Speedo goes backwards on engine braking!!!

[TV8]

Last Sunday, I thought my speedo was startin to play up as it seemed out of sync speedwise with the satnav on a couple of occasions on the tunnel run in London. I put this down to the satnav with all the tunnels and high buildings. However, using the car for the first time since then, I paid a bit more attention and watched the problem get worse on a trip out today!

If I use the engine braking, the speedo drops back a little bit, if I accelerate and back-off the throttle without changing up, the speedo drops back to 10 mph, or less!!!

Given that I have a hyperactive rev counter, is this an electrical problem or more likely to be something to do with the sensor? Car is a 93/4 Chim 4.0.

Thanks in advance.

Graham

[GunShot (Wound)]

Can't help with the problem graham, but engine braking is NOT a good idea in these cars.

Gunshot.

[Kitchski]

So what you're saying is that if you take your foot off the loud pedal, your speedo starts to rotate backwards?

Erm, mine does that too mate! think most people's do!

[TV8]

Kitchski said:

So what you're saying is that if you take your foot off the loud pedal, your speedo starts to rotate backwards?

Erm, mine does that too mate! think most people's do!

It didnt do it when I first got, so possibly I didnt notice until the tunnel run when high revs and over-run where called for!

[Ribol]

TV8 said:

Kitchski said:

So what you're saying is that if you take your foot off the loud pedal, your speedo starts to rotate backwards?

Erm, mine does that too mate! think most people's do!

It didnt do it when I first got, so possibly I didnt notice until the tunnel run when high revs and over-run where called for!

So a lower gear and even more engine braking from higher revs?

You might have the traction police on your back shortly talking about wet roads

[MikeTVR]

Sounds like the reaction forces of the transmission on the overun are causing a slight change in gap between the propshaft castelated rotor and the sensor.
Mike

[spend]

MikeTVR said:

Sounds like the reaction forces of the transmission on the overun are causing a slight change in gap between the propshaft castelated rotor and the sensor.
Mike

How does it do that then? are you suggesting the bracket or diff nose are bending???

More likely the wires or connector (its commonly just pins bonded to the sensor on this model I believe ie made in Hungary version) being bounced around IMHO.

[spend]

Simon Says said:

i thought that the sensor is sensitive to these kind of changes anyways

Complete bks IMHO, its the BTR ABS sensor that is gap sensitive, the early sensor on mine works fine with @1/4" gap! There is a treatise on wheels and sensors somewhere if you do your research

If your hypothesis bears any weight, I would suggest that the OP would be posting about grinding and banging rather than an inoperative speedo.

[Bluebottle]

My speedo does the same thing, the moment the engine comes off of load (ie foot off load pedal) the speed reading drops, and I also have a hyper active rev counter.
Mine also developed a new trick last winter, when i discovered that switching on the heater fan caused the speedo to read 25% slower...I'm guessing its an earthing stap issue in my case. The instrument earth strap is the one near the gear lever under the centre tunnel trim.

[TV8]

Bluebottle said:

My speedo does the same thing, the moment the engine comes off of load (ie foot off load pedal) the speed reading drops, and I also have a hyper active rev counter.
Mine also developed a new trick last winter, when i discovered that switching on the heater fan caused the speedo to read 25% slower...I'm guessing its an earthing stap issue in my case. The instrument earth strap is the one near the gear lever under the centre tunnel trim.

Thats interesting - my temp guage goes up a few degrees and I lose a few lb's of oil pressure when I switch the instrument lights on!!!

The centre tunnel earth strap looks a bit tricky to get to unless I have missed something or looking in the wrong place?

[TV8]

Finally fixed this yesterday with a new sensor. Last time the car was in the garage they tried adjusting the old one and it was no better. New one fitted, rock steady and stable. Happy days. Just need to stop the rev counter from being hyper now!

[Bluebottle]

Totally forgot about this thread...update on mine;
Turned out to be the earth strap by the gear stick which had a stripped thread on the chassis. And as a bonus it has also totally cured my hot start problem as well

[blitzracing]

TV8 said:

Finally fixed this yesterday with a new sensor. Last time the car was in the garage they tried adjusting the old one and it was no better. New one fitted, rock steady and stable. Happy days. Just need to stop the rev counter from being hyper now!

Magnecors?

[TV8]

blitzracing said:

Magnecors?

Not sure? The leads are red! I am thinking of trying the car without the extension caps and seeing if that makes a difference.

What is the problem with Magnecors please?