Intermittent speedo when driving, affects MPG

[Jay_G]

Hi guys, couldn't find anything similar do making a post.

Noticed it a few times now, driving along and my speedo would just drop to zero. And it would stay there for a random amount of time, then pop back up and carry on. While its at zero, my MPG drops rapidly (understandable), so I guess it's not the clocks, as I doubt the trip computer works out the MPG from where the dial is, so I guess it's a sensor.

Any idea where to look? I guess speedo sensor on gbox, but I couldn't find a definite answer whether my 172 has one there.

Also, is replacing the dials a good idea? Could rule it out I suppose?

Thanks guys and girls!

 

[Daniel]

How old is your 172?

 

[Jay_G]

Sorry, didn't even think to say. It's a phase 2. 51 plate

 

[Daniel]

No gearbox sensor on those, there's a pick up on the ABS iirc.

 

[TheEvilGiraffe]

Good ol' French electrics.

If the speedo/odo says no distance is travelling, but you're actually using enough fuel per second to maintain 30-70mph, it's going to decrease a lot as it thinks you're doing zero miles for x gallons.

 

[jon_r]

I've got the exact same problem on the phase1. I know what's causing it but it's doing wonders for my limited mileage policy.

 

[Jay_G]

No gearbox sensor on those, there's a pick up on the ABS iirc.

Sounds about right, cheers Dan. Although it Sounds s**t, annoying and expensive already. I guess if there was a dodgy sensor, my ABS would be kicking up a fuss to?

Good ol' French electrics.

If the speedo/odo says no distance is travelling, but you're actually using enough fuel per second to maintain 30-70mph, it's going to decrease a lot as it thinks you're doing zero miles for x gallons.

Haha yeah, that's why I didn't question it ;p, I just wondered if that made a difference with troubleshooting, as if, for example, the trip computer took an electrical signal from somewhere else, and the speedo fell to 0, if the mpg stayed constant I would probably blame the clocks, but as its falling,I guess the ecu/car as a whole thinks I'm not moving

 

[Jay_G]

Does anyone have more info? Mainly on the abs pickup if possible please

 

[Dan@SJM]

No gearbox sensor on those, there's a pick up on the ABS iirc.

Not always the case. Early ph2's are a parts bin special. Mine for instance has a speedo sensor in the box.

Speedo sensor is on the back of the diff housing, easiest way is look from above and find where the drive shaft goes in to the box, on the back of the casing there could be a black marker sized sensor.

 

[Brett]

It's not actually affecting your MPG though, just the MPG readout.