TR7 Speedo Screech

[TReslaV8]

Hello, So most times when starting from cold, I get a few yards down the road and the speedo starts to screech loudly and the needle spins round to the stop - nice to think I might be doing 130mph (I would be screeching if I were!) but clearly not, just 30 ish.  This goes on for 5 minutes then it stops and the needle reverts to a normal reading. Sometimes it does it again but once the car is warm it doesn't seem to do it at all. In normal operation the needle is quivering (ah, the excitement of a run out in winter!) rather than being steady but this may be normal.  The odometer works as per normal throughout. So to resolve, is this a speedo cable replacement ? Or is it likely to be the 'clock' itself? Ideas welcome.

David

[RobPearce]

That behaviour is, I'm afraid, not the cable but the speedo itself. My Spitfire's old speedo did it, until one day it broke completely.

The needle is connected to a sprung aluminium disk in close proximity to a magnet that is spun by the cable. Because they're not in contact, the needle is moved by the force from the induced current from the magnetic field. That's a pretty weak coupling. If there's a stronger coupling, such as a bit of grit or oil, then the needle swings right off the scale, often accompanied by a screech of the bit of grit rubbing on that disc.

It may be possible to fix it by dismantling and cleaning, but it's a delicate mechanism that's hard to get to and easy to break.

[PeteStupps]

Yeah to echo Rob's thoughts I had the same thing with my rev counter. It progressively got worse, during the 2018 RBRR I through Wales in the dead of night, until I disconnected the cable. By that point the coil spring had got stretched and it was useless, going off scale at tickover.

So don't leave it too long before doing something!

[TReslaV8]

Cheers, that will be the second speedo the car has had in only 40,000 miles! but it figures. I will have a go at trying to clear out any grit etc.

 

David