[davediesel]

2000 w mk2 ghia x td erratic speedo....changed clocks twice, speedo sensor (plastic thingy with wire that attaches to drive in box).
actual drive in box is turning and does not slip or anything.
i actually clocked 1 mile the other day while parked up with engine running!! mechanic has spent hours checking over and has drawn a blank....

 

[mondeoman]

Cracked track/s on the circuit board that is the backplane of the instrument cluster.

 

[davediesel]

just changed clocks out of another mondeo yesterday...same problem?
put back in other car still working ok so rules out the clock itself...

 

[guardtower]

Is the rev counter eratic too?

 

[davediesel]

no rev counter is as normal fuel guage temp warnings etc all as they should be....
what is confusing me is the fact it clocked up
while stationary could it be possible for drive in box to move while stopped?

 

[mondeoman]

MkII has electric speedo, so a series of pulses on a wire would be the drive mecahnism. It is possible that something is causing interference and a spurious signal is being induced on the speedo wire. Alternately a break in a wire or circuit board track that is highly intermittent could be made broken at a high rate by engine vibrations at idle which could possibly make it give non-zero readings while at rest.

 

[davediesel]

yes thats right it has a standard speedo drive on gearbox in that plugs a hexagon shaped plastic thing that converts movement in to pulse this has a wire on that connects to loom,have changed this for a second hand one which made no difference.
we put a tester on wire which showed no brake in wire.it is a taxi so inacurage odometer is causing major problems!

 

[LeeST200]

I've got the exact same problem! I've got a Mk2 1997 mondeo and the speedo jumps wildly. Sometimes it stays on zero and other times it jumps all the way to 130 even though I'm only going 30mph! I can also feel a slight fluctuation in the power of the engine when I'm driving as soon as the speedo problem starts (feels like when you turn your air con on - that slight loss in power).
Does anyone have a clue as to whats going on?!
LEE

 

[mikes]

Sounds to me like interfearence on the speedo pulse signal. This could come from many different places, but the best way to check would be to observe the signal at the actual clock end of the cable. If one of you guys has a multi-meter then have a look at the DC voltage on this cable, it should be a smooth pulse between a voltage (prob 12v DC) and a low which shold be 0v.

It sounds like somewhere the ionterference is stopping the "low" part of the signal from reaching 0v and this is causing the speedo unit to see pulse's which aren't actually there.

Once you have seen the problem on the cable there are a number of ways to potentially proceed.

I realise if you know nothing about electronics then this might all be a bit foriegn, just say if it is and i'll work out another way of testing it for you.

 

[davediesel]

ok the latest we changed the drive in the box did not look worn.....new one in same problem!

looks like when i put pressure on clocks with hand it settles down???

going to change plug on back in the morning.

already put 3 clocks in so cant be that

 

[LeeST200]

I've noticed that if I put blower on hot and full power, and turn it so its on the windscreen, the speedo seems to settle down and work ok? It only starts working OK when the blower is hot enough though?! Could it be moisture somewhere on something electrical?

LEE

 

[mikes]

That sounds alot like a break in the cable, somewhere near where the heater is. This is becuase as the cable warms up, because it is near the heater the copper inside will expand an amount, therefore briding the gap which has clearly formed inside the cable insulation.

You can solve this by running a new cable from the sender to the back of the clocks.

 

[LeeST200]

Thanks for that! :L I'll have to get the clocks out and have a look. Hopefully it won't be too much of a big job.

LEE

 

[mikes]

Good luck

 

[davediesel]

stripped off the plug on the back of the clocks and fitted a new one,made no difference....
garage put a meter on wires above speedo sensor....showing resistance..anyone know if its possible to rewire speedo as after hours of checking he cannot find the short?

on top of this the timing belt went yesterday

 

[davediesel]

could it be ecu fault...no other symptoms apart from speedo :headshake:

 

[mikes]

Did the mechanic check the continuity of the cable from where the sensor is to where the clocks are? You need to check the entire length of cable really! I'm sure the sensor is fine, and the clocks are obviously fine, as are the connectors, so it has got to be a fault with the cable.

It may not take the form of a short, it may just be that the copper has come apart in side the cable, or moisture has caused the cable to degrade?

 

[davediesel]

easier said than done as cables are in loom and loom is buried...will have to go to an auto spark i think..

 

[davediesel]

cured at last!! turned out to be wire in to fuse box pulled out slightly,caused by someone pulling down on fuse box.... anyone else with problem like this it is worth trying first.....

thanks for help on this and all the best..