IT CAN'T BE THAT FAST !!!

[s3c chris]

Hello again.
Following on from my recent post asking for a good used speedo, I have "sort" of made progress!
I had an old speedo already with a cracked glasswhich I had previously tried in the car with no joy.
As i had a brand new sender too I made a little test rig up and lo and behold it worked.
The original was soon removed from the car and the glass and bezels swapped over. Still no speedo.
The wiring was once again checked and found to be in order and when the new sender was connected and powered by a battery drill, yes, the speedo worked! I did rec heck the original which did prove to be U/S.....
It appears that the "new" sender the previous owner had fitted was not working so connectors duly swapped and my new sender was fitted.
I eagerly took the car out and the speedo was working.
However, all is not well.... I didn't think any S could be that fast and when checked with a Sat Nav, the speedo is consistantly reading twice as fast as it should be! No juddering, skipping or hopping, just 2X too fast!
Any ideas would be welcome please, at least I know it's not the sender!
I may still need a speedo after all.....
Regards Chris.

[v8s4me]

Wrong sender?

[phillpot]

Faulty Sat Nav ? ......

[Le TVR]

v8s4me said:

Wrong sender?

Most likely. The inductance of the sender is mis-matched with the speedo circuitry. Instead of seeing a square wave type of signal the mis-match causes 'ringing' which is always a harmonic of the original frequency, most often the 2nd. As you have found.

[s3c chris]

Hello and thanks for the replies.
I'm sure the Sat Nav is reading correctly and am also sure the sender is the correct one. Genuine Ford for a Granada.
There are two adjustment (I assume?) potentiometers inside the speedo, anyone know what they do?
It would be a bit of a pain to get the bezel off again though... Could a resistor be put in the pulse line?
I'm at a loss!
Chris.

[v8s4me]

As LeTVR says, there must be mismatch between the sender and the speedo. If youre sure youve got the right sender for your gearbox, where did your new speedo come from? Is there a choice of terminals on the back of the speedo; maybe it is designed to work with more than one type of sender?

If you need to loose half the signal then its probably not a resistor you need but maybe a transistor? Maybe someone on here will know how to make up a little IC board for you.

Good luck

[s3c chris]

Thanks for that, maybe someone can figure it out?
The speedo was a used one from David Gerald, cost 80 plus VAT. I tried it in the car last year as I assumed the sender was ok but obviously it didn't work so I took it apart to look inside, that's why the glass was broken...
I'm sure the sender is the correct one, it is brand new and identical to the one I removed which was a TVR part, I have the receipt for that.
Hopefully I will be able to sort something out but if anyone has any more suggestions, they would be greatly appreciated.
Regards Chris.

[Longers]

Is the speedo in km per hour?

[Le TVR]

If there is a mismatch it is probably the capacitance loading inside the speedo. I doubt whether adding resistors in the connection cable will do anything.
There always seems to be something with TVR instruments and senders, I had the same issue with the temperature gauge. Temperature sender was from TVR, looked identical but the gauge shot straight into the red after a few minutes! Caused no end of panic before I realised what was really happening.

[Barkychoc]

Anybody know what the voltage of the square wave is? It's pretty easy to make a frequency divider you would only need a chip that costs pennies. The output would be a 5 volt square wave which may work anyway. If it needs to be 12v it's getting more complicated and I think you'd be better off getting the speedo sorted.
I bought a new one from speedy cables and it was great. You send your current one to them and they build a new one with the same mileage showing. Odometer is digital with a trip meter