Euromagination

Hi all. I'm having a weird patterned speedometer problem and I need to learn exactly how the assembly works so I can go upon trying to fix it. I think once I learn how it works and how the speed is read, it will be an easy thing to diagnose.

My questions are:

--Where on the car is the speed reading taken?
--What other things does the signal go through before plugging into the back of the speedo?

Thnx,
Harry

JasonAndreas

For manual transmission MY89-90 the signal comes off an impulse sender mounted to the transmission and for MY91+ the left front wheel ABS tension ring. With a tiptronic gearbox the signal originates at the transmission control unit.


The wiring diagrams for MY89-90 have the signal going directly from the impulse sender to the speedometer. For MY91+ the signal travels through the ABS control unit and to a frequency converter (PWM converter?) in the fuse box, it looks like an ordinary relay.

Euromagination

Thank you. Hopefully I can get this figured out. Until then, the car sits!

JasonAndreas

The first thing you should do is pop the gauge out from the dashboard and then clean the pin contacts on the rear of the speedometer with a pencil eraser.

parsecnc4

Pencil eraser? This is the first I heard of this and is interested in trying. How hard is it to pull the speedo and does it need special tools or require extra care to avoid breaking tabs?

JasonAndreas

Over time the pin contacts have problems with carbon buildup or oxidation (possibly from frettage corrosion?) and eventually the resistance is high enough to stop the gauge(s) from working. With a pencil eraser you can remove the carbon from the pins and the instruments will operate as new.

It's not that difficult but the few times I've removed my gauges I started on the far left gauge and worked my way to the right. No special tools were required.

parsecnc4

That makes perfect sense.... carbon remove with eraser. If only we can remove the carbon in our engines with liquid eraser

So just pull the gauge straight out like a cork in a bottle? Any reason from left to right? Does it give you some clearance to reach inside the dash and push the gague out from back?

craig001

Tape up the end of a small putty knife to protect everything. Place edge of knife underlip of gauge rim and work the gauge out carefully. Go easy at first to make sure the rime and glass don't come flying off. You might find it easier to remove the clock and then reach in and push the speedo out.