12V+ at speedo sensor?

My 95 F355 speedometer isn't working, stuck on the peg at 10mph. I have read that most people say replace the sender, which I am about to do. However, I wanted to test my circuit first to see if everything is in order. I have tried unplugging and plugging in the gauge cluster a few times. All other gauges and lights are working so I don't think I have a bad connection there. Tried unplugging the sensor, and tried various ways to test it. It doesn't seem to produce any kind of signal that I can read with my basic DVM, even with the engine spinning the wheels on jackstands.

I read on my348 that there should be 12V+ present at the VSS. I don't have 12V+ there with the key on. I even tried supplying 12V+ to the green wire on the sensor and routing the other signal wire back to the plug and, presumably, back to the speedo or wherever it goes.

Bottom line question: Should there be 12V+ coming to the sensor?

 

According to the wiring diagrams, yes. The green/white wire on the sensor gets power from the fuse which supplies power to a range of items (oil temp, fuel quantity damper, airconditioning components), as you say, with the ignition on. It's the "AC system" fuse (fuse 24 or 25 [passenger footwell fuse/relay panel] in your manual, depending on the car).

Hope this helps
Cheers
Ian.

(EDIT) Fuse 25 for 1994 model, Fuse 24 for 1995 onwards. Not sure about Challenge and F1 cars.

 

Thanks again, Ian. You always seem to know the electrical issues. I suppose that means you've solved a few or your own!

Brian

 

Fortunately, not so many on my F355 (knock on wood). I just spent quite a few decades fixing aircraft electrics and many years fixing Lotus problems.

Having said that, redrawing and adding notes and pretty colours to most of the official F355 schematics, by hand, over several months did provide some insight into the inner workings of our cars. This also overcame the problem of aligning all those confusing lines on photocopied and stickytaped paper diagrams. I also had some help from the forum in identifying some of the more obscure electrical symbols

Anyway, I hope you find a simple fix to your problem (like a blown fuse)

Cheers
Ian.