Discussion
I did the above speed all the way to work this morning according to my speedo - even when stationary at traffic lights. The odometer has also stopped working. When the ignition is switched off, the speedo returns to 0mph; switch it on and it flies around to 195mph again. The odo is not on anything ending in **999 & I've searched for speedo problems on this forum but can only find threads on speedo's refusing to move or being inaccurate. It's a 1994 4.0l car.
Does anyone have any suggestions?
Does anyone have any suggestions?
did you manage to brake in time for that fence you can just see? 
When I had my 94 Chim, my odometer was the LCD type, and on one occasion when starting up, it zeroed !!!. The speedo still worked, and the odometer then started clocking up again, but obviously starting from zero miles. Every time I turned the ignition off, then on, it zeroed again. I spoke to my local dealer, who said they could help, so I took the speedo out, gave it to them, and they fixed it, tok about a week. Apparently they explained it as a dry joint, means nothing to me, but they charged me around £45, they returned the odometer to it's correct reading, and from then on it worked perfectly.
Thought I had the old sticking @ 999 problem with mine, but it cleared on it's own.
When had the problem I reported it to the garage they said that Smiths will repair them, but you have to send them away and can take a few weeks to come back. They said it is obviously illegal to drive without a speedo, but I said I had a roadangel which allegedly gives you a more accurate speed reading than the speedo.
Anyone know if this would satisfy the boys in blue?
Could be useful information for anyone with similar problems.
When had the problem I reported it to the garage they said that Smiths will repair them, but you have to send them away and can take a few weeks to come back. They said it is obviously illegal to drive without a speedo, but I said I had a roadangel which allegedly gives you a more accurate speed reading than the speedo.
Anyone know if this would satisfy the boys in blue?
Could be useful information for anyone with similar problems.
Tafford said:
When the ignition is switched off, the speedo returns to 0mph; switch it on and it flies around to 195mph again.
Does anyone have any suggestions?
The speedo is electric and repsonds to the frequency of impluses generated by the toothed wheel and pick up on one of the drive shafts. If it is permanently at full deflection with the ignition on it suggests that it is either being fed with 12v constantly or the signal generator - amplifier has gone mad and is sending a constant high-frequency output to the speedo. I'm not sure if the first scenario is possible tho' as it may be that the speedo needs an alternating signal to function and that 12v DC would do nothing depending on its mode of operation. Either way it sound like theres a problem with the supply to the instrument.
It might be worth visually checking the pick-up and reluctor / toothed wheel first as these are accessible under the rear by the diff'. As a guess I'd be looking for a misplaced pick-up with reduced air gap or contact with the reluctor or debris, (posssibly metallic) on the pick up. If you happen to have an oscilloscope you could check the operation of this directly. Otherwise you'll probably have to take the dash top off and check out the wiring / voltages at the speedo and the instrument itself.
tahiti450 said:
Either way it sound like theres a problem with the supply to the instrument.
But if the speedo is working properly and merely reacting to a spurious incoming signal, wouldn't the reorded mileage be incresaing at the rate of 195mph or more)? I think it would also raise the engine's idle speed too, as the same signal is sent to the ECU.
Also, the speedo would fall to 0mph with the ignition switched off, as that cuts the supply to the instrument, irrespective of any signal being received.
But Andy....;
As regards the odometer, IIRC there is a separate motor which drives this and the speedo needle is more like an linear / induction motor, I don't think there is a mechanical connection between them. I agree that if there was a pulsed signal then you would expect the odo to be going as well, so maybe DC? The effect on the idle speed is quite subtle, but as you say if there was a constant in-motion signal to the ECU the idle would be slightly elevated, say 2-300 rpm.
>> Edited by tahiti450 on Monday 3rd October 11:01
Tafford said:
When the ignition is switched off, the speedo returns to 0mph
As regards the odometer, IIRC there is a separate motor which drives this and the speedo needle is more like an linear / induction motor, I don't think there is a mechanical connection between them. I agree that if there was a pulsed signal then you would expect the odo to be going as well, so maybe DC? The effect on the idle speed is quite subtle, but as you say if there was a constant in-motion signal to the ECU the idle would be slightly elevated, say 2-300 rpm.
>> Edited by tahiti450 on Monday 3rd October 11:01
petea said:
is the digital mileage retained?
