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Thanks all for these tips.

My 1991 LHD Spider Veloce's speedometer appeared to have failed (along with the odometer) and I normally use a He-Man HUD thing or my Tom Tom to keep me honest on speed.

Recently, however, I noticed that it started working again above 40 mph but only when the aircon was on. The only other change I had made to the car was to put the hardtop on it, but I had not connected the rear screen heater. (The hardtop was on in high summer because I was taking the car back to the UK for an MOT and respray before anybody asks:))

Very odd but I can only replicate this intermittently. I suspect a bad earth somewhere.

The car is an automatic if that makes any difference.

Peter
 

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I don't think I know any more than you about these, Dom, apart from some hands on experience!

The problem is unlikely to be a bad earth as the pulse generator earths through its casing. If you compare it to a manual cars generator you will see that there are only two wires, vs. three for the manual car. One of the three in the manual car is an earth wire.

One of the two wires on the automatic car generator carries the signal (blue grey colour, from memory) to the speedo head, the other (pink, again from memory) feeds 12v to the generator. The wires can fray so its worth connecting up separate wires to see if that is the problem. You can get to the connectors if you can get under the car, the generator is screwed into the side of the gearbox.
Thanks. I have a lift at my second home, so I can get under the car quite easily once I have the S4 back there. I shall do as you suggest.

Peter
 
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