Cause of my Speedo Needle dropping to zero ?

[Error404]

check the earth pins at the bottom of the A-pillar in the drivers footwell for corrosion - they are up behind the bonnet pull

[macspring]

Important Update

Not more inforamtive I'm afraid, just as confusing it not worse.

Measured the voltage at the fuel pump, and it is 7 volts. so wired up a swtched positive supply from the battery, and went for a drive. It now will go past 4k rpm and onwards to 7k rpm, but still is reluctant to go past say 4k rpm, the speedo drops from it's indicated speed as one goes wot from say cruising at 4k rpm in say fifth or sixth. Like as if there is enough electrical power to go around, Is it a bad or ineffective earth issue?

Where are the various earths that deal with the engine and the instrument cluster ?

I have a spare speed sensor to swop out of the gearbox, and will also check the condition of it's connector plug and its's adjacent section of wiring.

Bit of a pain in rear, as the urS6 revs to the limiter no problem. Just want this car to be able to do the same, with it's much fresher engine.

Checked the spark plugs for tightness, the rubber boots are very fresh, I have the urS6 POS units to swop in to make sure it is not not them.

Is it a permanent + power to the fuel pump, with a switched - earth, or the other way around ?

Will post up what I find when I've relayed the fuel pump and swopped in the spare, known to be working POS.

Any suggestions most welcome.

Thanks.

Update.

Firstly I was measuring the voltage at the fuel pump incorrectly, so ignore the 7 volts comment.

Also I ran a separate new earth cable to each of the two HID headlights which also gives a good earth to the main beam bulbs also. No more HID issues.

I ran an additional heavy duty engine earth cable. Make sure yours is been refitted if your engine was out, and that you are not relying on the battery earth in the boot Avant, under rear seat in Coupe.

But the most important item I found and 90% solved my engine issues is I looked at the two big connector plugs mounted on the on the bulkhead of a RHD car. i.e. behind the ABS unit in the engine bay. Hopefully someone can post a suitable photo of them.

The engine bay had been power washed years ago and won't start afterwards, and like most people I assumed it was a water ingress issue with the coils.

I don't believe it was now, as when I opened one of the two connectors the pins were all green and corroded like a copper roof on a building.

I've tried to clean it as best as I can, and sprayed in electrical contact cleaner. I feel cleaning and spraying this connector and the less affected one beside it has made a huge difference.

The engine now revs to limiter until wot now as intended.

Just need to figure out how to clean the connector pins back to as new condition, either in situ, or by taking them out of the individual connector housings ?

Any ideas ?

As it is such an easy thing to check and plays havoc with the coils, therefore the sparks to the engine, giving all sorts of bad / hard to fathom symptoms, I would urge everyone with an ABY / ADU engine to open the two connector plugs and check they are making a good electrical contact.

Important tip, don't power wash your engine bay.

HTH someone else. Worked for me.

[Error404]

Try this stuff as recommended by Newsh, its is well worth the money. Then apply some dielectric grease to the pins to prevent moisture problems in future