Yup, shouldnt make any difference, if yours is the same as mine, the odometer is basically a traditional mechanical counter with revolving drums etc - as with a mechanical speedo but instead driven by an electric motor. Sorry I just note that an LCD odo is mentioned above, (I think the earlier cars had these?)I have no experience of this speedo so i wouldnt like to say for sure. Having said that the other cars with electronic displays i've had haven't zeroed when the battery is disconnected, so i would doubt that there would be a problem (would make fiddling the mileage a bit simple....).
>> Edited by tahiti450 on Tuesday 4th October 11:11
Ok, now have dash off. There are three wires going to the back of the speedo - black, green and blue/yellow.
Following is with iginition on : Green shows 12v when checked to earth so assume that is power. Black continous to earth so negative/earth. Mulitmeter between green and black also ahows 12v as expected. However, reading between green and blue/yellow also shows 12v - I would have assumed blue/yellow to be the pulsed feed which would show nothing when car stationary. Any thoughts?
Added : Black to blue/yellow - 0V, 1000ish ohms resistance
>> Edited by Tafford on Tuesday 4th October 11:44
Following is with iginition on : Green shows 12v when checked to earth so assume that is power. Black continous to earth so negative/earth. Mulitmeter between green and black also ahows 12v as expected. However, reading between green and blue/yellow also shows 12v - I would have assumed blue/yellow to be the pulsed feed which would show nothing when car stationary. Any thoughts?
Added : Black to blue/yellow - 0V, 1000ish ohms resistance
>> Edited by Tafford on Tuesday 4th October 11:44
I don't know for certain about the wiring, but in my understanding of how it works I would have expected three wires: an ignition-switched live (12V) a ground and a pulse. It looks like that is what you've found.
Nor do I know how the sender functions, but the results you have found with your multimeter would indicate you've found the wire from it, although I would have thought it was an inductive device (and so could not provide a voltage if the car was stopped): for it to show 12V with the car stopped, as it appears to have done, is a surprise. Similarly, like Tahiti450, I would be surprised if the speedo showed a maximum reading if a permanent 12V was supplied on the pulse line.
Maybe the way the speedo works is that it shows 0mph with no voltage on the signal line, maximum speed with a permanent 12V and all the readings in between when there is a pulsed supply, but that the odometer only counts up while there is a pulse, or maybe it moves up slightly each time 12V is applied ie. it is calibrated to know that, say, 100 pulses = 0.1 miles?
It wouldn't seem to me to be the best design if that is the case and I would expect to see the speedo needle jump each time the ignition was switched on. All in all, it takes you back to square one: either it's the sender or the speedo! Hopefully, someone will come along who can say for certain how it works.
Nor do I know how the sender functions, but the results you have found with your multimeter would indicate you've found the wire from it, although I would have thought it was an inductive device (and so could not provide a voltage if the car was stopped): for it to show 12V with the car stopped, as it appears to have done, is a surprise. Similarly, like Tahiti450, I would be surprised if the speedo showed a maximum reading if a permanent 12V was supplied on the pulse line.
Maybe the way the speedo works is that it shows 0mph with no voltage on the signal line, maximum speed with a permanent 12V and all the readings in between when there is a pulsed supply, but that the odometer only counts up while there is a pulse, or maybe it moves up slightly each time 12V is applied ie. it is calibrated to know that, say, 100 pulses = 0.1 miles?
It wouldn't seem to me to be the best design if that is the case and I would expect to see the speedo needle jump each time the ignition was switched on. All in all, it takes you back to square one: either it's the sender or the speedo! Hopefully, someone will come along who can say for certain how it works.
Again, thanks for all your help. I sent the speedo to Speedy Cables who confirmed there was a fault but advised they needed 6 - 8 weeks to get parts. Neil at tvrcarparts.com (01884 266755)had meanwhile worked out that it was an ETB speedo (cream face, black bezel), and that they could fix it in about a week. Speedo thus dispatched to ETB www.etbinstruments.com/index.htm and returned repaired for approx £100. Apparently circuit board had burnt out.
Top service from Neil at tvrcarparts.com and ETB Instruments - thanks to both!
After driving without a speedo for a while (on private roads of course) it is very noticable that I was going slightly faster than I thought......
Top service from Neil at tvrcarparts.com and ETB Instruments - thanks to both!
After driving without a speedo for a while (on private roads of course) it is very noticable that I was going slightly faster than I thought......
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