Speedometer on '88 does not work

[red901]

Hello all,

Last year, the speedometer worked intermittently, then died. I sent it out for a rebuild and replaced the sender in the gearbox.

When I reinstalled the speedo, it still did not work. On an '88, the insert for the gearbox only goes in one way, so it is not installed incorrectly.

At the gauge, the voltage is 7.83 V, but the battery reads 12.3. At the connectors inside the tunnel at the shifter linkage, I have the same 7.83 V.

Can someone confirm the wiring in back of the gauge for me?

Thanks in advance, Tom

[island911]

easy to hook up wrong. It might seem that the brown/red wires go together on one terminal, and the ground wires (brown) on the other. But, those wires are split to each. -the signal terminal has a brown/red wire and a ground, and the ground terminal has the other pair (of brown/red & brown wires) ...iirc.

Because the speed sensor is just a switch, it doesn't mater if the pairs are switched.

expect porwolf to chime in with more on this - it's one of his things. ..or search.

[red901]

Thanks Island911,

Any idea on the placement of the wires on the back of the speedo?
There are the following wires and the labels on the gauge (current location):
Brown (2); "-"
Green (2); 31b
White/Red (2); A
Red with slight black stripe; "+"

Thanks for your help, Tom

[island911]

Green wire?

ah, green (Carrera) was previously brn/red (SC's)


pic from Carrera odometer stopped working. Speedo fine. = plastic gears?

[red901]

I Googled '88 speedo and found this photo. Seems like all my wires are in the correct location, as they are in the same place as this photo.

What about my 7.83 V on the green wire? Or, how do I test the hall generator in the trans?

[island911]

For refnce, Kurt/Rot 911 posted this '86 Speedometer Problem

The Carrera is different that the SC, for sure.

The sender, at least on the SC, is not a hall effect, but rather a mechanical switch, opened and closed by a magnet (Hall effect measures a voltage due to electron drift in the presence of a field) This means a simple ohm meter can measure the sender. Mine cycles with every qtr turn of the rear wheel.

[island911]

Here we go...

Originally Posted by Early_S_Man

... so I guess Bentley will have to do ...

From Urgent!! - Need Gauge Electrical Help!!

Looks like you have it wired right. Fwiw, my sender was causing me problems - turned out to be exposed wire grounding out. (my guess is this wire-pair often gets forgotten on engine/trans drops - abuse) If you need a new sender, they are ~$150
Edit, lucky you, the later Carreras used a less expensive sender ~$90

fwiw2 - My Bentley manual, for the SC, has the same schematic of the sender - which is misleading on the operation for the SC.

[DRACO A5OG]

Post a pic of the sensor on the gear box. I am sure you installed it correctly but let's just make sure.

In fact, let's test it. Remove it and with a magnet, run it over the magnet. Have a friend look at the speedo while doing this, if it bounces then sensor is good. If not, you need to check the connection at the tunnel access area in the rear seat footwell center.

Next thing would be is to clean and polish the contacts at the speedo, especially the ground contact.