From: mlschmidt@sprintmail.com
Sent: Tuesday, July 30, 2002 1:35 AM
To: 928
Subject: [928] Re: Speedometer sticks until I hit 40 mph

>My 88 SE does that too sometimes. I always put it down to the sensor in the
>transmission dying. (Like the ABS sensors that I think have died causing ABS
>warnings)

There really shouldn't be too much that can go wrong with the actual sensor
at the transmission. The sensor is a Hall effect type that sends electrical
pulses to the gauge. There's a ring with I think eight magnets in it's
circumference located inside of the transmission. The sensor attached to
the transmission cover is just a coil that detects the magnets spinning
past it through the transmission cover. The number of pulses required to
register one mile on the odometer should be written on the speedometer
housing. It may vary depending on the size tire that the particular model
year car originally came with, but on my '88 it's 6,673 pulses per mile.
The faster the pulses, the higher the speedometer reads. If the odometer is
still working when the speedometer is stuck, the problem would be inside
the gauge, and most likely mechanical.

It's kind of a coincidence that you mentioned the ABS sensors. On the GTS
the pulsed signal for the speedometer/odometer is gotten from the ABS
sensors, and the ring with magnets and sensor at the transmission was no
longer used.

Mike Schmidt
'88 928S4 Auto Black/Black "PORSCHE" cloth
928 Owners Club Charter Member
PCA Chicago Region



From: Wally Plumley [mailto:wplumley@bellsouth.net]
Sent: Sunday, March 16, 2003 4:25 PM
To: 928
Subject: [928] Re: '89 Speedo T/U

At 03:22 PM 3/16/03, biguns928gt@charter.net wrote:
>Last night while coming home from a short in state trip in the rain, the
>speedo on my '89 GT started jumping up and down about 20 mph, then just quit
>working and dropped to zero.(all other guages working fine, but anything on
>the digital system related to speedo signal stopped also) I don't think that
>it is anything pod related, but it appeared to me to have lost an electrical
>connection somewhere, most probably at the sensor?? I have the manuals on
>CD, but am having problems finding where in the manuals the info on the
>speedo sensor/signal sender and where it's located. I was thinking that it
>was somewhere on top of the transaxle, but I'm not sure. Any help with
>either the speedo problem, or manual location of the info I seek would be
>greatly appreciated. TIA,
>
>Chris Lockhart
>'89 928 GT
>

Speedo/odo sensor is a reed switch mounted in the rear cover of the 
differential, operated by magnets on a ring mounted to the differential.

The most common problem is a bad connection at the connector in the forward 
right corner of the spare tire well, although a bad connection inside the 
pod can also cause this.

Wally Plumley
928 Specialists