Speedometer stopping Nightrider turbo boost

[Kyleonsite]

Bit of an odd one this one!

Driving to work and pretty much every day at roughly the same spot the speedometer 'dies' and drops to rest. The tripmeter doesn't increment either so it's the sender rather than the display/gauge. After - random time of ~30seconds to a minute ish the Speedo needle jumps back up and normality returns 

The thing is when the Speedo dies it's almost like a Nightrider style turbo boost happens! Whilst the Speedo is inoperable the car is definitely far more perky and responsive with a little extra power. The engine is a 2.0HDI.

It's almost like the Speedo takes 10bhp to operate and whenever it mechanically fails to work then instantly get a 10bhp extra power available.

I'm guessing the speedometer drive sensor is a hall effect digital output rather than a tachograph generator? Anyone know why the speedometer drive 'sucks' so much power?

[Sol]

It sounds more likely that most electrical loads are dropping out, except just enough to run the engine (ECU, fuel stop solenoid, pump etc) as th Speedo circuit wouldn't pull that much power.

Maybe there's a load shedding relay that is cutting out, taking most of the electrical load out and then cutting back in again.

I'd scan it for codes but a generic may not show much. Do you have Diagbox?

I'll have a look at the main circuits in Servicebox tomorrow, which power that section and see if anything jumps out.

[Kyleonsite]

Everything else electrical works okay when 'turbo boost' is happening (radio, headlights).

It happens roughly in the same place every day which is how I notice it. Alternator is ~6month old, battery is fine and starts the car normally.
Car has been running for 5-10minutes and done a few miles at motorway speed before it happens but it's always sooner into a journey rather than towards the end so possibly something that operates when a certain temperature (coolant / oil) is reached?

I don't have Diagbox/Lexia and my generic reader doesn't work on it.

[Sol]

No idea then, that's a mad situation I've never encountered.

[Kyleonsite]

No me neither, my GoogleFoo is pretty good but I've not been able to find similar instances of it occuring on any other M59's but thought I'd ask on the off chance!

[ffrenchie]

Have you thought of going a different way to work ?

[oilyrag]

Close Encounters was on over here at the weekend - was watching the bit where the big spacecraft comes in and poof!  Big power cut, dinner in the oven half-cooked.  Finished off with me in the rain, barbecueing and looking for bright lights. Might it be??

Maybe try unplugging the speedo and see if the 'boost' is still available.

[Kyleonsite]

"Have you thought of going a different way to work ?"

I did wonder about the 5G mast at the side of road around the point it does it - maybe it's not my brain cells it scrambles but the cars brain instead!

[Kieron G]

Do you drive past a power station maybe. Had this with a truck where gauges go haywire when passing the transformers at Drax power

[Kyleonsite]

No power stations and it does it on other journeys but always around similar distance.

It did it again this morning - radio unaffected, windows/horn/headlights still work.

Tripmeter decimal point miles don't increase whilst speedo isn't working.

Pulled over turned engine off and removed key, restarted and speedo still didn't work. Came back to life 1/2mile or so down the road.

I think I'll get a replacement sender for it.