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[prefacelift]

hello people hope all is well.
I'm sure ive seen a topic on this before but cant find it at the moment.
99 mfl 3.0 manual 94k on the clock.
so I'm driving along the motorway 70mph cruise on, then the speedo drops to zero cruise disengages then re-engages then speedo back to 70mph no warning lights, all normal again, any ideas?
regards
steve    

[Lincs Robert]

Do the ABS & TC lights come on?

The speedo is driven electrically from the ABS control unit & it’s a common failure mode for the symptom you have combined with these lights coming on.

If they aren’t coming on then I’d suspect a poor connection in the instrument cluster. It’s easy to remove, press home  hard & maybe clean the flexi plug in connector - often fixes the problem.

[prefacelift]

no warning lights came on, thanks Robert i'll take the cluster out at some point and have clean! 

[Lincs Robert]

It’s interesting that the cruise drops out as that indicates the speed signal to the cruise control unit is also dropping out.

It’s a blue/red wire, it originates at the ABS unit & goes into the pass footwell (where it feeds the satnav connector) the dash, feeds the stereo, upto the sunroof & then out to the cruise unit.

Could be a damaged wire - good luck.......

[TheBoy]

When it happens, do the fuel computer readings (such as Instantaneous Consumption) go to pot?  If so, suspect the ABS ECU.

[DrAndyB]

When it happens, do the fuel computer readings (such as Instantaneous Consumption) go to pot?  If so, suspect the ABS ECU.

Yep, as mentioned, Check all other speed functions. 

I had the same one day as the speedo just stopped working completely (everything else on instrument cluster was OK and no warning lights.

ABS and Traction control ALL WORKED fine, but Cruise Control didn't work not work as all gets signal from ABS ECU (Pin 9) - BLRT colour wire(Blue with Red Tracer).  I changed ABS ECU and normal service was restored  (after filling the swear box from the job of changing the ECU as the 6 torx screws are a TW#& to get to  but doable.

Check, as mentioned, if instrument cluster seated OK.  If cluster out is, then check continuity from ECU Pin 9 to Immobiliser, then other side of immobiliser to connector in passenger foot-well (Its from the footwell that it then goes to all the other functions (Cruise, Instrument, Climate control, Radio, sunroof etc).   If continuity OK, then its ABS ECU.

Pin 9 of ABS Ecu goes via a connector in engine bay by battery.  Its (RH I think) of the two small black torpedo shaped connected (small rubber boot on the top - couple inches diameter - Plug has a locking ring halfway on it from memory which will need to undo to access connections inside.

Good luck, and let us know how get on.

Andy.

[Chazza12]

if its been raining check the rubber seal on the abs unit were the connection fits. 

[TheBoy]

if its been raining check the rubber seal on the abs unit were the connection fits.

I admire your optimism