your speedo and odo are both sent a signal by a quarter sized sensor that plugs into the side of the transmission. The wires run up through the shift coupling tunnel. Possibly the sensor is going bad or the wiring is going intermittent.

[Lucky]

Ground strap. Start there.

A bad strap causes all sorts of weirdness... typically starting with low current (but voltage sensitive) items such as speedos and progressing to starting and running problems.

[BobNovas]

I can verify that Charlie S's speedo only got up to about 80MPH at MO. I'm not sure if that's the fault of the speedo or Charlie's driving.

[Vicegrip]

Flexation.
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.Sorry for the useless post, I just like you Targa sig.

[Marc A]

Steve:

Check the connection point right next to the shift coupler. There are 2 wires from the tranny as Dave says. Sometimes they get loose and when you hit a bump they make contact.

[kimlangley7]

I've had a similiar malfunction with my 80 911... speedo started going all over the place and then dropped to zero... odometer just quit... I'm replacing the sensor unit

[William Miller]

guess what didn't work after we pulled the harness back thru the tunnel?

When doing this we dislodged the connector and one wire was hanging out.
Must have been periodically because every once in a while the needle would bounce. The good news is that it was my first instinct and fixed easily.

While we're on it can anyone describe how it works.
I'm imagining a magnetic pick up like on a bicycle odometer.

It the pickup counting the turns of the final drive?
If so it would be independant of the actual gearing.
My ROW SC had it's spedo replaced when it came over the pond in 1985.
Are there different spedos for different gear ratios or is it like I said above.


I expect to hit 260,000 miles this week. Not a huge accomplishmment, cause most has been refurbished. (Also, I'm only responsible for the last 30-40K.)
Good luck Steve. Remember start with the simple stuff!
Did you replace your shift coupler?

It the pickup counting the turns of the final drive?
If so it would be independant of the actual gearing.
My ROW SC had it's spedo replaced when it came over the pond in 1985.
Are there different spedos for different gear ratios or is it like I said above.

I don't believe that the final drive has anything to do with the speedo. If you think about it, by measuring the number of times the final drive rotates will give you a certain speed based on tire size. This is why when you change the tire size the speedo accuracy can go out the window.

I sure hope I'm right here.

[William Miller]

I suspect you are right and it is independant of the final drive ratio.
At least it would be much easier to have one calibration for spedos instead if different for each final ratio.