Speedo Question - Input from Peter Morris if Possible

[cabrioman]

Hi Peter

How you doing?, missed most of the shows this year due to work commitments but still plodding away on the build when time permits.

Question for you regards speedo sensing. I have a CAi programable speedo which says it can take a pulse input from various sources which includes a connection for a hall sensor / ECU input. Looking at the wiring diagrams for the Sierra the speed sensor on the box is a simple setup connected to the original speedo and the ECU on pin 4.

Do you know if it is possible to successfully use the ECU to pulse an aftermarket speedo head, the CAi unit uses a series of dip swithes to calibrate the speedo based on wheel revolution/tyre size.

Your thoughts would be appreciated, I am wondering if I can save myself £40 on buying a prop sensor and the pain of fitting it by using the ecu signal.

Cheers

John

[cabrioman]

Re: Speedo Question - Input from Peter Morris if Possible

Some further information. I spoke to the technical engineering department at Caerbont Instruments who make the CAi programmable speedo. They confirm the speed sensor on the mt 75 box provides an ideal signal that can be used to drive the head, the 3 wires are black 12v+, brown 12v - and yell/brown signal (pin 4 ecu) which is a square wave output.

It apparently will involve some messing about with the switch values on the speedo head to get the calibration right so I will have to give some thought to how I do that, a sat nav calibration system would be fine if I could drive the car on the road and use the sat nav to provide the error correction value but I am not sure the boys in blue would be too keen on that pre sva.

I think I will give it a go to see if I can get it working from the speed sensor before I go down the route of a prop proximity kit etc.. if anybody has any pearls of wisdom or know its a waste of time please let me know.

Cheers

John

[pdm_engineer]

Re: Speedo Question - Input from Peter Morris if Possible

Hello John
Sorry for the delay with this reply but I have just returned from a week of driving my Cabrio around Cornwall.
Yes Caerbont are correct in saying the gearbox sensor is ideal for their speedo, and I would recommend that you try and get this to work before buying another sensor. What I can tell is that this sensor provides 8 pulses per revolution. Also the local Ford dealer told me that it is only used to help prevent engine stalling when the car comes to rest. The one I had was broken and replacing it made no difference to how the engine behaved.
You are going to have to calibrate your speedo no matter what sensor you use, so here is the theory if you don,t already know.
A) Find the rolling circumference of you wheels (push the car along a flat level surface and measure, over as many revolutions as you can, how far the car has traveled per wheel turn.(measurement in metre)
B) Multiply above by 1609.34 to give you the number of wheel turns per mile. (1 mile = 1609.34m)
C) Multiply above by your diff ratio, probably 3.92 in your case.
D) Multiply this by the number of pulses per prop revolution, in your case this would be 8.
E) This last figure will be you calibration number to use. Find the nearest in the chart provided for your Dip switch setting.
As for wiring you should be able to get away with disconnecting the ECU pin 4 connection and wiring this directly to your speedo input signal.
Please realise that I have not used this sensor myself as I chose to fit a proximity sensor on the propshaft, and BMW gearboxes don't have a speed sensor. (It's in the Diff)
If you need my artical for using a propshaft fitted proximity sensor let me know and will E-Mail to you.
Good luck with this John and let me know how you get on.
Don't forget that you can always E-Mail me directly for a quicker response. (My address is in pitstop and on this forum)
Regards Peter

[cabrioman]

Re: Speedo Question - Input from Peter Morris if Possible

Hi Peter

Hope you had a good holiday and managed to find some half decent weather on the way. You confirm my thoughts, I will try to get it going with the speed sensor signal. I know several people have had problems with the prop/proximity route although most seem to be the problem of the sensor hitting the prop when the drive train moves rather than getting the signal to work. Any way at £40 for a proximity sensor I think I may as well try the speed sensor route first.

I do have a copy of the article you wrote and thanks for the info on calculating the calibration, I'll let you know how it goes.

Anyway most go now as its 8.00 Sunday morning and the garage is calling!! Hope to see you at one of the shows towards the end of the year, havn't managed to get to any yet this year due to work and family commitments but such is life.

Cheers

John

[pdm_engineer]

Re: Speedo Question - Input from Peter Morris if Possible

John
Sorry but have just noticed my mistake with item B above. It should be "devide 1609.34 by your wheel circumference" to give number of wheel turns per mile.
I also think you may have a reduction drive in the gearbox which drives the speed sensor. I don't know the answer to this but maybe someone else does. If this is the case you will have to devide your final figure from the above by whatever this is.
For the record my proximity sensor points at a disc, sandwiched between the propshaft drive flange and the differential drive flange, and works perfectly.
I also do not recommend the magnet/reed sensor kits, partly for the reason you give, and also the reluctance for the reed sensor to switch fast enough at higher speeds.

Cornwall was mostly dry for me this week but was very windy. Must have been sunny too as I now have a red nose.

Peter