Speedo jump at mobile traffic lights

I generally slow down for road construction.

 

I have experienced the same issue in the past. I have not tried to correct it since it happens infrequently. I am certain the cause is a bad or loose ground or possibly bad solder joint on speedometer circuit board. I am interested to hear if others have experienced this issue and how they resolved it.

 

Funny you should mention it but on Sunday I went for a long drive, mainly freeway and then I got off and entered a small town where they were conducting roadworks and they had a mobile traffic light set up and then when I took off, I noticed my speedometer had stopped working.

I stopped at my destination for a few hours then when I got back in, it started working again but only for about 10 minutes or so and then died..

Tried again the next day and the same thing, would work for 10 odd minutes and then stop.

A mechanic friend of mine also has a 328 so we were able to swap speedometers to see if it was the speedometer but it wasn’t so it must be the dreaded Speed sensor unit which has shot up in price over the years..

 

Haven’t experienced this on the 328 but seen it last week I hole driving another car (Alfa Romeo SZ).

 

Could be the sending unit as well as I know the Alfa Romeo Spider Series 4 used the same sender so there’s a good chance the SZ does as well.